School Science Lessons
Topic 16d
2024-06-21
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Milk, cheese, dairy products
Table of contents
16.2.4 Cheese making
16.0.0 Milk testing and quality control
16.2.0 Milk and dairy products
16.3.0 Casein and caseinates, Milk proteins
16.3.1 A2 Milk
3.5.6 Casein, C81H125N22O39P
7.8.0 Colloids and crystalloids
4.7.1.3 Lactase
Lactic acid
4.3.19 Micro-organisms and milk quality
9.2.19.2 Prepare milk agar solution
19.2.10 Pasteurization of milk
19.1.13 Prepare carbon dioxide, sodium bicarbonate with vinegar
3.3.5 Prepare casein plastic from milk
12.7.10 Prepare lactic acid with milk
4.2.11 Prepare lactose from milk or whey, using immobilized lactase
3.2.7 Milk, invisible writing ink
11.3.9 Size of colloid particles
16.0.0 Milk testing and quality control
16.0.1 Milk sampling
Experiments
16.1.1 Organoleptic tests (Taste, sight, smell test)
16.1.2 Clot on boiling test (C.O.B. test)
16.1.3 Alcohol test
16.1.4 Alcohol alizarin test
16.1.5 Acidity test
16.1.6 Resazurin stain, test for milk
16.1.7 Gerber butterfat test
16.1.8 Lactometer test
16.1.9 Freezing point determination
16.1.10 Inhibitor test
16.1.11 Pasteurized milk test
16.2.11 Tests for milk fortification with calcium carbonate
16.2.0 Milk and dairy products
16.2.1a Regular cows milk, fresh milk, full cream milk, whole milk
16.2.2 Reduced milk, milk powder, condensed milk, evaporated milk
16.2.3 Butter, butter oil, clarified butter, ghee
16.2.4 Cheese making
Rennet (rennin or rennilase)
Permeate
16.2.5 Whey
16.2.7 Yoghurts
Yakult
16.2.8 Dairy desserts
16.2.9 Cream, regular cream and sour cream
16.2.10 Ice cream
12.7.10 Prepare lactic acid with milk
1. Leave a beaker of milk on a shelf for days until it has gone sour.
Pour off the watery liquid, leaving as much as possible of the white curd behind.
Filter the liquid and boil it in a beaker for a few minutes to precipitate any finely suspended matter.
Leave the liquid to cool and again filter it.
The filtrate is an almost colourless solution of lactic acid.
Tests it with blue litmus paper.
Form crystals by boiling the solution in an evaporating basin on a gauze and tripod until the volume of the solution has been reduced to one quarter.
Complete the evaporation by leaving the evaporating dish in a hot place.
2. Lactic acid, CH3-CHOH-COOH, forms in milk due to the action of fungi and bacteria acting on the lactose sugar, especially Lactobacillus bacteria.
The presence of lactic acid, produced during the lactic acid fermentation is responsible for the sour taste and for the improved microbiological stability and safety of the food.
Investigate the factors influencing the rate of formation of lactic acid with starter bacteria, e.g. plain yoghurt.
The possible factors include heat, amount of bacteria added, light, access to air, shape of container, sugar concentration, initial pH, amount of fat (normal, low fat, skim), degree of agitation.
3. Acidity of fresh milk may be measured by titration with a 0.1 M NaOH solution, and shows the reaction with NaOH up to a phenolphthalein end point.
Fresh milk contains almost no lactic acid and the reaction with NaOH is used to change the pH of carbon dioxide, citrates, casein, albumin and phosphates.
This indicates lactic acid concentration about 0.13% of the determination of "acidity" in fresh milk by means of titration with NaOH.
It is a measure of the buffer action of milk, so it is called the "initial acidity".
The "titrarable" acidity of fresh milk is not due to lactic acid only.
The acidity, the result of bacterial activity producing lactic acid during milk collection, transportation, and processing can be called the "developed acidity".
Measure "titrarable acidity" (not calling it "lactic acid concentration"), subtract the initial "acidity" to get a value for the "developed acidity".
An example, for the titratable acidity of milk:
Day 1: = 0.0288M (7.10 mL titre), initial acidity
Day 7: = 0.0815M (15.65 mL titre), titratable acidity
4. Titratable Acidity of Milk The "Titratable acidity test" measures if milk is of a high acidity to reduce its keeping quality and heat stability.
The "natural acidity" is caused by citrates and phosphates in the milk and dissolved carbon dioxide.
The "developed acidity" is caused by lactic acid produced action of bacteria on milk lactose.
Titratable acidity refers to the total acidity, i.e. natural acidity + developed acidity, measured by titration.
The method below measures the buffering capacity of milk and not the true acidity:
* Fill the burette with N/10 NaOH solution.
* Mix the milk sample, but do not add air.
* Use a pipette to transfer 10 ml milk to a conical flask and add an equal quantity of distilled water.
* Add 3-4 drops of phenolphthalein indicator solution (1 g phenolphthalein in 100 ml of 95 % ethyl alcohol) and stir with glass rod.
* Titrate with N/10 NaOH solution until change to pink colour remains constant for 10 seconds.
Percentage lactic acid = (ml of 0.1 N NaOH solution X 0.009 / weight of sample) X 100
5. Milk fermentation experiments
* Temperature is the independent variable, e.g. 0oC, 10oC, 20oC, 30oC, and titratable acidity is the dependent variable.
If measurement is done only after 1 week, "time" is a controlled variable, as are variables initial pH, sunlight, sugar concentration, aeration, exposed surface area.
So prepare a one line graph plotting "titrarable acidity" on the y-axis and temperature on the x-axis.
* Time is the independent variable.
Temperature remains as an independent variable, but measurement of titratable acidity is done every week at 0, 1, 2 and 3 weeks.
So, if using four different temperatures, prepare a four line graph plotting of titratable acidity on the y-axis and time on the x-axis to show the fermentation rate at each temperature.
16.0.1 Milk sampling
1. Liquid milk in cans and bulk tanks should be thoroughly mixed to disperse the milk fat before a milk sample is taken for tests.
Dippers used for sampling must be sterilized either in an autoclave or pressure cooker for at least 15 minutes at 120oC or use a 70% alcohol swab followed by flaming and scalding in hot steam or boiling water for 1 minute.
2. Milk samples kept for butterfat testing may be preserved with potassium dichromate solution (1 tablet or 1/2 mL 14% solution in 1/4 litre sample bottle).
Milk samples kept for butterfat determination in a refrigerator or an ice box must first be warmed in a water bath at 40oC, then cooled to 20oC, and mixed.
3. Milk producers should see their milk being tested and see the records.
4. Household hints: Use milk powder paste for mouse trap bait.
Use milk soured for eight hours to remove rust stains.
16.1.1 Organoleptic tests, (taste, sight, smell tests)
Abnormal smell and taste may be caused by the following:
1. Atmospheric taints, e.g. "barnyard odours" or "cow odours"
2. Physiological taints caused by hormonal imbalance or cows in late lactation causing spontaneous rancidity
3. Bacterial taints
4. Chemical taints or discolouring
5. Advanced acidification, where pH < 6.4
Experiment
The organoleptic tests are for immediate segregation of poor quality milk at the milk receiving platform.
No equipment is required, but the milk grader must have an experienced sense of sight, smell and taste.
Open a can of milk and immediately smell the milk.
Observe the appearance of the milk.
Taste the milk and do not swallow it, but spit out the sample to be flushed away.
Examine the lid of the milk can and check for cleanliness.
16.1.2 Clot on boiling test, (C.O.B) test
1. The C.O.B test is for acid milk that is too acid, pH <5.8 or abnormal milk, e.g. colostrum milk or mastitis-affected milk.
If a milk sample fails in the test, it contains either many acid and rennet producing micro-organisms or the milk has an abnormally high percentage of proteins, as in colostrum milk.
This milk cannot stand the heat treatment in milk processing and must be rejected for consumption.
Experiment
Boil a 5 mL sample of milk in a test-tube.
If clotting, coagulation or precipitation occurs, the milk has failed the test.
Heavy contamination in freshly drawn milk cannot be detected when the acidity is below 0.20-0.26% lactic acid.
2. Determine the stability of milk for heat processing.
Milk is kept at room temperature will increase in acidity, called "developed acidity'.
If acidity has increased by more than 0.2% percent, coagulation may occur during heat treatment, the result of dissociation of calcium caseinate.
So the heat stability of incoming raw milk must be determined.
Experiment
Put a 5 ml milk sample in a test-tube and put it in a boiling water bath for five minutes.
Remove the test-tube from the water bath without shaking it.
Note any acid smell or precipitated particles on the sides of the test-tube.
Samples showing precipitated particles are recorded as positive C.O.B. test and the milk must be rejected on the receiving platform.
16.1.3 Alcohol test
The test is based on the instability of the proteins when the levels of acid or rennet are increased and acted upon by the alcohol.
Increased levels of albumen in colostrum milk and salt concentrates in mastitis-infected milk results in a positive test.
Experiment
Prepare 68% ethanol solution with 68 mL of 96% (absolute) alcohol + 28 mL distilled water.
Mix 2 mL of milk and 2 mL of 68% of ethanol solution in a test-tube.
No coagulation, clotting or precipitation is observed in good quality milk.
Look carefully for small lumps.
The first clotting due to acid development occurs at 0.21-0.23% lactic acid.
16.1.4 Alcohol alizarin test
Experiment
Prepare alcohol alizarin solution by adding 0.4 g alizarin powder to 1 litre of 61% alcohol solution or purchase the solution ready-made.
Test results
Parameter
 Normal milk
 Slightly acid milk
 Acid milk
 Alkaline milk
pH
 6.6 to 6.7
 6.4 to 6.6
 < 6.3
 > 6.8
Colour milk colour
 red-brown
 yellow brown
 yellow lilac
Appearance
 no coagulation
 no coagulation
 coagulation
or
 no coagulation
"
 No lumps
 "
 small lumps
 mastitis?

16.1.5 Acidity test
Experiments
1. Bacteria in raw milk produce lactic acid.
Neutralize the acid with 0.1 M sodium hydroxide, measure the amount of alkali, then calculate the percentage of lactic acid.
Fresh milk has a "natural acidity", the natural ability to resist changes in pH.
The natural acidity of milk is 0.16 - 0.18%.
Figures higher than this signifies developed acidity due to the action of lactic acid bacteria group, mainly Lactobacillus, on milk sugar.
2. Put a 9 mL milk sample into a conical flask.
Add 1 mL (0.5% in 50% alcohol) of phenolphthalein solution.
From a burette, add 0.1 M sodium hydroxide solution until a faint pink colour appears.
% lactic acid = volume of sodium hydroxide solution (mL) / 10.
16.1.6 Resazurin stain, test for milk
Resazurin tablets (tests: oxygen in solution, freshness of food, e.g. milk, viability of cells), large dichromatism.
Purchase: Resazurin sodium salt (7-​Hydroxy-​3H-​phenoxazin-​3-​one-​10-​oxide sodium salt ), C12H6NNaO4, powder, BioReagent, suitable for cell culture.
Resazurin (7-Hydroxy-3H-phenoxazin-3-one 10-oxide), blue and weakly fluorescent, is reduced to pink and highly red fluorescent.
Prepare a 0.005% solution of the blue redox indicator dye resazurin by dissolving 1 tablet in 50 mL of deionized water.
Resazurin is pink when oxidized and colourless when reduced.
The Resazurin test for the quality of raw bulked milk is a rapid bacteria estimation, done either as a 10 minute pass or fail, or a 3 hour triple reading test, or as the standard and generally-accepted 1 hour test.
Resazurin gives milk a characteristic blue colour and the test is based on the ability of lactic acid bacteria group, mainly Lactobacillus, in the milk to reduce the blue dye.
The quality of the milk is judged by noting the degree of colour change, from blue through mauve and purple and pink and finally colourless, after a stated period of incubation, or the time required to reduce the dye to a predetermined colour.
Experiments
1. Resazurin is an indicator dye and can later be used to test for microbial activity in the milk sample.
The Resazurin test can be done as a 10 minutes test, one hour test, 3 hours test.
The 10 min Resazurin screening test can be done at the milk platform.
2. Prepare the Resazurin solution by adding one Resazurin tablet to 50 mL of distilled water.
Do not expose Resazurin solution to sunlight.
Use the solution within eight hours after preparation.
3. Mix the milk sample and use a sanitized dipper to put 10 mL of the milk sample into a sterile test-tube.
Add one mL of Resazurin solution.
Insert a sterile stopper.
Mix the dye gently into the milk and mark the test-tube before the incubation in a water bath.
Place the test-tube with the Resazurin disk in a Lovibond comparator, and compare the colours with a test-tube containing 10 mL milk of the same sample, but without the dye as a control.
Results of a resazurin disc test
Disc number
 Colour
 Grade of milk
 Evaluation
6
 blue
 Excellent
 Accepted
5
 light blue
 Good
 Accepted
4
 purple
 Good
 Accepted
3
 purple pink
 Fair
 Separate
2
 light pink
 Poor
 Separate
1
 pink
 Bad
 Reject
0
 white
 Very bad
 Reject

16.1.7 Gerber butterfat test
The fat content of milk and cream may determine the price paid to farmers, the feed ration for dairy cow's, the use of individual animals in breeding programs, the adjustment needed for butterfat percentage in standardized milk and milk products.
Experiment
Mix fresh milk samples at approximately 20oC.
If samples have been kept cool for days, warm to 40oC, mix gently and cool to 20oC before the testing.
Add 10 ml of Gerber sulfuric acid (1.82 g / cc) to a Gerber butyrometer, 0-6% or 0-8% BF, then add 11 mL of mixed milk.
Do not wet the neck of the butyrometer.
Add 1 mL of amyl alcohol, insert a stopper and shake the butyrometer until the curd dissolves and no white particles remain.
Place the butyrometer in a water bath at 65oC, until ready for centrifuging.
Place the butyrometer with the stem (scale) pointing towards the centre of the centrifuge.
Spin for 5 minutes at 1100 rpm.
Remove the butyrometer from the centrifuge, then put it in a water bath with neck pointing up, at 65oC for 3 minutes, before taking the reading from the butyrometer scale.
Read the fat column from the lowest point of the meniscus of the interface of the acid fat to the 0 mark of the scale, then read the butterfat percentage.
Empty the butyrometer into a container suitable for the acid milk solution, wash the butyrometer in warm water and leave to dry before the next use.
The fat column should have a straw yellow colour and free from specks and sediment.
The ends of the fat column should be clearly and sharply defined and the water just below the fat column should be clear.
The fat must be within the graduation.
A too lightly-coloured column or a column containing fat curds can be caused by the temperature of milk or acid, or both, being too low, acid too weak concentration, insufficient acid, milk and acid not mixed thoroughly.
A darkened fat column containing black specks at the base is caused by the temperature of milk acid mixture being too high, acid too strong, milk and acid mixed too slowly, too much acid used, acid dropped through the milk.
16.1.8 Lactometer test
The lactometer test is designed to detect the change in density of adulterated milk.
Together with the Gerber butterfat test, it enables the milk processor to calculate the milk total solids (% TS ) and solids not fat (SNF).
In normal milk, SNF should not be below 8.5% according to local standards.
Experiment
1. Mix the milk sample gently and pour it into a 500 mL measuring cylinder.
Let the lactometer sink slowly into the milk.
Read and record the last lactometer degree (oL), just above the surface of the milk.
If the temperature of the milk is different from the calibration temperature, calculate the temperature correction.
For each oC above the calibration temperature, add 0.2 oL to the recorded lactometer reading.
For each oC below calibration temperature, subtract 0.2 oL from the recorded lactometer reading.
Sample
 Milk
 Lactometer
 Correction
 True reading
1. 17oC
 30.6 oL
 -0.6 oL
 30.0 oL
2. 20oC
 30.0 oL nil
 30.0 oL
3. 23oC
 29.4 oL
 +0.6 oL
 30.0 oL

2. For the calculations, use lactometer degrees, L, and for the conversion to density write 1.0 in front of the true lactometer reading, i.e. 1.030 g / mL.
Check for adulteration by looking for unusual sediment from the milk at the bottom of the milk can and taste the milk to find out if it is too sweet or too salty.
Samples of milk from individual cows often have lactometer reading outside the range of average milk, while samples of milk from herds should have readings near the average milk reading.
Wrong feeding, may result in low readings.
A standard specific gravity of 1.026 -1.032 g / mL implies a lactometer reading range of 26.0-32.0 oL.
16.1.9 Freezing point determination
The freezing point of milk is regarded as the most constant of all measurable properties of milk.
A small adulteration of milk with water will cause a detectable elevation of the freezing point of milk from its normal value of -0.54oC.
Since the test is accurate and sensitive to added water in milk, it is used to detect whether milk is of normal composition or adulterated.
Experiment
Measure the freezing point of pure milk and milk mixed with water.
However, in the laboratory, a cryoscope is used for determination of the freezing point of milk.
16.1.10 Inhibitor test
Milk collected from producers may contain drugs and / or pesticide residues.
Residues when present in significant amounts in milk may inhibit the growth of lactic acid bacteria group, mainly Lactobacillus, used in the manufacture of fermented milk such as cheese and yoghurt, besides being a health hazard.
The suspected milk sample is subjected to a fermentation test with starter culture and the acidity checked after three hours.
The values of the titrate acidity obtained is compared with titrate acidity of a similarly treated sample that is free from any inhibitory substances.
Experiment
Put 10 mL of test samples of milk in three test-tubes and put 10 mL of control samples ("normal" milk) in three test-tubes.
Heat all test-tubes to 90oC by putting them in boiling water for 3 - 5 minutes.
Leave the test-tubes to cool to the optimum temperature of the starter culture, 42oC.
Add 1 mL of starter culture to each test-tube.
Mix the contents then incubate them for 3 hours.
After each hour, one test-tube is from the test sample and the control sample is determined.
If the acid production in the test sample is the same as in the normal sample, then the test sample does not contain any inhibitory substances.
If the acid production in the test sample is less than in the normal milk sample, then the test sample contains antibiotics or other inhibitory substances.
16.1.11 Pasteurized milk test
Milk is pasteurized at 63oC for 30 min in batch pasteurizers, or 72oC for 15 seconds in heat exchangers (continuous flow pasteurizers), to destroy all pathogenic bacteria and render milk safe for human consumption.
Also, some enzymes that might affect milk flavour are destroyed, e.g. milk phosphatase.
Milk phosphatase is measured to determine whether milk has been adequately pasteurized.
A positive test means the pasteurization process was inadequate, the milk may not be safe for human consumption and it will have a short shelf life.
Before the test, all glassware must be rinsed, cleaned, rinsed in chromic acid solution and boiled in water for 30 minutes.
However, chromic acid solution is dangerous and other glass cleaning detergent solutions should be used.
Experiment
1. Prepare a buffer solution, by mixing 0.75 g of anhydrous sodium carbonate and l.75 g of sodium bicarbonate in 500 mL distilled water.
Prepare a buffer substrate solution, by placing 0.l5 g of di-sodium paranitrophenylphosphate (the substrate) into a clean 100 mL measuring cylinder and adding the buffer solution to the 00 mL mark.
Store this buffer substrate solution in a refrigerator and protect it from light.
After one week of use, prepare fresh buffer substrate solution.
2. Pipette 5 mL of buffer substrate solution into a test-tube, stopper and warm the solution in the water bath at 37oC.
Add to the test-tube 1 mL of the milk to be tested.
Insert a stopper, mix well and place in a water bath at 37oC.
Prepare a control blank sample from boiled milk of the same type.
Incubate the test sample and the blank sample at 37oC for 2 hours.
After incubation, remove the test-tubes and mix them thoroughly.
Place one sample against the blank in a Lovibond comparator using the A.P.T.W. disc and rotate the disc until the colour of the test sample is matched and read the disc number.
3. Disc reading after 2 hours of incubation at 37oC
0-10
 Properly pasteurized
10-18
 Slightly under pasteurized
18-42
 Under pasteurized
> 42
 Not pasteurized

16.2.1a Regular cows milk, fresh milk, full cream milk, whole milk
1. Fresh milk
Fresh milk has a rich and creamy texture and consists of solids (milk fat, protein, lactose and minerals), in water, with water about 87% of the volume.
It contains about 3.8% milk fat, and no less than 3.2% milk fat.
Milk is a rich source of calcium, phosphorus, potassium, riboflavin, vitamins A and B12, magnesium, carbohydrate, protein and zinc.
Milk contains fat, milk sugar, lactose, that gives milk its sweet taste, and casein.
When milk is left to stand, the fat rises to the surface as cream.
In the manufacture of butter and cheese the cream is separated.
The remaining watery liquid (skimmed milk), is a solution of milk sugar and casein.
Fresh (pasteurized) milk is heated to 74oC for 15 seconds.
In the days of milk bottles and aluminium tops, cream would separate and rise to the top of the bottle.
Homogenization passes the milk under pressure though very fine nozzles, evenly dispersing the fat globules to create a smooth, creamy texture and taste.
Milk is a poor emulsion.
The forces of cohesion between the emulsified cream droplets are greater than the forces of adhesion between the milk and the cream, so the cream floats above the milk.
In homogenized milk, the droplets have been broken into smaller particles and dispersed to form one phase.
Experiment: In fresh milk, droplets and suspended particles reflect light in all directions so the milk appears white and opaque.
As milk freezes and transparent ice crystals form, the remaining liquid has a higher concentration of riboflavin, so the milk appears yellow.
2. Cultured milk
Cultured milk has a tangy flavour similar to natural yoghurt and is excellent for baking.
A special starter culture is added to the pasteurized milk to develop the flavour and acidity.
3. Flavoured milk
Flavoured milk has flavours are added to regular milk, reduced fat milk, skim milk, modified milk or long life milk.
It may have added calcium and have a higher kilojoule level or be sweetened with low kilojoule sweeteners.
4. Fortified milk
Fortified milk may be whole or lower fat milk that has had nutrients added such as calcium, vitamin D and omega-3 fatty acids, besides the nutrients naturally found in regular milk.
5. Lactose-reduced milk
Lactose reduced milk or lactose free milk is suitable for people who are lactose intolerant.
6. Long life milk
Long life milk is heated to 140oC for two seconds and then packaged aseptically.
This ultra heat treatment (UHT), kills micro-organism spores and inactivates heat resistant enzymes.
Long life milk can be kept unopened in the pantry for up to six months, but once opened, UHT milk must be refrigerated and used within seven days.
7. Low fat milk
Low fat milk has less than 1.5% milk fat and the same nutritional benefits with a boosted calcium content.
8. Modified milk
Modified milk may be protein enriched, high in calcium, iron fortified, or low in lactose, to cater for a range of dietary requirements.
9. Reduced fat milk
Reduced fat milk has approximately 2% milk fat and it may have extra protein and calcium added.
10. Skim milk
Skim milk has no more than 0.15% milk fat.
Milk solids may be added to optimize the taste.
Skimmed milk can have the water evaporated to form dried milk powder.
Also, the casein is used to make casein plastics.
11. UF milk
Ultra filtration (UF) milk is enriched with protein and calcium.
12. Baked milk
Baked milk is still popular in Russia where by simmering milk on low temperature for many hours.
A brown sweet liquid with a crust forms.
Heat milk to form a skin of protein and calcium compounds.
13. Homogenized milk
Homogenize milk by breaking the fat globules.
Homogenized milk label: "Devondale", Australia, Milk, Full Cream, No preservatives or additives, 150 mL.
Devondale homogenized milk is pasteurized at an ultra high temperature and packed in a sterile Tetra Pak package so it stays fresh for up to 9 months without refrigeration.
Serve within seven days after opening.
Table 16.7.23 Homogenized milk and salt reduced butter
Contents
 Milk per 100 mL
 Butter per 100 g
Energy
 269 kJ
 3030 kj
Protein
 3.3 g
 < 1g
Fat, Total
 3.4 g
 81.5 g
Fat, Saturated
 2.3 g
 53.8 g
Carbohydrate
 5.1 g
 < 1 g
Sugars
 5.1 g
 < 1 g
Sodium
 40 mg
 350 mg
Calcium
 120 mg
 -
Potassium
 -
 28 mg

16.2.2 Reduced milk, milk powder, condensed milk, evaporated milk
1. Milk powder
Milk powder may be whole milk powder (WMP) or skim milk powder (SMP).
Milk Powder (dehydrated milk, dried milk), has a long shelf time and is easily transported.
It is used to make ice cream, infant formula and milk chocolate.
It is used by people who live in remote areas, bush walkers and foot soldiers.
An evaporated pasteurized milk concentrate, contains about 40% milk solids, is dried to reduce the moisture content to about 3% and prevent particles from clumping together.
Instant milk powders use additional techniques such as coating the dried milk particles with lecithin to reduce clumping.
2. Condensed milk
Condensed milk is a high fat product obtained by removing water and adding sugar.
Sweetened condensed milk is popular in homes especially where refrigeration is limited.
The added sugar acts as a natural preservative.
Adding the sugar is controlled carefully to maintain a smooth texture.
3. Evaporated milk
Evaporated milk (unsweetened condensed milk) is where less water has been removed so it is sweet tasting, but no added sugar.
For evaporated or condensed milk, the total milk solids are increased to 28% by the gentle evaporation of water.
Skim evaporated milk has total milk solids of about 20%.
After concentration, the milk is canned and sterilized to destroy bacteria and enzymes to ensure a long shelf life.
It also causes a slight caramelization of the milk sugar (lactose) giving the evaporated milk its typical colour and flavour.
16.2.3 Butter, butter oil, clarified butter, ghee
1. Butter
Butter is produced by churning the cream from milk to form butter and buttermilk, the remaining liquid.
It is thicker than milk, but not necessarily fatter.
Butter made at may have as little as 65% fat and 30% water, but butter made by machines has much less water.
Butterfat is a mixture of triglycerides, a triester derived from glycerol and three of any of several fatty acid groups.
Butter becomes rancid when these chains break down into smaller components, e.g. butyric acid and diacetyl.
The density of butter is about 0.911 g / cc, about the same as ice.
To compare the densities, float identical small cubes of ice, manufactured butter and home-made butter in water.
Cultured buttermilk is made by adding a lactic acid bacteria group, mainly Lactobacillus, to milk to start a fermentation process.
It can be used in bread making or ice cream making.
2. Butter oil
Butter oil (anhydrous milk fat, AMF), is butter with the moisture and the non-fat milk solids of butter removed.
It is used for bakery and confectionery.
Butter oil is an efficient and economical means of transporting butterfat.
Butterfat in milk contains different fat lipids, each with its own melting point.
The main fraction contains triglycerides of fatty acids.
The melting point for milk fat is 37oC.
However, butter oil starts to solidify at < 17oC.
Butter oil is easy to use in liquid form in food recipes.
Spreadable butters and vegetable oil-based dairy blends are easier to spread and are lower in saturated fat.
3. Clarified butter
Clarified butter is butter with most of the water and milk solids removed, leaving almost pure butterfat.
To make clarified butter, heat butter to its melting point, then allow it to cool slowly, so that after settling, the components separate into layers of different density.
Whey proteins form a skin that can be removed, and the resulting butterfat poured off from the mixture of water and casein proteins that settled to the bottom.
4. Ghee
Ghee is clarified butter brought to higher temperatures about 120oC after the water is removed to form brown milk solids.
This process gives ghee its flavour and produces antioxidants to help protect it longer from rancidity.
So ghee can keep for 6-8 months under normal conditions.
Experiments
5. Make butter
To make butter, use cream at room temperature, about 20oC.
Half fill the jar with the cream, put the lid on and shake it for up to one hour, or use a food processor with plastic blades or an electric whisk.
When the mixture suddenly becomes firm, reduce the speed of the whisk.
The cream suddenly becomes yellow, then bits of butter and a thin liquid, the buttermilk, appear.
Then butter forms as a clump separated from the buttermilk.
Drain off the buttermilk.
Wash away any remaining buttermilk with cold water in a low speed food blender.
If any buttermilk remains, the butter will quickly go rancid, so wash the butter seven times.
Use wooden paddles or the back of a spoon to press and shape the butter and squeeze out any moisture.
Either freeze the butter unsalted, or add a half teaspoon (2-3 mL) of salt for each 250 g of butter.
6. Heat butter.
The fat separates from salt and water.
Heat too hot, as in deep drying.
The fat cracks to form the unsaturated hydrocarbon acrid smelling tear-producing acrolein.
16.2.4 Cheese making
1. Cheese may be classified as cheddar cheese, semi-hard cheese, hard grating cheese, fresh cheese, and mould cheese.
The specific names include, Brie, Camembert, Cheddar, Cottage Cheese, (Cream Cottage Cheese), Coulommiers, Cream Cheese, Danbo, Edam, Emmentaler, Extra Hard Grating Cheese, Gouda, Havarti, Spreadable Processed Cheese, Provolone, Saint Paulin, Samsoe, Tilsiter and Whey Cheeses.
2. Cheese-making processes
Cheese is produced by coagulation of casein when milk is acidified and then coagulated by the addition of rennet that contains a proteolytic enzyme.
The stage of the cow's lactation, the breed of cow, the state and composition of the pastures affect the fat to protein ratio in the milk.
3. Cheese-making pre-concentration
Pre-concentration ahead of cheese production where whole milk is concentrated with Reverse Osmosis (RO) to levels of 25-30% total solids (TS), i.e. a 2-2.5 X volume concentration factor (VCF).
RO membranes have pore sizes < 0.001 microns, so separation occurs based on molecular size and chemical interactions between the membrane and fluid components in contact with the membrane.
Pressures from 450-600 psi pushes water molecules through the pores of a membrane while retaining the colloidal solids and salts.
4. Cheese-making standardization
Standardization for the ratio of proteins and fats in the milk to be adjusted to a preset value to ensure the cheese composition is uniform.
5. Cheese-making pasteurization
Pasteurization where the milk is pasteurized by being heated to 72oC for 15 seconds, then rapidly cooled.
However, hard cheeses matured for more than three months may be made from unpasteurized milk.
6. Cheese-making starter cultures
Cheese starter cultures for each style of cheese are acidifying starters that produce lactic acid from the milk sugar (lactose).
Some cheeses have additional cultures to assist during maturing.
Mould spores are sometimes used in cheese making, depending on the type of cheese being made.
Penicillium candidum grows as a white mould on Brie and Camembert.
Penicillium roqueforti spores are the blue mould spores that promote blue mould growth in blue vein cheese.
The gas producing starter, Propionibacterium shermanii, "Props bacteria", is a gas-producing bacterium that causes the eye formation in Swiss cheeses.
Aroma cultures, Brevibacterium lines are used for rubbing the surface of washed rind cheeses to produce colour and flavour effects.
Geotrichum candidum is used on several surface-ripened cheeses to modify the flavour, aroma and colour of the cheese.
Coagulation of the milk by the lactic acid from the starter cultures.
Rennet, (rennin or rennilase)
1. For matured cheese, rennet, from the abomasum of unweaned calves is added to the milk used to form the curd.
Adult cattle do not consume milk in their daily diet.
Most young mammals secrete rennilase in their stomach so that the milk consumed from their mothers will coagulate and create curds and separate.
Rennet (rennin or rennilase) is a protease enzyme added to milk, along with certain bacteria, which coagulate milk proteins, producing the curds.
This then separates from the liquid, whey.
Rennet can be formed from non-animal sources, e.g. the fungus Rhizomucor miehei and genetically modified Escherichia coli, Aspergillus niger and food yeasts.
When the milk is "set", the curd releases whey to concentrate the curd.
Rennet contains the proteolytic endopeptidase enzyme chymosin rennin, and other important enzymes, e.g., pepsin and lipase.
The active enzyme in rennet is called chymosin or rennin.
Rennin is available as rennet powder, rennet tablets, and junket tablets.
Rennet assists in the breakdown of milk --> caseinogen --> casein.
Junket tablets are a common source of rennet for home cheese makers.
Rennilase, an endopeptidase, hydrolyses casein in a similar manner to chymosin.
About 90% of hard cheeses, e.g. cheddar, are made from chymosin produced from genetically modified yeasts.
2. Cutting the curd to release of moisture, called syneresis.
A finely cut curd has a large surface area and releases more whey to produce a drier cheese.
The curd for Parmesan cheese (low moisture) is cut the size of rice grains.
The curd for a Brie or Camembert (high moisture cheeses), is cut to about 2 cm cubes.
3. Stirring the curd to keep the cut curds apart and to release more whey.
Soft cheeses require less stirring than harder cheeses.
4. Heating the curds gently to remove more whey.
Cheddar is heated to 38oC, Romano to 46oC, and Parmesan and Gruyere to 54oC, but most fresh cheeses are not "cooked".
5. Salting
Salting improves the flavour and preserves the cheese by reducing the moisture level to restrict the growth of undesirable bacteria.
Most cheeses are brine-salted, except Cheddar types, which are dry-salted by adding salt to curd chips before hooping.
The cheese is placed into a brine solution of 20 to 26% salt for a fixed time, depending on the cheese size and desired salt level.
Some cheeses also have their surface (rind) washed with a brine solution during maturation to restrict mould growth and aid the development of the rind.
6. Hooping
Hooping is used to place cheese into hoops or moulds for up to 16 hours to form the shape of the cheese.
Pressing of semi-hard to hard cheeses mechanical presses to assist curd fusion, close the texture and remove more whey.
Most soft cheeses are not pressed.
7. Maturing
Maturing of rindless cheeses in cool rooms, e.g. Cheddar at 8 to 10oC, for 3 to 24 months.
Cheeses with rinds require humidity, e.g. white mould cheeses, at 95% humidity, and 11 to 14oC.
During maturation the enzymes in the cheese break down the fats and proteins allowing textural and flavour characteristics of the cheese to develop.
The main enzyme sources are the milk, starter and rennet.
Hard Italian style cheeses may also have lipase added to accelerate fat breakdown and contribute to their unique flavour.
8. Wrapping
Wrapping by packaging fresh soft cheese as soon after it is made.
White mould cheese must still breathe through its wrapping.
Blue cheese is wrapped in laminated foil to prevent the rind from drying.
Cheddar is wrapped in a vacuum-packed bag.
More traditional methods such as waxing and wrapping in cloth are used for speciality cheddar.
9. Assessment of cheese
For assessment by colour, know how the cheese should look at that stage of maturity.
For assessment by smell, a washed rind cheese has an earthy, meaty aroma and a Brie may smell of ammonia its final stage of fermentation.
The strength of the aroma indicates the age of the cheese.
For assessment by taste, flavour descriptions may include acidic, nutty, milky, salty, mouldy, fruity, sweet, yeasty, earthy, bitter, sour, ammoniated or fatty.
The strong taste of Blue Cheese may be an acquired taste.
For assessment by texture, descriptions may include firm, coarse, gritty, soft, springy or elastic.
10. Curdling
Vinegar provides the acid that causes the milk to curdle and produces the acid flavour.
However, the traditional method of producing the acid is to use a culture of acid-producing bacteria.
This method is more complicated and takes longer, but as it is a live method so the cheese will continue to improve in flavour with age.
Rennet is an enzyme that produces a harder curd.
It originally came from calf stomachs, but nowadays it is synthesized and is available in liquid or tablet form.
Cheese is made by coagulating milk and removing a liquid called whey.
Rennet may be added to this remaining solidified form of the milk, the curd, to make it stable.
The cheese can then be left for some time to "ripen", creating taste with the help of bacteria and sometimes mould fungus.
The whey is used in baking or for other foods.
Muscle builders who use gymnasiums may consume huge amounts of whey protein to increase the size of their muscles.
11. Permeate
Permeate is a greenish waste product from the production of cheese to change fat levels in milk.
It may form up to 16% of the fresh milk in Australia, but recently the permeate is being removed from milk sold through retail outlets and labelled "permeate free".
12. Cottage cheese
Cottage cheese is a loose "curd" that still holds much whey.
It is usually mild in flavour, but it can be made with or without rennet with different results.
To make cottage cheese, boil 1 litre of full cream milk, then when cool squeeze in lemon juice from a fresh lemon until you see curds form.
Transfer the mixture to a sheet of cheese cloth.
Sprinkle on salt to taste.
Squeeze the curds by twisting the cheese cloth.
Put the curds still in the cheese cloth in a refrigerator.
Another method is to add live yoghurt to the boiled milk.
Experiments
1. Cream cheese is formed when lactic acid bacteria group, mainly Lactobacillus are added to a milk and cream mixture through different temperatures changes to achieve the creaminess.
To make cream cheese, put plain, Greek-style yoghurt in cheese cloth or in a strainer lined with a paper coffee filter and let the whey drip out overnight.
The whey can be used to make sauerkraut or many other lacto-fermented foods.
When the cream cheese and whey are separated, scrape the cream cheese out of the cloth with a spatula, and salt it to taste, and place it in a refrigerator.
2. To make cottage cheese, heat the 40 litres of 2% milk to 90oC or turn off the heat just before the milk begins to boil.
Add 2 mL of vinegar and leave to cool.
Pour the curds and whey into a colander and drain off the whey.
Pour the curds into a bow, sprinkle on the salt, and mix well.
Some people add some cream for a silky texture.
3. Experiments with three rennet enzymes
Enzyme 1. Calf rennet essence (sold in supermarkets or health food shops as junket tablets)
Enzyme 2. Novo Nordisk Rennilase (a fungal protease)
Enzyme 3. Gistbrocades Maxiren (pure calf chymosin from a genetically modified yeast).
Dispense 10 mL of milk into test-tubes.
Add 1 cc of rennet enzyme.
Record the time taken for the milk to coagulate.
It may be possible to distinguish between a "partial set", i.e. when particles stick to the side of the test-tube when it is gently rocked from side to side, and a "full set", i.e. when a solid curd is formed.
The enzyme first breaks specific bonds in a glycopeptide on the surface of soluble calcium caseinate particles in the milk.
Relatively insoluble calcium paracaseinate is formed, and in the presence of calcium ions this coagulates to form a continuous curd.
The products of this investigation should not be consumed.
4. Investigate the effect of temperature on the time taken for the curd to form.
Experiment
Compare the activity of different types of enzymes.
Add calcium chloride to the milk, to investigate the effect of calcium ion concentration on the setting rate, e.g. Maxiren requires a slightly greater calcium ion concentration than other enzymes.
Alter the pH of the milk.
1 cc of 1M hydrochloric acid + 10 cc of milk + enzyme --> pH of about 1
1 cc of 1M citric acid + 10 cc of milk + enzyme --> pH 5
1 cc of 1M sodium hydrogen carbonate + 10 cc of milk + enzyme --> pH 8
1 cc of 1M sodium hydroxide + 10 cc of milk + enzyme --> pH 14
1 cc of distilled water + 10 cc of milk + enzyme --> pH 7
Dilute the enzyme with distilled water to investigate the effect of enzyme concentration.
Cheese reactions
Cheese is made from coagulated casein protein of milk from cows, sheep, buffalo or goats.
The animal used is not as important as the fat and protein content of the milk.
Various strains of mould can be used to create special cheeses through the by-products of the biochemical pathways of these moulds during respiration.
Reactions 1
* Milk sugar + bacteria (special sort) --> milk acid
* Lactose (milk sugar) + type of lactic acid bacteria (LAB) --> lactic acid (milk acid)
* Lactose (combination of galactose and glucose) + Streptococcus thermophilus (type of LAB) --> 2-hydroxypropanoic acid (lactic acid)
Lactose C12H22O11
See diagram 16.3.1.4b: Lactose molecule.
Reactions 2
* Cheese protein (liquid) + enzyme (special sort) --> curds + whey
* Type of protein (liquid) + protease (coagulating type of enzyme) --> type of protein (solid) + milk plasma
* Casein (l) (type of protein) + rennet (containing chymosin, an aspartic acid protease)
Paracasein (s) + milk plasma (contains lactose, vitamins, minerals and traces of fat).
Reactions 3
Chymosin reacts with casein (l) by cleaving the protein between two of its residues, producing two fragments:
* The soluble fragment (residues after cleavage site) becomes part of the milk plasma.
* The insoluble fragment (residues before cleavage site) causes the coagulation of the micelles in casein (l) to form paracasein (s).
Reactions 4
* Curds + mould (special sort) --> cheese
* Type of protein (solid) + white mould spores --> soft cheese
* Paracasein (s) + Penicillium candidum (white mould spores) --> Camembert cheese
Penicillium candidum discourages the growth of spoilage micro-organisms.
As cheeses age, their long chain C18) fatty acids are converted into short chain C4-C8) fatty acids, via b-oxidation.
It is the unique combination of these short chain fatty acids, which give each cheese its distinct flavour and smell.
Cheese preparation
* Sterilize all equipment with 4% sodium hypochlorite bleach.
* Add 6 mL of bleach to 1 litre of cold water.
Store equipment covered in cling wrap.
* Make rennet solution by diluting rennet by 10 × its volume of cooled boiled water.
Store rennet in refrigerator.
* Make brine solution by adding 200 g salt to 800 mL boiled water, chilling in refrigerator, and then adding a speck of white mould spore powder.
Store brine solution in refrigerator.
5. Cheese making
A.
1. Pour 3 L milk into pot.
2. Place pot on hot plate.
3. Heat milk, whilst stirring with plastic utensil.
4. Wait until milk reaches 40oC.
5. Add a few grains of Type E starter.
6. Add 1/10 th of teaspoon white mould spore powder.
7. Mix well.
8. Remove pot from hot plate.
9. Leave for 45 minutes.
B.
1. Pour solution from pot into big plastic container.
2. Spread 1 mL rennet over entire surface of liquid.
3. Mix well for 1 to 3 minutes.
4. Allow solution to set for 30 minutes.
C.
1. Cut curd into 2 cm2 cubes.
2. Curd will be allowed to sit for 30 minutes and turned for 3 minutes three times.
D.
1. Pour curds and whey through strainer over sink.
2. The curds will be collected in the strainer.
The whey will pass through the strainer.
3. Repeat steps 1 and 2 until all of the big plastic container's contents have been strained.
4. Dry curd by pressing curd down with plastic utensil.
E.
1. Use plastic utensil to scoop curd into individual plastic containers.
2. Invert individual plastic containers onto drying racks.
3. Leave overnight.
F.
1. Place cheese in snap lock bag.
2. Fill snap lock bag with cold brine solution 3.
Leave for 30 minutes.
G.
1. Remove cheese from brine.
2. Place in clean snap lock bag.
3. Cheese after-care:
4. Keep cheese at room temperature for 24 hours.
5. Keep cheese in refrigerator for 4 weeks.
6. Eat cheese!
16.2.5 Whey
Whey is a by-product of the cheese making process.
Whey powder used in ice cream, bakery products (cakes, biscuits), chocolate flavouring, infant formula, yoghurt, beverages and processed meat.
Industrial uses include animal feed (for pigs, horses and poultry), calf milk replacer and even as a carrier for herbicides.
Whey protein concentrates are used in snack foods, juices, confectionery, ice cream, biscuits, processed meats (milk) protein drinks for weight lifters, desserts, infant foods and dietetic products.
Products such as cosmetics, skin creams, bath salts and detergents also contain protein concentrates.
16.2.7 Yoghurts
1. Yoghurt is created by fermenting milk with the help of certain bacteria.
The process creates lactic acid.
It holds much longer than ordinary milk.
Cultured dairy foods, like yoghurt, are made by the addition of live starter cultures to milk.
These bacterial cultures cause fermentation of lactose (the sugar found naturally in milk), and give yoghurt its great taste and aroma.
Cultured foods are those containing a living culture of healthy bacteria, the result of fermentation.
Live yoghurt contains enzymes and a living culture of bacteria.
Enzymes are the biological catalysts of the body, transforming one substance into another without molecular change.
They are vital for healthy digestion.
Like cream, yoghurt can curdle if overheated.
So simply stir in just before serving or mix in a little cornflour before adding to a hot dish.
2. Types of yoghurt
2.1 Natural yoghurt
Natural yoghurt with no other flavours or sweeteners, so have the pure yoghurt taste and no extra sugars.
Flavoured yoghurt, where flavours are added to the yoghurt at production stage to give a greater variety of tastes and increased sweetness.
2.2 Fruit yoghurt
Fruit yoghurt where fruit pieces or pulp are added at the production stage to give a greater variety of tastes, increased consumer appeal and increased sweetness.
2.3 Probiotic yoghurt
Probiotic yoghurt, where probiotic bacteria, e.g. Lactobacillus acidophilus, are added to the yoghurt to give added dietary and therapeutic effects for human health.
Probiotic Bacteria are live bacterial cultures that may be added to yoghurt for their health promoting benefits.
Probiotics are "friendly" bacteria that promote intestinal health by restoring harmony between "good" and "bad" bacteria in the gut.
The most commonly used probiotic strains are Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus GG (discovered in 1985 by Sherwood Gorback and Barry Goldin, University of Boston), and Bifidobacterium species.
Yakult fermented milk drink, made under licence in Australia, "Yakult, Honsha, Japan", contains water, sugar, skim milk powder, dextrose, flavouring, and live "L. casei Shirota strain".
Grameen Danone Foods, Bangladesh, makes a special type of yoghurt that is inexpensive to produce and affordable to the poorest families, resulting in a substantial improvement in the health of children, who eat it during the last four years.
2.4 Set yoghurt
Set yoghurt is produced when fermentation at controlled temperatures sets the yoghurt in tubs.
It characteristically may have a flat surface with a thin "skin" on top.
Usually natural or vanilla flavoured or with fruit or flavourings at the base, set yoghurt is thick in texture and without whey separation (free moisture).
2.5 Stirred yoghurt
Stirred yoghurt is made when fermentation is done in bulk and not in individual containers.
Once the fermentation reaches the desired level, the yoghurt is pumped through a cooler to stop fermentation.
Only then is any added fruit or flavouring stirred in.
2.6 Drinking yoghurt
Drinking yoghurt is produced similarly to stirred yoghurt.
The body is then diluted and mixed with a blend of flavours, fruit or berry juices, to provide a delicious, nutritious drink.
2.7 Soft serve / frozen yoghurt
Soft serve / frozen yoghurt results when a blend of sugars, stabilizers, emulsifiers and flavours are added to natural stirred yoghurt.
It is ideal as a frozen dessert as it maintains a smooth texture upon freezing, e.g. yoghurt in a tub with a spoon.
2.8 Labna
Labna is yoghurt drained of whey to form a fresh yoghurt "cheese".
Add a pinch of salt to natural set yoghurt before placing it in a double thickness of muslin cloth and suspending over a bowl, or placing in a strainer over a bowl, to catch the excess whey.
2.9 Ayran
Ayran is a drink mix of yoghurt, water and salt.
Lassi is an Indian dry yoghurt drink.
It is a mix with yoghurt, milk, spices and sometimes fruit.
In the 1970's in the US "frozen yoghurt" was invented.
Experiment
Prepare yoghurt
To make yoghurt, the milk must be of the highest bacteriological quality to ensure the uninhibited development of beneficial yoghurt culture.
Add skim milk powder to pasteurized milk to increase the protein content and helps produce the smooth texture and characteristic viscosity of yoghurt.
The milk is then homogenized and pasteurized to help achieve the desired yoghurt texture as it gives consistency to the particle (milk fat) sizes in the milk.
This destroys all unwanted bacteria that may cause undesirable flavours.
It also aids gel formation.
Starter cultures are added to convert the lactose in the milk into lactic acid that, in turn, aids the setting of the yoghurt.
Store the yoghurt in controlled temperatures (42oC to 43oC) for an incubation period of between four and six hours.
Add fruit or flavourings to enhance the taste and provide a wider range of products for consumers.
16.2.8 Dairy desserts
Dairy desserts are mousses, cream caramels and fromage frais, flavoured custard, dairy dips
Dulce de leche is caramelized milk used in desserts in South America.
16.2.9 Cream, regular cream and sour cream
1. Regular cream
Regular cream is used for direct consumption.
2. Crème fraiche (fresh cream)
Crème fraiche (fresh cream) from France is a mildly acidic, high fat, slightly nutty tasting thick cream.
It is made by letting unpasteurized milk ferment during 12-24 hours.
If using pasteurized milk add Lactobacillus bacteria (normally from yoghurt).
Crème fraîche is a soured cream containing about 28% butterfat and with a pH of around 4.5.
It is soured with bacterial culture, but is less sour than sour cream.
Before dairy products were first pasteurized, crème fraîche developed naturally when the bacteria present in cream fermented it, causing it to thicken naturally.
It is still available in this natural form in France and other European countries.
However, in Australia and the United States, laws require all commercial dairy products to be pasteurized so crème fraîche is manufactured through artificial fermentation, then re-pasteurized to halt the process, similarly to sour cream.
Crème fraîche and sour cream can be used interchangeably in some recipes, but crème fraîche can also be whipped like cream and does not split or curdle when boiled.
3. Sour cream
Sour cream is similar to crème fraiche, but has a lower fat level.
The bacteria used to create fermentation is from buttermilk, different thickening agents can be added.
Lactic acid-producing bacteria is added to cream to produce the slightly tart, thick, sour cream.
The butterfat content must be > 18% for products labelled as sour cream.
Experiment
To make sour cream, combine 1 cup heavy cream with 1 / 4 cup sour cream or buttermilk or white vinegar.
Shake the ingredients in a screw top jar and let it stand, covered, at room temperature for about 24 hours or until it becomes very thick.
Store the sour cream in the refrigerator for up to one week.
The sour cream must be well chilled before using.
Make lighter sour cream by using whole milk instead of heavy cream.
4. Clotted cream
Clotted cream is used in Devon, South England.
The Devonshire Tea method is to split a scone in two, cover each half with clotted cream, and then add strawberry jam on top.
Clotted cream is made by heating unpasteurized milk and leaving it until the cream rises to the surface.
16.2.10 Ice cream
| Make ice cream 1
| Make ice cream 2
1. Ice cream is a foam preserved by freezing.
Under a microscope you can see solid globules of milk fat, air cells, ice crystals, solution of concentrated sugars, salts and suspended milk proteins.
The ice crystals were formed by water freezing out of the solution to a point where the lowering of the freezing point caused by the concentration of the remaining solutes in the water corresponds to the freezer temperature.
Manufacturers can expand the ice cream with air to double its volume.
Expanded ice cream feels fluffier and has a warmer taste.
Ice cream containing less milk fat has bigger ice crystals, coarser texture and colder taste, but the addition of emulsifiers and stabilizers can mask these low fat properties, but prepare the ice cream sticky.
If not stored at a low enough temperature, partial thawing causes the smaller crystals to melt and later refreezing to larger crystals.
Ice cream on the tongue crystallizing out the lactose, milk sugar, which stays on the tongue after the ice has melted leaving a sweet taste.
2. A traditional method to make ice cream is to slowly freeze flavoured cream, with eggs sometimes used to emulsify it.
Ice cream is sold as stick line, or single serve, segments, home tubs, 2 to 10 litres packs, ice cream blocks, ice cream bricks, cones or cups in multiple flavours, ice cream bars with a sweet or chocolate coating, three dimensional stick novelties appealing to children, large cylinder tubs for scooping.
3. In some countries ice cream has the following composition:
* > 10% milk fat by legal definition (10% to 16% fat),
* 9% to 12% non-fat milk solids (caseins, whey proteins and lactose from milk),
* 12% to 16% sweeteners, e.g. sucrose and glucose-based corn syrup sweeteners,
* 0.2% to 0.5% added stabilizers and emulsifiers,
* 55% to 64% water from milk.
4. The sugars, including the lactose from the milk components, contribute to a depressed freezing point so that the ice cream has some unfrozen water.
The result is that at typical serving temperatures, -15oC to -18oC it is not too hard to scoop.
The colligative property of freezing point depression of a solution is greater with lower the molecular weight molecules, so the monosaccharides fructose and glucose produce a softer ice cream than the disaccharide sucrose.
Polysaccharides stabilizers add viscosity to the unfrozen portion of the water so that it cannot migrate within the product to produce a coarse and icy ice cream that is less firmer to the chew.
The smaller ice crystals are less detectable to the tongue.
Stabilizers prevent icy tasting.
5. Refrozen ice cream, that has not been stirred, becomes a mixture if ice crystals and dehydrated ingredients.
So it is harsh on the tongue and may be susceptible to infection by bacteria.
6. The types of ice cream include the following:
* Regular Ice Cream (no less than 10% milk fat),
* Reduced Fat Ice Cream (approximately 7% milk fat).
* Low Fat Ice Cream (< 3% milk fat),
* Soft Serve Ice Cream is similar in composition to reduced fat ice cream, it is aerated and frozen immediately before sale giving a frozen, but fluid ice cream.
* Gelato (Italian congelato, frozen), different frozen ice desserts, e.g. granita, sorbetto (sorbet), semi-freddo.
The gelato is lower in fat and sugar than the ice cream sold in Western Europe.
7. About 70% of the ingredients in ice cream are derived from milk.
The mix may also contain sugar and fruit.
Small quantities of stabilizers, flavours, colourings and emulsifiers may also be added to enhance flavour, texture or appearance.
There are no added preservatives as ice cream is preserved naturally by freezing.
As it freezes, the mixture is whipped and blended to give its smoothness.
The whipping blends air with the ice cream.
This increases its volume and reduces the kilojoules per serve.
It also helps to give ice cream its structure and a warming "mouth feel".
Milk fat gives ice cream its smoothness and creaminess, the higher the milk fat content the richer the ice cream.
It is added as cream, butter or pure milk fat.
Non-fat milk solids, nutritional milk proteins and lactose (milk sugar), give "body" to ice cream and help develop its smoothness.
Other milk solids include vitamins and essential minerals such as calcium.
Too much non-fat milk solid causes a "sandy" texture due to the formation of lactose crystals.
Cane sugar (sucrose) or glucose syrups are added to ice cream to give sweetness.
Sugar enhances flavour.
It also lowers the temperature at which ice cream freezes and melts to well below the freezing point of the water.
This makes ice cream easier to eat as, once taken from the freezer, it melts quite quickly.
Experiments
Make ice cream 1
To make ice cream, fill a pint size plastic food storage bag, e.g. Ziploc bag, half full of ice, and add 6 tablespoons rock salt.
Seal the bag.
Put 1 cup milk, 1 / 4 teaspoon vanilla, and 1 tablespoon white sugar into this small bag.
Place the small bag inside 1 gallon size plastic food storage bag.
Seal the bag.
Shake the bags for 5 minutes until the mixture is ice cream.
Wipe off the top of the small bag, then open it carefully.
A 1/2 cup milk will make about one scoop of ice cream.
Make ice cream 2
Use a coffee tin with a plastic lid.
Half fill the coffee tin with 250 mL of double cream, 250 mL of whole milk, 2 teaspoons of vanilla extract.
Firmly attach the plastic lid and use adhesive tape to attach the lid more tightly.
Put the coffee tin in a plastic bucket with a tight lid.
Add 10 cm of ice cubes to the space around the coffee tin then 200 g of rock salt to the same level as its height.
Then almost fill the bucket with ice cubes and attach the bucket lid and use adhesive tape to attach the lid more tightly.
Turn the bucket on its side and roughly roll it forwards and backwards for 10 minutes.
Open the bucket and coffee tin and observe the layer of frozen ice cream lining the inside of the coffee tin.
Friction between ice cubes in the rolling bucket causes some ice to melt but not refreeze, because the salt has lowered the freezing point.
The super-chilled water so formed cools the walls of the coffee tin.
The ice cream mixture starts to freeze, but does not form big crystals, because of the motion of the bucket.
The stirring from the rolling motion incorporates air and to prevents large ice crystals from forming to produce a smoothly textured, semi-solid foam that is malleable and can be scooped.
The salt water is cooled by the ice, and the action of the salt on the ice causes it to partially melt, absorb latent heat and bringing the mixture below the freezing point of pure water.
The immersed container can also make better thermal contact with the salty water and ice mixture than it could with ice alone.
16.2.11 Tests for milk fortification with calcium carbonate
Experiment modified by Adrian van Ravels
See diagram 16.2.11: Milk fortification
1. Use a 3 mL transfer pipette to transfer 6 mL of "XYZ brand Low Fat Milk" into a 50 mL centrifuge tube with a lid that has a flexible tube connected into it.
2. Use a 3 mL transfer to transfer 5 mL of 4.73% w / v aqueous solution of barium hydroxide into a second 50 mL centrifuge tube.
3. Use a 3 mL transfer pipette to transfer 10 mL of 6 M acetic acid into the XYZ brand Low Fat Milk.
Close the lid of this centrifuge tube and arrange the end of the flexible tube to sit below the level of the barium hydroxide solution in the other centrifuge tube.
Shake this centrifuge tube with back and forth shaking or finger tapping and allow any gas produced to bubble through the barium hydroxide solution.
4. Leave the solution to stand, then shake it again until no more bubbles appear out of the end of the flexible tube.
The bubbles of carbon dioxide produce a white flocculent precipitate in the bottom of barium carbonate.
5. So a carbonate exists in the XYZ brand Low Fat Milk.
Any carbonate reacts with acetic acid to produce carbon dioxide and calcium acetate.
CaCO3 + 2CH3COOH --> Ca(CH3COO)2 + H2O + CO2
Carbon dioxide reacts with barium hydroxide solution, Ba(OH)2, to form a white flocculent precipitate of barium carbonate, BaCO3.
CO2 + Ba(OH) 2 --> BaCO3 + H2O.
16.3.0 Casein and caseinates, Milk proteins
Casein and caseinates are ingredients in noodles, chocolate, sweets, mayonnaise, ice cream and cheese manufacture.
They are used as binding ingredients, emulsifiers and milk substitutes in processed foods.
Industrial uses of casein and caseinates include, plastics (buttons, knitting needles), manufacture of synthetic fibres and chemicals, (plants, glues, glazed paper, putty and cosmetics), as a reinforcing agent and stabilizer for rubber in tyres of motor cars, as a nutritional supplement and binder in calf milk replacers.
Milk proteins
About 80% of milk protein is the four types of casein as calcium salts, which all survive pasteurization at 62-71oC.
1. alpha S2 Casein, the bulk of the caseins, contains antioxidants
1a. alpha S2 Casein, antibacterial
2. beta Casein, many biological functions
3. kappa Casein, functions in milk clotting
The other proteins, the whey proteins, are denatured by pasteurization at 62-71oC.
4. beta lactoglobin, about 10%
5. alpha albumin, about 25
6. immunoglobin about 25
7. serum albumin about 1%
8. other proteins.
16.3.1 A2 Milk
16.3.1 A2 Milk
Nowadays, most milk contains roughly equal amounts af alpha and beta caseins.
However, according to recent research, the original milk of dairy cows contained only beta casein and no alpha caseins, which appeared in milk as a result of genetic mutations in European dairy herds.
People who suffer stomach ache after drinking milk are supposed to be susceptible to the digestive breakdown of the alpha caseins, possibly the opioid peptide BCM-7.
Researchers have discovered how to identify cows with milk containing only beta casein and no alpha caseins using a simple DNA test.
So commercial milk producers have kept separate the milk from such cows and have marketed it as "A2 Milk".