MILK AND MILK PRODUCTS
Keywords or Contents or Home
Lactic acid bacteria
Total count test
15(ii) Hygienic control of milk
15(iii) Raw milk
15(iv) Processing of milk
15(v) Checking and controlling the processing of milk.
15(vi) Storing or packing processed milk
15(vii) Testing of milk
15(viii) Spoilage of milk
15(ix) Dried milk
15(xiii) Ice cream
Milk is produced by cows which have recently given birth to a calf. The cows used to be milked by dairymaids and the milk was then collected into metal buckets which were then emptied into churns. These churns of raw milk were then taken to the dairy for heat processing. Now most milking is done by machine and the milk is transported to the dairy by bulk vacuum tanker.
At the dairy the milk is pumped into chilled holding tanks, then cleaned (either by being filtered or centrifuged) before being heat processed after which it is either filled into bottles or cartons or transferred to another holding tank to await further processing. The further processing might include removal of fat (skimming), drying to make dried milk, churning to make butter, or controlled fermentation as in yoghurt manufacture. The heat treated milk might be transported to another factory for making cheese, ice-cream, or chocolate or even soup or pudding manufacture.
Milk is a rich source of nutrients so it is prone to microbial attack. However sometimes controlled cultures of beneficial bacteria are added, on purpose, to milk as in yoghurt or cheese manufacture.
15(ii) Hygienic control of milk
Milk is a very nutritious food but it is equally nutritious to microbes and can easily become contaminated. Hence from cow to consumer care has to be taken to ensure that the milk does not become a vehicle for food poisoning or food borne microbes.
15(iii) Raw milk
a) The cow
Milk from a healthy cow contains few microbes, but if the cow is suffering from an infection the milk could be contaminated whilst being made inside the udder of the cow. Tuberculosis and brucellosis can be transferred into the milk if the cow is infected. Hence only healthy cows should be milked.
Milk should not be taken from cows recovering from disease, as the antibiotics used to treat the cows can pass from the blood into the milk. In England and Wales there is a legal limit for the amount of antibiotic, milk can contain.
Infections of the cow's udder can lead to the milk being contaminated with Salmonella and Staph. aureus food poisoning microbes. Cow faeces (dung) containing Salmonella can also lead to the milk becoming infected.
b) Milking machines and milking halls
Hand milking of milk has been superseded by milking machines. The milking machine extracts milk from the cow's udder by a vacuum pump which also removes air from the system whilst sucking the milk from the cow. The vacuum is also used to pump the milk into a bucket or churn or more frequently these days, into a refrigerated holding tank.
The main source of contamination at this stage is milk residue within the milking machine. To control these residues a daily cleaning schedule is essential. Also the pressure of the vacuum pump needs to be monitored to ensure that all the air is removed from the system to prevent air borne microbes contaminating the milk. Air leaks can suck air back into the system so these need to be frequently checked for and if found, fixed.
The milking hall should also be kept clean to control air borne contamination. The water used to clean milking machines and the milking hall should be chlorinated to prevent contamination via the cleaning water.
The handlers on the farm and especially in the milking hall can contaminate the raw milk with Staph. aureus either directly or via the equipment or via the cow if care is not taken.
Hence raw milk is often contaminated even before it leaves the farm so is unsafe to drink.
Legally (Warning UK only), milk has to be cooled below 4.4ºC within 30 minutes of milking and kept at or below this temperature. Usually the milk holding tank is cooled by water from an ice tank, circulating round an outer jacket of the milk holding tank and the temperature is monitored by a thermometer attached to the tank and a temperature / time control graph. The milk should be stirred by an agitator so that it does not separate and to ensure that the temperature of the milk remains the same throughout the holding tank.
c) Transport of raw milk
Usually transport of milk from the farm to the dairy is by bulk vacuum tanker. The tanker should be lined with stainless steel so that it is easy to clean. Before collecting the milk, the tanker should be cleaned and disinfected and once emptied at the dairy, the tanker should again be cleaned and disinfected. Ideally the tanker should only be used for transport of milk and to avoid chemical contamination, the tanker should only be used for the transport of food. The tanker should be well insulated for short journeys (hence the use of vacuum tankers) or be fitted with a cooling system for longer journeys so that the temperature of the milk remains below 4.4ºC.
15(iv) Processing of milk
When the milk arrives at the dairy it is chilled again through a heat exchanger then pumped into a holding tank. The milk in the holding tank is kept cool by circulating chilled water through an outer jacket. Yet again the temperature of the milk is monitored by a thermometer attached to the holding tank. Again the milk should be stirred by an agitator to prevent separation and to maintain uniform temperature.
In older systems the milk is filtered through cloth filters to remove any dirt and physical contamination. The filters need to be kept clean otherwise they could provide a good home for microbes.
In modern systems the filtration process is replaced by clarification in which the milk is span round at a fast speed (that is centrifuged). The sediment and most of the bacteria, being heavier than the milk, falls to the bottom of the milk container which is the furthest away from the centre of the centrifuge where it is collected in collecting bowls. For the clarification to be successful the collecting bowls should be regularly emptied, so that they do not overfill and be cleaned and disinfected after each emptying so that microbes do not accumulate.
The milk should be heat processed immediately after filtration or clarification so that the residual bacteria do not cause the milk to go sour.
b) Types of heat processing
There are three main types of heat processing: Pasteurization, Disinfection and Sterilization.
Pasteurization is named after the French chemist Louis Pasteur who discovered that the souring of milk is caused by microbes and that microbes can be killed by heat treatment. The principle of pasteurization is to heat milk in order to kill all the harmful microbes in raw milk. The process kills all the pathogens but some spoilage microbes survive along with the spores and toxins. Hence pasteurized milk has a short shelf life, the 'Use By' date being about 7 days and has to be stored chilled.
In pasteurization the milk is heated to at least 72ºC for at least 15 seconds. However due to problems with a food borne bacterium known as Listeria monocytogenes, which causes a disease called listeriosis, some dairies, especially in America, have increased the pasteurizing temperature to 77ºC.
In disinfection the milk is heated to 100ºC, producing a more stable product than in pasteurization. However disinfected milk is not stable, as the spores survive the heat treatment so disinfected milk has to be kept chilled. Disinfected milk is mainly used for milk products such as dried milk or yoghurt.
In sterilization the milk is heated to a high enough temperature to kill all microbes, dormant spores as well as active (vegetative) microbes. Sterilized milk has a shelf life of 6 months and can be stored at room temperature. The shelf life is determined by flavour deterioration due to chemical changes rather than microbial action.
c) Methods of heat processing
Milk is normally pasteurized or disinfected by a plate heat exchanger. In this method the raw milk is first heated by processed milk, in the regeneration section of the heat exchanger. The plate heat exchanger consists of stainless steel plates with a spiral channel on either side through which liquid can flow. The spiral channels are coated with rubber so that when two plates are pressed together an air and liquid tight seal is produced, thus the liquid can flow through the spiral channels but cannot escape between the plates. In the regeneration section raw milk flows on one side of each plate and processed milk flows on the other side of the plate. The heat travels from the processed milk through the metal plate into the cold raw milk on the other side.
After being heated in the regeneration section the raw milk goes to the hot section where the milk is heated to temperature (72ºC or 77ºC for pasteurized milk; 100ºC for disinfected milk) by hot water (or steam/ water mix for disinfected milk). The principle is the same as in the regeneration section; raw milk on one side of the plate and hot water/ steam on the other side.
When the milk has reached temperature it is held at that temperature for the requisite time in stainless steel holding tubes.
The milk which is now deemed heat processed returns to the regeneration section where it sheds some of its heat to the raw milk before going onto the cold section where it is cooled to below 5ºC by chilled water, yet again by heat exchange across stainless steel plates.
The milk is then pumped either to a filling machine or to another holding tank. If a filling machine is used there needs to be a return pipe to a balance tank. This is to keep the milk flowing even if the filling machine breaks down, otherwise the milk in the hot section would get burnt, ruining its quality and causing solid burnt deposits on the plates, reducing the efficiency of the processor.
There also needs to be a divert valve just before the holding tubes to ensure that any milk which fails to reach the required temperature (for pasteurization, disinfection or sterilization) does not enter the cold section of the heat exchanger and contaminate it together with the line pipes, filling machine or holding tank for heat processed milk. The divert valve is linked to a divert flow pipe which ensures that any divert milk returns to the balance tank.
The first method of sterilizing milk was filling it into heat resistant glass bottles, sealed with an air tight, pressure resistant cap, then heated in a retort (a commercial pressure cooker) to temperatures between 115ºC and 123ºC for between 12 and 20 minutes. Basically the higher the temperature used the less time is needed. The process and associated problems are similar to canning.
However this method is virtually obsolete and has been replaced by the U.H.T. (Ultra High Temperature) methods. In the U.H.T. process the milk is heated to 133ºC for one second. There are two ways of achieving this; either the direct process or the indirect process. In the direct process, live steam is injected into the milk, until the temperature and time have been reached, then the extra water is evaporated off, as the milk cools. This method is also dying out as most U.H.T. milk is processed by the indirect method in which case a plate heat exchanger is usually used. The plate heat exchanger for processing U.H.T. milk is similar to that for pasteurized and disinfected milk but is more complicated, with more regeneration sections, a homogenizer, in which the fat particles in the milk are reduced in size to stop them separating out and sometimes a retarder section (or vacuum chamber) where the milk is held at 53ºC for 6 minutes to improve its protein stability. However the main difference is that the plate heat exchanger has to be able to withstand the high pressures needed to heat milk to a temperature over 133ºC. The milk is cooled to 10ºC before being packed. U.H.T. milk is reckoned to be of higher quality than bottle retorted sterilized milk as U.H.T. milk does not have the burnt taste characteristic of the more primitive method of sterilizing milk.
15(v) Checking and controlling the processing of milk.
a) Processing and cleaning
Before the milk can be processed the stainless steel pipes to the plate heat exchanger, the exchanger itself, and the pipes to the filling machine or holding tank for processed milk need to be cleaned and disinfected (sterilized in the case of U.H.T. milk) together with the filling machine or holding tank.
Normally cleaning and sterilization is by a C.I.P. system. In cleaning the equipment is first flushed with hot water followed by caustic solutions then a further hot water rinse to remove the caustic, then an acid solution to remove calcium deposits from the milk (milkstone), followed by a further hot water rinse to remove the acid, whilst sterilization is either by chlorine solutions or more commonly by water above 100ºC, eg water at 121ºC for 15 minutes. Sterilization is essential for U.H.T. milk but for pasteurized or disinfected milk it is adequate to disinfect the equipment (which does not kill the microbial spores) by using water at 100ºC for 15 minutes.
When the steel pipes, plate heat exchanger, holding tank or filling machine has been cleaned and either sterilized or disinfected, the temperature of the hot section of the plate heat exchanger is reduced to the processing temperature and chilled water is passed into the cold section. The water flowing through the equipment is checked for residual chemicals before milk is allowed into the system and replace the circulating water.
The temperature of the system is checked with on line thermometers one being located at the end of the holding tubes, before the milk returns to the regeneration section and another one being placed in the pipe in which the milk leaves the heat exchanger. The first thermometer ensures that the processing temperature is correct and the second thermometer ensures that the milk leaves the heat exchanger at the correct temperature, below 5ºC for pasteurized and disinfected milk and about 10ºC for U.H.T. milk. During sterilization of the equipment both these thermometers should be reading 121ºC if sterilization is by water/ steam mix.
The processing and cooling temperatures should also be monitored by a temperature/ time recording chart.
Normally there are also on line thermometers checking the temperatures of the hot water or steam entering the hot section and the chilled water in the cold section.
The divert temperature should be set at 1ºC below the processing temperature. Hence if the milk is being processed at 72ºC the divert temperature would be set at 71ºC. (As 71.7ºC (Warning UK only) is the legal minimum pasteurization temperature for milk, the divert temperature is normally set at 72ºC and the processing temperature at 73ºC).
b) Processing problems
i) Temperature dropping
If the temperature drops during processing and the milk passes through the divert pipes, strictly speaking the plate heat exchanger should be resterilized or redisinfected, before processing resumes. In the case of U.H.T. milk there is no alternative. However, in the case of pasteurized milk, going to a filling machine, many dairies resume processing without redisinfecting. If this occurs, a definite temperature needs to be set below which the plate heat exchanger must be redisinfected. Also before processing can resume, all the milk in the plate heat exchanger, which has not been adequately heat treated must be sent down the divert pipe, back to the balance tank to be reprocessed, rather than allowed to be packed out or held as processed milk.
ii) Problems with air in the system
There are three ways in which air can enter the system:
2) Balance tank running dry
3) A leak in the system
Foaming is when air is mixed in with the milk producing a froth. This normally occurs either due to the milk being stirred too much or due to a sudden drop in pressure in the milk flowing through the heat exchanger.
Air provides protection, especially for air borne spores, as the temperature of the air in the bubbles is slightly lower than in the surrounding liquid. If the microbes remain dry more heat is needed to kill them.
Fortunately in foaming the air is trapped within the liquid making it difficult for the enclosed surviving microbes to contaminate the heat exchanger or pipes, so as long as the frothy milk is discarded it is safe to continue production.
If the milk is going to a filling system, foaming could cause a sudden reduction in weight. These low weight bottles or cartons should not be sent out to the customer.
If the milk is being processed into a holding tank, processing should stop and the milk in the holding tank should be reprocessed.
2) Balance tank running dry
If the level of milk in the balance tank falls below the pipe which carries the milk to the heat exchanger air is sucked into the system. Even if the temperature does not drop, the system must be disinfected or sterilized again before processing can continue.
3) A leak in the system
The most common places for leaks are the plates in the heat exchanger, the valves or the joints between pipes, but leaks can occur anywhere in the system.
When there is a leak in the system where milk flows out, air is sucked in. This air might contain microbes which will contaminate the system. The risk can be reduced by pumping milk through at a greater pressure than atmospheric but even for a small leak the risk can never be eliminated. Hence even if the leak is stopped by tightening a joint or the plates, the system should be disinfected or sterilized again before processing.
If the plate heat exchanger has to be opened up to replace a faulty plate, or a section of pipe, or a valve has to be replaced, or the system has to be opened up for any reason, once repaired, the system should be cleaned before being disinfected or sterilized. Opening the system causes a large number of microbes to enter the system and they can be protected by any milk solids or other debris clinging to plates or pipes so these plates or pipes need cleaning before the microbes can be killed.
The most dangerous type of leak is when raw milk in a plate escapes and contaminates the processed milk flowing through the base of a plate in the regeneration section of the plate heat exchanger. If the milk remains contained within the heat exchanger and no milk is seen seeping out between the plates, this type of leak can only be detected by testing the finished product. This could be very expensive if a large quantity of milk has been processed through the heat exchanger so the best option is to avoid this fault by properly maintaining the heat exchanger according to the supplier's advice. Frequently strip cleaning the plate heat exchanger and routinely checking the rubbers as well as replacing and rerubbering the plates on a regular basis will help to avoid this fault.
15(vi) Storing or packing processed milk
a) Storing heat processed milk
If a holding tank is used to store processed milk it should be clean, kept cool at or below 5ºC, by circulating chilled water round an outer jacket and containing a stirring mechanism to stop the milk separating out. The same holding tank should never be used for heat processed milk as for raw milk to prevent raw milk being mistaken and being used instead of heat processed milk.
b) Bottling of milk
The traditional method of delivering milk from the dairy to the consumer is in returnable glass bottles via a milk float.
When the empty bottles are returned to the dairy, they are rinsed, cleaned with a detergent, rinsed again, disinfected with hot water at or above 80ºC, then given a final rinse with cold water before being refilled with milk. Even so the milk bottles are susceptible to two sources of contamination, as the dairy looses control over its bottles once they have been delivered and cannot prevent them being treated unhygienically.
The first source of contamination arises from people using milk bottles to store chemicals. The chemicals involved can be any liquid, although chemicals such as turpentine or cresols, which are insoluble in water create the biggest hazard. This is because the cleaning process is not effective at removing these types of chemicals so residues can remain when the bottle is refilled, contaminating the milk. In the worst cases this could lead to an outbreak of chemical food poisoning.
Another major problem which might arise from reusable bottles is biological contamination. If milk bottles are not cleaned when empty, but left open with milk residues, flies might find them an attractive place in which to lay their eggs. To the fly, the bottle provides protection and the milk residue provides a source of food and moisture. The fly's eggs cling to the glass. The dairy's cleaning procedure is not always successful in washing away these eggs.
After cleaning the bottles are refilled with milk in the bottling room. The main problem at this stage is caused by broken glass. Occasionally the bottles are smashed and glass splinters fly all over the place so care has to be taken to avoid contamination with broken glass.
c) Cartons and plastic bottles
Over the last twenty years more milk is being packed in cartons and plastic bottles at the expense of glass bottles. The main hygiene problems associated with these types of packaging are related to pack integrity. Leaks in the cartons and plastic bottles may cause spillages which attract microbes. Air holes in sterile or U.H.T. packs, which are stored at room temperature, may result in spoilage due to air borne microbes entering the pack via the air hole.
Other hygiene problems may occur during the manufacture of the packaging. If carried out under poor conditions, dust and even insects might be incorporated into the packaging.
During storage the packaging should be covered to prevent contamination by dust, insects and microbes.
15(vii) Testing of milk
Normally pasteurized milk is tested for a total count of microbes; contamination by gut microbes (a coliform test); that sufficient heat treatment has been provided (a phosphatase test); and the number of aerobic bacteria present ( a dye reduction test).
Legally (Warning UK only) pasteurized milk must pass the total (or plate) count test, the coliform test and the phosphatase test. There is no legally required test for sterilized or U.H.T. milk but the usual methods of testing are the turbidity test and incubating samples.
a) Total count or plate count test
This test is used to work out the total number of microbes in 1ml (one thousandth of a litre) of milk. The legal limit is 20,000 microbes per ml of milk.
Basically this test consists of diluting 1ml of milk in 9mls of sterile solution, then taking 1ml of this solution and further diluting it in 9mls of sterile solution. 1ml of this is added to a petri dish (a small flat lidded dish commonly called a plate) then mixed with agar (a substance with similar setting properties to jelly but obtained from seaweed), enriched with chemicals to aid the growth of microbes. The petri dish is kept for 48 hours (this is called the incubation period) at 32ºC (this is called the incubation temperature and the place where the plate is kept is called the incubator). After 48 hours has elapsed since the plate (or petri dish) was prepared, the number of microbial colonies on the plate is counted. This number is multiplied by 100 to give the total or plate count. eg a count of 23 colonies gives a plate count of 2,300.
Generally the lower the total count, the better the keeping quality of the pasteurized milk. However this test does not ensure that the milk is free from pathogens. The phosphatase test is better for determining that.
b) Coliform test
Coliforms are a group of bacteria which live in the gut of people and animals and are transferred from one animal to another via contaminated sewage, or faeces (dung) which are washed into the water supply. In natural environments animals pick up coliforms when drinking from rivers, lakes or ponds.
Basically samples of milk are added to a purple broth in a test tube with a very small inverted tube at the bottom of the test tube. After 48 hours the test tubes are inspected. Coliforms produce acid which causes the colour of the broth to change from purple to yellow. Any gas produced causes the small tube to rise from the bottom of the test tube.
The presence of coliforms means that pathogens may be present in the milk. However a positive coliform test usually indicates a breakdown of hygiene in the dairy, either due to inadequate cleaning of equipment, poor maintenance of equipment or unhygienic handling of milk or equipment by dairy staff.
c) Dye reduction tests
The most common dye reduction tests are the Methylene blue test and the Resazurin test. In these tests a sample of milk is added to a coloured liquid. If there are any aerobic bacteria present in the milk they use up the oxygen in the liquid causing the dye to change colour. In the Methylene blue test the dye changes from blue to colourless and in the Resazurin test the dye changes from violet to pink then fades to colourless. The faster the dye changes colour, the more bacteria there are in the sample, hence in the milk.
Not all the bacteria in milk cause the dye to change colour, but all the pathogenic and most of the spoilage microbes would use up the oxygen, thus cause a colour change.
d) The phosphatase test
Phosphatase is an enzyme (a biological catalyst which is protein in nature) produced by most animals. Phosphatase passes into milk, probably from blood, during the milk's formation in the udder of cows.
The phosphatase enzyme is destroyed (or more accurately temporarily made non-functional) at temperatures around 71ºC, so can be used as in indicator to ensure that the pasteurization process has been successful. If there has been a temperature fault and insufficient heat has been supplied, or the processed milk has been contaminated by raw milk due to a fault in the heat exchanger the phosphatase test will give a positive result.
The phosphatase enzyme takes about 2 to 3 days to become functional again after being de-activated by heat, so this test cannot be used after 2 days to test sterilized or U.H.T. milk.
In the phosphatase test any active phosphatase left in the milk is used to produce a yellow chemical from a colourless liquid. The colour change is then compared to a standard on a disc colour comparator. The compulsory, legal test in the UK takes two hours but a modified version gives results in about half an hour and this can be used in addition to the legally sanctified test.
e) The turbidity test
This test is used to ensure that sterilized or U.H.T. milk has received adequate heat treatment. Basically some ammonium sulphate is dissolved in a sample of milk, the mixture is then filtered into a tube, which is then placed in boiling water for 5 minutes, then cooled with cold water. The mixture is then checked to see if it is clear or cloudy (or turbid). If the mixture is clear the milk has received sufficient heat treatment but if it is cloudy the heat treatment has been inadequate.
15(viii) Spoilage of milk
Traditionally when milk went off, it went sour but during the last twenty years milk, especially raw milk, has been held more successfully at lower temperatures. This has both reduced the amount of spoilage and changed the type of spoilage caused by microbes, from microbes which prefer warm temperatures to those which can cope better at lower temperatures. Hence spoilt milk is now more likely to have an off taste like a metallic or rancid taste rather than a sour taste.
U.H.T. and sterilized milk usually spoil due to chemical changes in the fat component rather than by microbial attack. If the milk is roughly treated in the dairy by being pumped through pipes with numerous bends the fat component is disturbed and the milk spoils faster.
15(ix) Dried milk
Milk used to be dried with roller driers but this has been superseded by spay driers which produce a more even product and unlike roller dried milk reconstitute without leaving any sediment.
If whole milk is being dried it needs to be pre-heated before drying to stabilize the fat component. Pre-heating is usually done by heating the milk to 107ºC for about five seconds in a process similar to disinfecting milk. The milk is then condensed to a moisture content of 65% under vacuum before being spray dried.
The spray drier is a chamber in which the condensed milk is injected into, as a fine spray. As the spray of milk falls down the chamber it is dried by hot air at 150ºC, which is blown across the chamber. The milk powder is collected at the bottom of the chamber.
If skimmed milk is being dried it should be pasteurized (or disinfected), skimmed then dried as the spray drying process is not effective at killing microbes and there have been outbreaks of food poisoning caused by Staph. aureus bacteria which have survived the drying process. This is because dry heat is not as good as moist heat at killing microbes, as the microbes can use other particles as shade against the heat. Even when unprotected, bacteria can withstand higher temperatures if the heat is dry rather than moist. In a similar way we feel the heat less when it is dry than when there is high humidity.
If the dried milk is not stored properly it may deteriorate due to chemical changes or become contaminated with microbes or be attacked by pests, especially rodents.
In dried milk the microbes are dormant rather than dead, so if the moisture content increases by storing in a humid atmosphere, the dried milk might be attacked by moulds or even bacteria. The moisture content of dried milk should not exceed 3.5%.
In the presence of moisture, chemical changes take place which might cause the dried milk to go brown, stale or the powder might turn to cake. These changes reduce the quality of the product as well as making the milk harder to reconstitute.
If the dried milk is stored in the presence of air, the fat component might go rancid. This problem is more serious in dried whole milk than in dried skimmed milk. Hence dried milk should be stored in air tight containers, preferably from which the air has been removed, at a low humidity and at a low temperature.
If the dried milk is stored in sacks they should be inspected for spillages and any spillages found should be swept up, before rodents are attracted by the dried milk odour.
Cream used to be produced by hand skimming warm milk but this has been superseded by centrifuging milk which has been heated to 45ºC. The milk should be of good quality and stored for the shortest possible time at 5ºC before it is needed for cream manufacture. The milk should be rapidly heated from 5 to 45ºC, then the cream is separated from the milk, after which the cream is further heat treated. Neither the milk nor the cream should be held for any length of time at 45ºC, which is in the danger zone. Like milk, cream needs to be heat treated to ensure microbial safety, either by pasteurizing at 79ºC for 15 seconds or by sterilization at 140ºC for 2 seconds. The equipment used is similar to that for milk and the same hygienic control needs to be applied for cream as for milk. If cream is sterilized it needs to be packed in an aseptic (or microbe free) environment.
Most milk separators are now self cleaning, to a certain extent, with the sludge dropping to a bowl at the bottom of the separator. Immediately that the bowl is full, or just before for safety sake, production should stop, the bowl emptied, then the separator put on a C.I.P. clean. However every couple of months (or according to the manufacturer's recommendation) the separator should be stripped, inspected and cleaned.
Unless the cream has been sterilized, then aseptically packed, it should be stored chilled and only sold by retailers who have facilities for chilled storage.
Yoghurt was first produced in the Middle East, several thousand years ago from naturally soured milk. However the addition of fruit and flavourings to yoghurt is a recent innovation and was only introduced to Britain after the second world war.
Yoghurt is an example of a food which uses bacteria for its manufacture. The bacteria used are strains of lactic acid bacteria and when added to milk are known as a starter culture. These lactic acid bacteria are harmless (some people believe that they are beneficial to health) and are eaten along with the yoghurt.
b) Manufacture of yoghurt
The milk is either heated to 85ºC for 30 minutes or to between 90 and 95ºC for 5 to 10 minutes to disinfect the milk and improve the texture of yoghurt. The milk used can be whole, skimmed, or semi-skimmed. After disinfection the milk is cooled to 40 to 42ºC and the starter culture of lactic acid bacteria is added at the ratio of 2l of starter culture to every 100l of milk. The fermentation is completed in about 4 hours.
After fermentation, the yoghurt is cooled in two stages to stop the yoghurt from separating out. In the first stage the yoghurt is cooled through a heat exchanger to 15 to 20ºC, then left in a cold store to cool slowly to 5ºC. The yoghurt should be filled into containers as soon as possible after cooling. This is usually done by pumping the yoghurt into plastic pots which are then lidded.
Sugar, emulsifiers and stabilizers may be added to the milk after it has been disinfected and before the starter culture is added. Artificial flavours and colouring compounds, if present, are normally added after the disinfection stage but do not affect and are not affected by the fermentation so can be added at any stage. Fruit jams, nuts or muesli mixes are usually added to the cooled yoghurt after fermentation.
c) Hygienic control of yoghurt manufacture
The milk used should be of high quality and free from antibiotics. Any antibiotics in milk could kill some or most of the lactic acid bacteria in the starter culture, leading only to a partially fermented yoghurt, which would then be vulnerable to attack by more antibiotic resistant types of microbes, some of which might be pathogenic or cause spoilage.
The fermenting activity of the starter culture needs to be checked daily as a poor starter culture will not only form a poor yoghurt but will also provide an opportunity for more active microbes to take advantage of the 4 hour fermentation time, leading to possible health risks. The pH of the yoghurt needs to be carefully checked for each batch to determine when the fermentation has ended. The final pH should be between pH 4.2 and 4.4. A pH lower than 4.0 leads to a taste which is too sour but a pH which is too high can be a health hazard. If the pH is above 4.5 there is a risk that food poisoning bacteria (includingClos. botulinum if the yoghurt is packed aseptically or under low oxygen tension) can survive and grow in the yoghurt.
The heating and cooling of the milk and yoghurt, together with the plate heat exchanger should be monitored in the same way as when heat processing milk.
Any emulsifier, stabilizer, sugar, flavouring or colouring added to the yoghurt, should be stored under the appropriate conditions as outlined by the supplier. These ingredients need to be checked to ensure that they are free from harmful microbes and are of good microbial quality (i.e. contain no or only a few microbes and are within specification). After the milk has been disinfected the yoghurt is not further processed in a way that can kill pathogens. Adding too much sugar to the yoghurt mix can cause poor fermentation leading to a high pH in the final product. There have been cases of Staph. aureus food poisoning caused by yoghurt in which too much sugar was added (not properly regulated) causing a high final pH (which was not checked!).
Special care needs to be taken with low acid ingredients (i.e. a pH above 4.5), including nuts, muesli, honey and chocolate, due to the risk from Clos. botulinum food poisoning. These ingredients should have undergone a botulinum cook process, then aseptically sealed before being supplied to the yoghurt manufacturer. Also the yoghurt manufacturer needs to practise a high standard of hygiene to prevent contamination with Clos. botulinum.
The area used to manufacture yoghurt should be kept clean. To prevent microbial and physical contamination, the milk in the fermentation vessels and the yoghurt produced should be kept covered.
The yoghurt is normally packed into pots. Preferably these pots should have tamper proof seals, to protect the yoghurt from human pests. Also the pack integrity of the pots should be checked for puncture holes or poor seals which might cause leaks. The seals of the pots should be air tight as mould spores could enter along with the air, causing spoilage of the yoghurt.
Unless the yoghurt is aseptically packed, the pots should be stored between 2 and 4ºC and the pots should be labelled with a 'Use By' date. The main spoilage risk for chilled yoghurts is from moulds and the main spoilage risk for aseptic yoghurt is due to chemical changes. The shelf life of yoghurt is determined by these factors.
It is believed that the discovery of cheese was a lucky accident which occurred over 10,000 years ago when nomads carried milk in calf's stomachs. However the making of cheese remained small scale until this century. There are many different varieties of cheese and these depend on:
1) Mix of lactic acid bacteria starter culture
2) Means of ripening
3) Moisture content
4) Type of mould or bacteria, if any, added during ripening
Hard cheese, such as cheddar, has the lower moisture content (between 26 and 50%) and soft cheese, such as camembert, has the higher moisture content (between 48 and 80%).
As in the manufacture of yoghurt a starter culture of lactic acid bacteria is used in making cheese. This is usually supplied by a specialist laboratory, either as a powder, liquid or frozen liquid.
Milk should be pasteurized at 78ºC for 15 seconds to kill Salmonella and Listeria bacteria. Some people claim that this vigorous heat treatment leads to a flavour which is too mild, but responsible manufacturers should put safety first. A bout of food poisoning, or a miscarriage due to Listeria, is too great a price for a stronger flavoured cheese.
b) Manufacture of hard cheese
Care must be taken when pasteurizing milk for cheese making. Below 77ºC there is a risk that some Listeria bacteria might survive and above 80ºC, the milk is chemically altered so that good cheese can not be made.
After the milk has been pasteurized, it is cooled to 30ºC, then pumped into a cheese making vat before the starter culture is added. The milk is stirred and the acidity and pH are measured. When the acidity reaches 0.10 to 0.20% lactic acid or the pH drops to 4.6, rennet is added. This normally takes about 20 minutes. Rennet is an enzyme or biological catalyst taken from a calf's stomach. In about 45 minutes to an hour the milk separates into the curd and the whey. Curd is basically a mixture of milk proteins which go from the liquid state to the solid state as the pH falls (or acidity rises). These proteins trap some water, fat and salts when changing from liquid to solid. The part of the milk which stays as a liquid after separation is known as whey. The whey consists mainly of water but contains some proteins, sugars, fats and salts.
The curd is then cut into small pieces to release some of the trapped water after which the curd/ whey mixture is heat treated in the cheese making vat so that most of the liquid trapped in the curd leaves and joins the whey. This heat treatment is known as scalding. The scalding temperature depends on the type of cheese being made. The higher the scalding temperature the lower the moisture content in the cheese. The scalding temperature varies from 32ºC to 80ºC and the scalding time (the time the curd/ whey mixture is held at the scalding temperature) varies from about an hour to an hour and a half. When scalding starts the temperature should slowly be raised from 30ºC to the scalding temperature and the curd/ whey mixture should be stirred during scalding.
After scalding the whey used to be drained out of the cheese making vat leaving the curd in the vat, to be treated. In more modern methods both the curd and the whey are pumped out of the cheese making vat into a tray or vat below. The whey is then drained off leaving the curd.
After the whey has been removed the curd is turned and pressed. (If making cheddar cheese the curd is piled as well as turned and pressed.) The curd is then rested for a couple of hours before it is milled and salted. During resting the curd becomes drier and its texture becomes less rubbery. During milling the curd is cut into smaller pieces, making it easier to absorb the added salt.
After salting the curd is filled into a mould, usually now rectangularly shaped, and pressure is applied. The pressure is slowly increased until it is about 1000Kg (a metric tonne). The pressure is applied for about three days changing the curd into an unripe cheese.
The ripening of cheese can take anytime from 2 weeks to 2 years depending on the variety cheese. Normally the cheese is ripened in a store room between 5 and 12ºC. If a blue vein cheese is being made, the ripening cheese is injected with needles which allow air containing mould spores to enter the cheese. Once there has been sufficient mould growth the cheese is wrapped in tin foil, which stops air from entering, thus halting the spread of mould.
c) Manufacture of soft cheese
Making soft cheese is similar to that of hard cheese except that there is no scalding and little or no cutting and the curd is not pressed. Hence the moisture content of soft cheese is much higher than hard cheese. Also the curd formation takes longer (about 2 hours), but more significantly the ripening period is much shorter, being only between 1 and 7 days.
In soft cheese the mould culture is added to the milk and not at the ripening stage as for hard cheese.
d) Hygienic control of cheese making
The main risk from cheese is if it is made from raw or under pasteurized milk in which case Salmonella and Listeria microbes can grow whilst the cheese is being made. If the milk has been properly pasteurized then the hygiene risks come from Staph. aureus, if the cheese is badly handled and Listeria if the milk, curd or cheese is allowed to become recontaminated.
The milk used for cheese making should be of good quality and free from antibiotics otherwise antibiotic resistant pathogens can be incorporated into the cheese. Similar problems can arise if the lactic acid bacteria starter culture is of poor quality.
The cheese making vat should be kept covered, for as long as possible, to prevent foreign bodies entering the cheese. A series of plastic panels could do this job. In modern vats, with knives and stirrers suspended from the roof, the vat could be completely enclosed by using plastic side panels. Just because the vats have been left exposed in the past, it is no excuse to continue this bad practise. The curd should be mixed and stirred by automated stainless steel equipment and not by the traditional wooden utensils.
The handlers should touch the curd or cheese as little as possible and when they do so, disposable gloves should be worn to prevent Staph. aureus contamination.
The cheese making vats and curd/ whey separating trays should be cleaned and disinfected after each batch.
Nitrates, such as potassium nitrate, should not be used as a preservative for cheese, as these chemicals can react with proteins to form harmful substances which might cause cancer.
Whilst ripening, the cheeses should be inspected for blown cheeses and these should be rejected as blown cheeses are rich in coliforms, hence are a possible health risk.
To prevent recontamination of milk, curd or cheese after heat treatment there needs to be a high standard of factory hygiene. Listeria can grow in wet sites, so the walls, floors and ceilings in cheese making areas should be kept clean and dry. Condensation should be avoided by good ventilation and by lagging all pipes. Also floor drainage should be rapid. The use of high pressure hoses should be avoided in cheese making areas to prevent air borne contamination with Listeria bacteria.
15(xiii) Ice cream
The first iced desserts, created by mixing snow with fruit or fruit juices, originated in ancient China. However the idea of using frozen milk and dairy products is only under three hundred years old. Originally ice cream was made with a hand operated ice cream maker in Europe but it was first made on an industrial scale in America in the 1850s
Basically ice cream consists of milk solids (usually referred to as milk solids non fat), fats, sugars, water and air. The air in ice cream, which is called overrun, is very important as it normally makes up about half its final volume. Usually emulsifiers, stabilizers flavouring and colouring are also added to ice cream. All the ingredients except the flavourings and colourings are mixed together, heat processed then cooled to 4ºC. Some of the flavourings and colourings might be destroyed or evaporated by heat so they are added at a later stage. When the mix has been chilled to 4ºC, it is held for a few hours to age, then blended with the colourings and flavourings. The ice cream mix is then frozen to about -10ºC, incorporating air whilst being frozen. After freezing the ice cream is extruded onto a conveyor belt then cut into either small blocks for individual portions or larger blocks for litre containers or catering packs. Layered products are made by using one extruder for each layer, so the bottom layer is extruded onto a conveyor, then the next layer is extruded on top of it and so on. After portioning the ice cream is further frozen to a temperature around -18ºC in a tunnel. This process is known as hardening. The ice cream is then packed into a container and taken to a cold store where its temperature further drops to between -25 to -30ºC.
b) Raw materials
All raw materials should be supplied by a reputable supplier and be accompanied by quality certificates. Samples of raw materials should be checked to ensure that they contain no or only a small number of microbes and no pathogens. The raw materials should be stored under the appropriate conditions.
Liquid milk solids non fat is usually either whole or skimmed milk and should be stored in a holding tank below 5ºC, away from tanks used to store heat treated ice cream or finished product. The liquid milk should be stored for the shortest possible time and never more than 20 hours. The holding tanks and associated pipe work should be cleaned and disinfected at least once a day.
Dried milk solids non fat normally consists of skimmed milk powder, whole milk powder, whey powder or milk substitute powder. All these products should be stored under cool dry conditions to prevent absorption of water leading to mould or bacterial growth. These powders should be used within their shelf life. Care needs to be taken with opened bags, which should be resealed and returned to the cool, dry store as soon as possible and not left in wet places where they can take in water and deteriorate. Emulsifiers and stabilizers should also be stored in the cool dry store.
The storage conditions for cream is similar to that of liquid milk. Vegetable fats are normally supplied in blocks which should be stored in a cool dry area to prevent them going rancid.
When water is used as a raw material it must be of a good microbial quality and should be tested before use. As an added precaution the water could be pre-heated to 85ºC, then cooled before being added to an ice cream mix.
The air in ice cream, is a possible source of contamination. If a compressed air supply is used, the air should be filtered to remove microbes and the air filters need to be kept clean. For small freezers the air valve should be cleaned and disinfected before use and the freezer sited where a clean air supply is available.
In small scale ice cream making raw eggs used to be added into the ice cream mix to act as an emulsifier and stabilizer. This is very risky as it could lead to an outbreak of Salmonella food poisoning. If egg products are used in an ice cream mix they need to be heat treated in a similar way to milk products.
c) Mixing and heat processing
The mixing should take place using stainless steel equipment and the mix should be kept for under an hour, below 7ºC before heat processing. The mix should be heat processed either by pasteurization at 82ºC for 20 seconds or by disinfection at 100ºC for 10 seconds. After heat processing the mix is cooled to 4ºC. Normally a plate heat exchanger, similar to the one used for pasteurizing milk is used and the hygiene requirements are the same.
d) Aging and blending
If the temperature of the mix goes to over 7ºC, during aging, or the mix is not frozen within 24 hours, the mix will have to be heat processed again.
After aging the ice cream mix is blended with colourings and flavourings which might include sauces, fruits or nuts, prior to freezing. These colourings and flavourings have to be virtually free of microbes, as they are being added to a high risk food which will not be receiving any more heat treatment. These flavourings and colourings should be kept covered at all times, even when in hoppers and feeders, to prevent contamination.
The air incorporated during freezing should be filtered in two stages. The first stage is to remove extraneous matter, such as oil particles and water and the second stage is to remove microbes.
The freezer pumps, valves and pipes need to be checked for oil or product leaks.
e) Extrusion and packing
The extrusion machines, cutting machines, conveyors, packing machines and all other on line equipment should be kept clean. The walls, floors and ceilings of the processing and packing areas should be kept clean and well maintained. All equipment and conveyor lines should be kept covered, to protect the product.
Packaging material should be left open or exposed for a minimum time before filling and be kept covered at all other times.
Manual touching of the product should be avoided to prevent Staph. aureus food poisoning. Tongs should be used to realign product on conveyors and clean, disinfected equipment should be used to put the lid on plastic containers of ice cream rather than handler's fingers.
f) Finished product
The finished product should be stored in a cold store used solely for finished product (no raw materials) at -30ºC. The cold store should have two separate compressor units each capable of holding the temperature below -20ºC, if the other compressor fails.
The ice cream should be delivered to the retailer at a temperature not higher than -20ºC, in a delivery van refrigerated to -25ºC.
All ice cream packs and containers must have a 'Best Before' date, a batch code and information about storage conditions.
The manufacturer and retailer should both practise stock rotation.
Milk is a very nutritious and versatile food which can be used as a raw material for many types of food including butter, cream, yoghurt, cheese, ice cream, puddings and soups. The disadvantage is that microbes also find milk an attractive source of food.
Milk should only be taken from healthy cows and not from ill cows or cows recovering from illness, which are still on antibiotics. Raw milk is unsafe to drink as it might be contaminated with Salmonella, Staph. aureus or Listeria. Hence milk needs to be heat treated before being drunk or further processed. The types of heat treatment include pasteurization, disinfection and sterilization.
Modern methods of packing milk, such as cartons or plastic bottles, are safer than the traditional glass bottle.
In dried milk the microbes are dormant so it needs to be stored in a cool dry place.
Starter cultures of lactic acid bacteria are used in the manufacture of yoghurt and cheese. The pH of the yoghurt needs to be checked before it is filled into containers, because if the pH is above 4.5, it is a high risk food and can support the growth of pathogens. If yoghurt is sterilized then packed aseptically it is essential that the pH is below 4.5, otherwise it might allow growth of Clostridium botulinum.
Due to risk from Listeria, Salmonella and Staph. aureus, cheese should not be made from raw milk and care needs to be taken that the milk, cheese or curd is not recontaminated during the cheese making process.
Unless the temperature is carefully controlled during the making and storage of ice cream, there is a hygiene risk from Staph. aureus food poisoning.
© R. T. L. BERG 1999