9 Hygiene of Production – Processing of Other Foods This chapter aims to outline the dairy production chain, introducing the reader to the process steps involved and examining public health and hygiene issues throughout. Milk is produced in the udder of all mammals, in order to feed and nourish the newborn of the species. It is composed primarily of water, containing proteins, carbohydrate, minerals and fat. The proportion of each of these nutrients present in milk from different species of animal varies, and has evolved in order to provide the correct balance required for the development of the newborn of that species. For example, the protein content of human milk is substantially lower than that of other species, and the lactose level quite high. These differences mean that the formulation of milk replacers, used extensively in the rearing of infants and other newborn animals, is an exceedingly complex task, and also mean that milk replacers formulated for one species are not appropriate nutrition for another. There are two groups of proteins present in raw milk, the caseins and the whey proteins. Caseins form over 80% of the total protein in milk, and do not possess an organized molecular structure, so cannot be denatured in the conventional sense. Within the casein molecule, there are areas that are highly hydrophobic, and other areas that are extremely hydrophilic. Cross links between these areas form readily, and these cross links form the basis of cheese formation. Whey proteins, which include various enzymes, are organized, globular proteins and are readily denatured, e.g. by the application of heat. The primary sugar in milk is lactose, comprising around 4.5% of the total milk solids. Lactose is a disaccharide of glucose and galactose, which itself is a reducing sugar, and reacts with the free amino acids present in milk producing a brown discolouration. The fat content of milk is very variable, from low fat in the horse (around 2% of milk solids) to high in sheep (7%) and buffalo (8%), and is comprised of globules of triglyceride surrounded by a phospholipid membrane. In the early part of the 20th Century, milk and dairy products probably constituted the most dangerous parts of our diet, being the vector for thousands of cases of diseases such as brucellosis, paratyphoid fever, Bovine Tuberculosis (which at this time often led to the death of the affected individual), as well as food poisoning. Human illness and deaths as a result of milk-borne disease have fallen dramatically since the 1920s. Disease due to bovine tuberculosis and brucellosis has been controlled through milk pasteurization, and through the implementation of eradication programmes in livestock in the latter half of the 20th Century, but milk-borne infection with Salmonella and Campylobacter still occurs, causing substantial morbidity and subsequent cost to society. Most of these infections originate with raw or improperly pasteurized products, or through recontamination of a pasteurized product, for example wild birds pecking through the bottle tops on doorstep-delivered milk to reach the cream below can introduce organisms such as Salmonella or Campylobacter into the bottle contents, thus presenting a risk of infection to the householder consuming that product. Recontamination may also occur from a food handler carrying and excreting pathogens, and summer-long enteric disease outbreaks in popular seaside holiday resorts may commonly be traced back to a particular ice cream vendor who happens to be excreting, for example, Salmonella paratyphi B. Globally, dairy products are responsible for approximately 21% of outbreaks of infectious intestinal disease where a food vehicle has been identified; 8% is attributable to milk and milk products and 13% to cakes and ice cream (Fig. 9.1). For comparison, 23% of outbreaks are attributable to eggs and egg products, and 15% to meat and meat products. Milk is first produced on the farm, and in the majority of cases is removed in a raw state to a collection centre or processing plant. There do exist small producer–processors, where the milk is harvested from the lactating cattle, sheep, goats or buffalo, and processed on the farm of origin for direct sale to the final consumer. In countries with a highly industrialized dairy industry, these private operations are in the minority, but in other countries they may form the greater proportion of the dairy supply. Equally, although this chapter considers milk produced by cattle and sheep, throughout the world the milk of many species is harvested and used for human consumption. The hygiene considerations however, are similar. Fig. 9.1. Estimated contribution of different food groups to outbreaks of food-borne illness. In an industrialized system, the milk is removed from the farm in bulk tanker vehicles to a collection point, where it is mixed with milk from many different farms, standardized and stored under refrigeration until processed. Most milk is pasteurized, before packaging or bottling and distribution, or further processing into dairy products (Table 9.1). Some products and some markets, however, demand raw, unpasteurized milk. Table 9.1. Production of milk for human consumption. Site Stages in production On-farm Milking Cooling Transportation Processing plant Storage in silos Clarification Pasteurization – heating and holding Cooling to below 10°C Pre-filling holding tank, agitated Filling cartons or bottles Chilling Distribution to cold store and retail outlets Primary production occurs on the farm, and farm and livestock management can have a significant impact on the productivity of the herd. There is also a genetic effect on milk yield from individual cows, but, using the UK as an example, improvements in genetic quality, health and nutrition of dairy cows over the last 20 years of the 20th Century have resulted in an increase in average milk yield per dairy cow from 3900 l per annum to 6500 l per annum. To put this figure into perspective, 6500 l would be sufficient to supply 26 families with their liquid milk requirement for 1 year. On average, in the UK, each person drinks around 1.5 l of liquid milk each week. On the dairy farm, cleanliness and the use of good farming practices are paramount. Cleanliness of the premises, personnel, animals and equipment will not only protect public health, by reducing the risk of milk contamination, but also protect the health of the animals, by reducing the risk of mastitis. A reduction in mastitis in the dairy herd also results in improvements in milk quality as measured by the somatic cell count (SCC) in the milk. Also, healthy animals are capable of maximum performance as regards milk yield, and thus the overall farm income is supported. One of the hidden costs of endemic and subclinical illness in dairy herds is a reduction in milk yield, and this reduction often remains unobserved until the disease problem is solved, and milk yields rise, because the reduction in yield is often insidious in onset rather than sudden. Milk is highly vulnerable to contamination by both pathogenic and spoilage organisms, so it is important that the premises are laid out in such a manner that there is complete separation between animal/‘dirty’ operations and food/‘clean’ operations. When the dairy is first constructed, regard should be given both to the topography of the land and to the services available. Flood plains and sewage outlets should be avoided, whilst a sufficient supply of potable water and power is essential. Sources of contamination, such as manure heaps and tanks, should be separated from the animal housing, and particularly from the milk-handling and storage areas, as should lavatories. The premises should be well drained and ventilated, to prevent excessive humidity and the build-up of moisture in animal bedding, and dung channels should be cleaned and droppings removed regularly. Within the animal housing, the premises should be kept in such a manner that the animals themselves remain clean and healthy. There should be isolation facilities for sick animals, and species constituting a high risk of carrying food-borne pathogens, for example pigs and poultry with Salmonella or Campylobacter, should be housed separately from lactating dairy animals. Similarly, animals and substances that may pose a risk of tainting the milk should be separated from the lactating animals, and the dairy parlour itself. Breeding rams and billy goats have been known to impart an odour taint in sheep milk, whilst certain chemicals or combinations of chemicals may result in tainting: for example the use of phenolic detergents alongside hypochlorite results in the liberation of chlorophenols, which would taint milk. The choice of chemicals used on the dairy farm should be chosen with care to avoid problems with taints, and also to avoid residues of unauthorized substances in milk. In fact, any activity which may pose a risk to the hygiene and quality of the milk, for example feeding, mucking out or cleaning, should not be carried out in the same place or at the same time as milk harvesting is under way. Feedstuffs should be kept separate from the milk production and storage areas. Animal feeds may contain components that carry a risk of tainting milk, and also feeds have been shown to be a source of contamination for the farm with food-borne pathogens such as Salmonella. Even feeds purchased from a reputable supplier and tested for certain pathogens are not likely to be sterile, and so present a risk of introduction of spoilage and other organisms if kept close to milk. Equipment used on the farm should be kept clean and well-maintained, and store rooms and feed stores clean and pest-proof. Vermin should be actively discouraged throughout, and there should be hygienic arrangements for the disposal of waste materials and discarded milk. Personnel should be provided with clean protective clothing to be used only within the milking parlour, separate from clothing to be used in the animal housing, and facilities for washing and drying hands should be available. The milking parlour itself should be easy to clean, with good drainage and ventilation, and good lighting is important so that cleanliness of both equipment and animals can be adequately assessed. Chemicals used in cleaning, and medicines used on the farm, should be correctly stored in lockable facilities. Traditionally, milk was harvested by hand, into open buckets, which would then be covered for transportation to the milk store, where the buckets would be tipped into milk churns for collection or processing. In a modern facility, the milk is collected automatically through a milking system comprising a cluster of teat cups that attach to the udder, drawing the milk off by a pulsing vacuum system into a primary receiver bottle. The bottle is then emptied via a milk transfer line into the bulk milk storage tank, where it is held under refrigeration (6°C or less) until collection by the milk tanker vehicle. The bulk tank should be housed in a separate room, considered to be a ‘high-risk’ area, which is used for no other purpose. In some areas, the ‘milking bail’ is commonly used for milk harvest while cows are at summer pasture. The ‘milking bail’ is a mobile milking parlour which is positioned at a site close to the cows, often in a corner of the pasture, close to the gate. When choosing the site for a bail, the farmer should take into account the same factors as when building a permanent dairy parlour, such as potential for flooding, position of manure heaps, sewage outflows and drainage and cleanliness of the land. Adequate protection for the milk is important, and the bail must be kept clean. The farmer will need to bring to the site a power supply, potable water and storage/transport containers for the milk produced, and the equipment must be cleaned as thoroughly as the equipment in the permanent facility. An outline of a milking system is given in Fig. 9.2. Where a farm is a producer–processor, and the milk is to be further processed on the same site, the processing unit should be a separate building, accessed via dedicated hygiene facilities, and using separate equipment and protective clothing from the rest of the operation. Milk may be transferred from the dairy parlour to the processing unit using milk churns or via hygienic pipes. The latter arrangement is more suitable, as it involves an entirely closed system, so the milk will not be exposed to external sources of contamination. The equipment used in the dairy plant must be smooth in nature, to prevent the build-up of milk residue, which would be an ideal medium for the growth of microbes. It must be corrosion resistant and easy to clean and disinfect, and also must be made of a substance which itself will not cause tainting of the milk. Metal piping is commonly used, usually a steel–nickel alloy, or stainless steel. Glass tubing may be used, but with this material there is the risk of breakage, resulting in foreign body contamination of the milk, and also safety issues with personnel. Rubber is often used to form corner-pieces, gaskets to provide good seals, and for teat cup liners. All these items are therefore prone to deteriorate with time as the rubber slowly perishes. Perished rubber is characterized by a myriad of tiny cracks, and these allow harbourage of microorganisms including spoilers, human pathogens, and mastitis-causing organisms. Cleaning of a dairy facility usually involves a clean-in-place system, where hot water – or a solution of hot water and chemical – is flushed through the pipes, effecting cleaning through the turbulence of the liquid in the pipes. Where dead areas occur in the system, such as where the layout of the pipes has been modified, leaving a closed branch, the cleaning liquid often bypasses this area, allowing deposits of milk to build up, and thereby giving harbourage to microbes. Once cleaned, the system should be rinsed with potable water to prevent taint occurring. The outlet valve on the bulk tank should be left open to allow complete drainage and drying of the tank, as pools of residual liquid can present a reservoir of contamination to the subsequent batch of milk stored in the tank. Fig. 9.2. Schematic diagram of milking system, showing clean-in-place wash line. Milk for human consumption should never be harvested from animals showing clinical signs of infectious diseases that may be transmitted to humans through ingestion of contaminated milk, for example tuberculosis or brucellosis, or from animals suffering from enteritis, mastitis or metritis. Sick animals may be treated with therapeutic medicines, and during and after treatment their milk should be discarded, until such time has passed that there is no risk of residues of those medicinal products being present in the milk. When harvesting milk, it is important that the udder and teats are clean. They should be cleaned and dried each time the animal is milked, and the foremilk drawn off, prior to application of the teat cups, or drawing off of the milk. This action will not only aid in prevention of contamination of the milk, but will also assist in the prevention of mastitis, and the discarded foremilk can be examined as part of the herd mastitis screening programme. However, rather than focusing solely on the udder, the stockman should assess the cleanliness of the cows as a whole, as there is a correlation between cow cleanliness and milk hygiene. Clean cows also tend to have a lower incidence of mastitis, and hence lower somatic cell count, meaning that the milk is of a higher quality grade. Assessing the cleanliness of cows objectively can also assist the stockman in identifying possible management problems, and thereby to take appropriate action to correct these. The primary cause of coat soiling in dairy cattle is loose faeces, but other factors also play a part in the overall appearance of the cows on a farm. Where cows have dirty legs, the passageway in the cow house is often insufficiently cleaned, and the cows may be walking through slurry. Modern dairy cows are substantially larger than their counterparts of 30 or 40 years ago, consume more food and water, and subsequently void more faeces and urine. A dairy cow can produce up to 30 l of each of these daily, and the practice of scraping passageways twice daily is no longer sufficient to adequately remove this waste. All too often, scraping produces a tidal wave of slurry that slops back over the scraper and into the beds. Cows with dirty tails may be housed in cubicles that are too small. Again, the modern dairy cow, being larger than her predecessors, requires a larger bed, and where the bed is too small, her tail and udder may hang over into the dung passage and become contaminated. Tails may also be excessively dirty where the faeces are too loose. A healthy faecal consistency, where the dropping is moist but forms a distinct pat on the ground, indicates cows that are on a well-balanced diet. Cows which are fed diets with insufficient long fibre and a crude protein content greater than 18% often show sloppy faeces and dirty tails. A dirty tail not only indicates health and diet problems in the cows, it also poses a significant risk of contamination in the dairy parlour, the tail will flick faecal matter onto the equipment and personnel surrounding the cow and faecal contamination of the milk may result. Where the flanks of the cows are identified as being dirty, this suggests that the beds are dirty and damp. Bed cleanliness is affected by the substrate used, drainage and the humidity of the cow house. Unused bedding should be stored under cover to keep it dry, and its moisture content should be maintained at less than 20%. Wet straw wads together, becomes mouldy and loses absorbency; wheat straw has a higher lignin content than barley straw, so is less absorbent; and all natural beddings such as straw, paper and wood shavings may support microbial growth and survival. Approximately half of the milk produced by dairy cattle is ultimately sold as liquid drinking milk, the rest being processed into milk products such as butter, yoghurt or cheese. Milk is transported from the farm of origin in specially designed refrigerated tankers that are used solely for milk transportation, to prevent the risk of taint. The tank is often constructed of stainless steel, and designed in such a way as to allow thorough cleaning and complete drainage, as residual fluids can be a source of contamination for subsequent batches of milk. Tankers are often cleaned using a clean-in-place system, where superheated steam is circulated through the milk lines and the tank until sterilization temperatures are achieved. A tanker may collect milk from up to eight farms on a circuit before returning to the dairy processing plant to be emptied and cleaned. When the milk arrives at the processing plant, a sample of the milk is taken and tested for odour, temperature and microbiological status. At this stage a rapid indicator test is used as a primary screen, and the sample removed to a laboratory for full microbiological analysis, which takes a matter of days. The milk is stored in a refrigerated silo prior to processing. On average, silos in use in modern dairy plants would hold the contents of three milk tankers, around 60,000 l. On arrival at the collection point, the milk should be at a temperature of no more than 10°C, and is cooled to below 6°C for storage prior to processing. Microbiological and quality standards to which the raw milk must comply have been set by the competent authority in most countries. For example, in the European Union (EU), raw cows’ milk must have a somatic cell count (SCC) of no more than 400,000/ml, and some dairy processors pay a premium for milk of SCC < 100,000 cells/ml. Microbiologically, raw cows’ milk is required to have an Aerobic Plate Count (APC) at 30°C of no more than 100,000 organisms/ml, whilst milk from sheep, goats or buffalo must have an APC of no more than 1,500,000 organisms/ml if the milk is going to be heat treated, and no more than 500,000 if it is to be sold without heat treatment. Raw milk may undergo a number of treatments such as filtration, clarification and homogenization, as well as pasteurization, whilst at the processing plant. Usually the milk is passed from the silo through a coarse filter to remove large particles and foreign bodies. Further filtration may occur before the pasteurization process begins, whilst the milk is cold, during pasteurization, or immediately after pasteurization. Where filtration is carried out during pasteurization, this is effected by filters placed in the milk line, usually after pre-heating. A disadvantage of warm filtration is that some undesirable material, for example some dirt particles, becomes soluble when heated, and therefore will not be trapped in the filter. Cold milk filtration, however, reduces the butterfat content of the milk, and thereby affects its composition. After pasteurization, ultrafiltration is often used to concentrate the proteins in skimmed milk, and the retentate from this process can be used as a concentrate in the process of milk standardization. The process of clarification involves the use of centrifuges to spin off foreign material. Centrifuges are also used to separate milk in modern processing plants, to remove the cream and for production of skimmed or semi-skimmed milk. The process of clarification is very important when the milk is homogenized, as it removes leucocytes and epithelial cells from the milk. These cells are naturally sloughed from the udder during lactation, are not removed by filtration and are present in raw milk. In milk that has not been homogenized, the cells remain in suspension and are not detected by eye, but in homogenized milk, they precipitate out and settle at the base of the container, leaving an unsightly, though harmless, grey sediment. Homogenization is carried out by forcing the milk through a tiny aperture under high pressure (around 210 kg/cm2 or 3000 lb/in2). This process breaks up the fat globules in the milk to give an even and stable dispersion of the butterfat, and breaks down any cross-links formed between caseins, reducing the curd tension of the milk. This makes the milk easier to digest and more palatable, but it can activate lipase in the milk, which will impart a rancid flavour. Lipase can be denatured by heat treatment, so homogenized milk is immediately pasteurized. Milk may be heat treated by one of a number of ways – holding method pasteurization, High Temperature Short Time (HTST) pasteurization, Ultra High Temperature (UHT) treatment or sterilization. The aim of these treatments, of pasteurization, is the application of sufficient heat for sufficient time to destroy pathogenic microorganisms. This heat treatment, in a modern plant, is normally a batch process, with milk travelling through a long pipe in a heat exchanger. In the heat exchanger, the milk line is tightly coiled round a pipe containing superheated water or steam, so thermal energy is transferred from the steam to the milk without direct contact. The speed of flow of the milk and the length of the milk line is such that the milk remains in the heat exchanger for sufficient time to achieve the desired temperature and holding time. In a small dairy, the pasteurizer may be a small tank with a heated jacket and agitator, or a tank with rotating heated coils, stirring the milk continuously to ensure even heat distribution. When tank pasteurization is used, it is common practice to preheat the milk to reduce the energy load on the pasteurizer. Holding method pasteurization, developed in the late 19th Century, requires milk to be heated to between 62.8°C and 65.6°C, and held for 30 min at this temperature. HTST pasteurization requires a temperature of 71.7°C, with 15 s holding. This method was developed from a technique called ‘flash pasteurization’, used in the early part of the 20th Century, where milk was heated to around 72°C with no holding period. The holding period of 15 s was added when it was realized that ‘flash pasteurization’ without holding often gave an inadequate kill of pathogenic microorganisms. UHT milk has been held at 135°C for one second. This process is effected by indirect or direct heating. Indirect methods would use a heat exchanger as described above, while direct methods may involve passing steam directly through the milk, similar to the preparation of certain speciality coffees, e.g. caffe latte, or by passing an electric current through the milk. Sterilized milk has been heated to above 100°C. This tends to apply to canned products; milk in filled and seamed cans is placed in a retort and steamed at 115–120°C for 15 to 20 min, dependent on can size. After pasteurization, it is important that the milk is protected from recontamination from any source. Pasteurization is the Critical Control Point in a HACCP for liquid milk. It should be cooled immediately to below 10°C, and packaged without delay. Processors should keep records of the pasteurization process, and plants are often fitted with automatic fail-safe devices, where milk that has not achieved proper pasteurization is diverted back to the raw milk silo for re-pasteurization, and there can be no cross-contamination of treated milk with untreated milk. Pasteurization denatures milk enzymes, and this can be used as a test of correct pasteurization. Phosphatase is a natural milk enzyme, and it is used as the indicator enzyme. Properly pasteurized milk is phosphatase negative, and this is often used in legislation as a milk standard. To test for phosphatase activity, milk at 37°C is mixed with a phenylphosphoric ester, and a colour change indicator (2,6-di-bromoquinonechlorine) added. If phosphatase enzyme is present, phenol is liberated and the indicator develops a blue colour. This test is sensitive enough to detect 0.1% raw milk added to pasteurized milk, a 5-min reduction in holding time if using the holding method of pasteurization, or a 1°C loss in temperature during the pasteurization process. Heat treatment also reduces the Vitamin C content of milk by around 20%, and the Vitamin B12/Thiamin content by around 10% (this has no significant effect on our dietary intake of these vitamins, as milk is not an important source in a balanced diet); this causes a slight disaggregation of the fat globules in the milk, giving a reduced cream line (this too has no nutritional significance). In UHT and sterilized milk these effects are increased due to the higher temperatures achieved, and also there is some caramelization of the sugars in the milk, giving it a sweeter flavour. An outline of process control for milk production is given in Table 9.2. Cream is the portion of milk, rich in butterfat, that rises to the surface when milk is allowed to stand. Commercially it is separated from milk by centrifugation. Different types of cream are sold dependent on the butterfat content. For example, half cream has at least 12% butterfat, single cream at least 18% and double cream at least 48%. Cream sold as whipping cream has a butterfat content of 35% or more, and clotted cream 55%. Cream is pasteurized at a slightly higher temperature than milk; the holding method uses 63°C over a 30 min holding period, whilst the HTST method uses 73°C, held for 15 s. Once again, the phosphatase test is used as the indicator of correct pasteurization, and the cream must be immediately cooled and protected from recontamination. Cream is considered to be sterilized if it has been held at 108°C for 45 min, and UHT treatment of cream involves 140°C for 2 s. Table 9.2. Process control in milk production. Process step Hazards Controls Arrival of raw milk Raw milk may be contaminated with pathogens Check bacterial quality of raw milk Keep raw milk separate from pasteurized milk Pre-processing storage below 5°C Growth of psychrotrophic bacteria Limit length of holding time Thorough cleaning of equipment between batches Pasteurization Failure to destroy pathogens Record time/temperature and flow rates Test milk – phosphatase test should be negative Clean and disinfect thoroughly between batches Cooling to below 10°C Bacterial growth Recontamination Chill rapidly Holding Bacterial growth Recontamination Temperature control Prevent contact with raw product Thorough cleaning and disinfection between batches Filling Recontamination from cartons Store cartons hygienically Cold Storage Bacterial growth Temperature control Ice cream is a frozen product made from a combination of any of the following ingredients: • eggs; • sugar; • cream; • butter or butter oil; • milk yoghurt; • skimmed milk; • evaporated milk; • dried milk; • dried skimmed milk; • condensed milk; • sweetened condensed milk; • condensed skimmed milk; and • sweetened condensed skimmed milk. Flavourings and colourings are usually added to the ice cream base after pasteurization. The ‘milk’ in ice cream may begin in a number of forms, but once the base is prepared, it should be pasteurized at 65.6°C for 30 min, 71.1°C for 10 min or 79.4°C for 15 s. The ice cream base may also be sterilized using 148°C for 2 s. Immediately after heat treatment, the ice cream base is homogenized and cooled. Flavourings and colourants are added in a storage vat, mixed with a mechanical paddle – either during the initial cooling phase or once the holding temperature of 7°C is reached. Ice cream must be cooled to below 7°C within 90 min of heat treatment, and held at this temperature until freezing begins. A temperature of below 4°C will improve the shelf life of the ice cream. Solid items such as fruits or cookies may be added to the base during cool storage, or at primary freezing. Primary freezing involves the cool ice cream mix being sprayed onto a cold surface and this soft-frozen mix is scraped off and placed into the final containers, which are transferred to the freezer store for final freezing to below −2.2°C. If the temperature of the product exceeds −2.2°C after this point, the ice cream must be re-pasteurized before sale. Ice cream is a high-risk food product, as pathogenic organisms may be introduced to the base mix with ingredients such as fruit and cookies added after the pasteurization step, and some ice creams are made with pre-pasteurized milk, and no further pasteurization step after the eggs and gelatin have been added to make the base. The critical controls in ice cream production are pasteurization of the base (monitored using the phosphatase test) and strict temperature control thenceforth, combined with great care over the status of added ingredients (Table 9.3). Milk used for further processing into milk products must achieve certain minimum standards prior to processing. In the EU, immediately before processing, raw cows’ milk must have an APC of less than 300,000 organisms/ml, whilst previously pasteurized or heat-treated cows’ milk must have an APC of less than 100,000 organisms/ml. When milk arrives at the processing plant, it must be refrigerated and held at no more than 6°C until processed, unless processed within 4 h of arrival. If milk is to be stored for long periods (up to 48 h) it should be refrigerated to below 4°C. Milk for further processing may undergo a process called ‘thermization’, where it is warmed to 57–68°C and held for 15 s. This treatment improves the keeping quality of the milk pending processing, but is insufficient to destroy pathogens, and the phosphatase test will yield a positive result. This treatment cannot be used to ‘improve’ poor-quality milk – if the APC is greater than 300,000 organisms/ml, the milk must be discarded. Table 9.3. Outline of production process for ice cream. Process stage Notes Mixing of base ingredients, e.g. milk, cream, sugar, eggs Ingredients are potentially contaminated Pasteurization or sterilization Temperature control, negative phosphatase test Homogenization Cooling to below 7.2°C within 1.5 h of heat treatment Temperatures of below 4°C will improve keeping quality Addition of flavourings and colourants Primary freezing to below 2.2°C Added ingredients may carry contamination Primary freezing to below 2. The temperature must remain below −2.2°C henceforth; if the temperature is allowed to rise, the product must be re-pasteurized Addition of fruits, nuts, cookies Added ingredients may carry contamination Package Ensure packaging is stored hygienically Hardening Cheese is produced by coagulating the caseins in milk, using rennet or other suitable enzymes, or by the development of lactic acid produced during bacterial fermentation. Commonly, cheesemaking involves a combination of enzyme action and bacterial fermentation. The curd produced is then modified using heat, pressure, special moulds, ripening ferments and seasoning to produce cheeses of many flavours and textures. Cheese may be classified based on the raw material used in production: for example ‘whole-milk cheese’, ‘skim-milk cheese’, ‘pasteurized cheese’ or, more commonly, as hard or soft cheeses. Hard and soft cheeses can be further sub-classified based on the ripening process (Table 9.4). Table 9.4. Examples of types of cheese. Cheese type Sub-type Examples Hard cheeses Without gas holes Cheddar Red Leicester With gas holes Emmental Jarlsberg Semi-hard cheeses Ripened by moulds Roquefort Stilton Ripened by bacteria Edam Gouda Soft cheeses Ripened by moulds Camembert Gorgonzola Ripened by bacteria Limburger Brie Unripened Cottage cheese Fromage frais Very hard cheeses would traditionally be made using partially skimmed milk, but at least 32% fat must be present to impart flavour to the cheese, whilst soft cheeses would traditionally be made with full-fat milk. However, in recent years, the use of skim milk in the production of soft cheese has resulted in the availability of numerous low-fat varieties. In cheese manufacture, a starter culture of selected microbes is added to the milk to assist the action of rennet, to expel whey due to acidification of the curd, to inhibit undesirable organisms, and to assist in curing of the cheese. In Cheddar cheese, the predominant organisms in the starter culture are Streptococcus lactis and Lactobacillus casei. In other cheeses, S. thermophilus and L. bulgari may be the organisms of choice, imparting a different flavour. Organisms that produce propionic acid as a by-product of fermentation are used to impart a particular flavour and for holes in certain cheeses. Rennin is added, and the caseins in the milk begin to form cross-links and coagulate. As the proteins coagulate, particles of curd are produced, and the acidification within these particles, caused by the starter culture organisms, pushes the whey out so that the curds shrink and become firm. Hard cheeses are traditionally made in large, open tanks, which contain up to 10,000 l of milk. As the curds develop the cheese is scalded, causing a marked contraction of the curd. During scalding, the curd is stirred or ‘raked’ to prevent it coalescing completely, and to allow the whey to be expelled. Cheddar cheese is scalded to 40°C, whilst Emmental cheese is scalded to 53–60°C, which alters the microbial population in the cheese and thereby alters the flavour and textural characteristics of the cheese. The scalding process takes around 35 min, after which the whey is drained off through a strainer. The curds are then piled onto the sides of the tank, where they form a uniform mass. This mass of curd is then repeatedly cut and turned to encourage further shrinkage and whey expulsion, a process known as ‘cheddaring’. In a hard, rennet cheese, ripening occurs uniformly throughout the curd, so large cheese wheels can be produced, and the cheese has good keeping quality. In soft cheese, where microbial growth and acid fermentation are important, the wheels must be small, as the microbial growth occurs on the surface, and the acid must diffuse into the centre of the cheese. If a soft cheese is over-ripened, abnormal flavours develop, and hence the shelf life of soft cheese is short. If blue-mould cheeses are made, the moulds (Penicillium roquefortii or P. glaucum) are added during preparation of the curd. It is vital to add S. lactis in the starter culture to produce sufficient lactic acid to control putrefactive spoilage organisms. The P. roquefortii breaks down the fats and proteins in the milk into simpler compounds, which gives the cheese its characteristic flavour. Salt is added to soft cheeses at 2.5–3.0% to restrict growth of spoilage organisms, and the cheeses are ripened at 95% relative humidity and 10°C for 60 days. Outlines of process controls for hard and soft cheeses are given in Tables 9.5 and 9.6. Fromage frais is an unripened soft cheese, which may or may not be produced using pasteurized milk. There is a heating stage in the process, but it is insufficient to kill pathogenic organisms, and the ripening phase is insufficiently short to allow acidification of the product to a level where microorganisms will be inhibited. Fromage frais is a short shelf life, potentially high-risk product. An outline of process controls for fromage frais is given in Table 9.7. Rennet, used in cheese manufacture and also in the manufacture of powdered milk puddings and desserts, contains the enzyme rennin, which is the active ingredient, causing the coagulation of casein in milk which results in the formation of an insoluble calcium complex. Natural Rennet is harvested from the stomach lining of unweaned calves at slaughter, as rennin is the main digestive enzyme in the calf stomach. Calf stomachs are harvested at slaughter, dried and finely ground. The resultant material is added to a vat of water and acid, and slowly stirred for a period of 24 hours. The mucin is separated from the ground stomachs and rises to the surface, where it is skimmed off and dried before packaging. Each batch of rennet produced undergoes laboratory analysis for rennin activity, to allow a standardized product to be marketed. Rennet itself has a rather low proteolytic activity, so is commonly combined with, or replaced by, other combinations of enzymes such as papain (harvested from the papaya tree), ficin (from figs), pancreatin or pepsin (other digestive enzymes), and commonly by synthetic enzymes. Table 9.5. Process control in hard cheese production. Process step Hazards Controls Standardization of cheese milk Raw milk may be contaminated with pathogens Check bacterial quality of raw milk; keep raw milk separate from pasteurized milk Pasteurization Failure to destroy pathogens Record time/temperature and flow rates; test milk – phosphatase test should be negative; clean and disinfect thoroughly between batches Addition of starter culture Slow acid development may allow growth of bacteria including pathogens Obtain starter cultures from a reliable source; check acid development Addition of rennet Rennet may be contaminated with pathogens Obtain rennet from reliable source Cutting Contamination from equipment Clean and disinfect thoroughly between batches Scalding at 40°C No incoming hazard, but note that this temperature is not sufficient to destroy pathogens Draining whey off; cheddaring at Recontamination Check pH to ensure that fermentation is proceeding normally Note that salt will assist in the suppression of bacterial growth Clean and disinfect thoroughly Ripening Outgrowth of contaminants Clean and disinfect thoroughly between batches; monitor environment and product Listeria monocytogenes is the organism most commonly associated with food-borne disease from cheeses, particularly from unpasteurized soft and unripened cheeses. L. monocytogenes is a soil-borne organism that can multiply at temperatures between 0 and 3°C, and can persist in the environment in dairy plants, acting as a source of recontamination post-pasteurization. The competent authority may set limits on Listeria contamination in cheeses; for example, in the UK, each batch of cheese other than hard cheese must be tested for L. monocytogenes five times, and each of these samples must be negative, whilst other milk-based products need be tested only once. Dairy products must also be tested and found negative for the presence of Salmonella, and there are maximum limits set for Staphylococcus aureus and Escherichia coli. For these two organisms, in the UK, a scale of acceptable, marginal and unacceptable has been set, and five samples are tested, of which at least three must comply with the acceptable standard, and all five must be below the unacceptable level. Table 9.6. Process control in soft, mould-ripened cheese production. Process step Hazards Controls Standardization of cheese milk Raw milk may be contaminated with pathogens Check bacterial quality of raw milk; keep raw milk separate from pasteurized milk Pasteurization Failure to destroy pathogens Record time/temperature and flow rates; test milk – phosphatase test should be negative; clean and disinfect thoroughly between batches Addition of starter culture Slow acid development may allow growth of bacteria, including pathogens Obtain starter cultures from a reliable source; check acid development Addition of rennet Rennet may be contaminated with pathogens Obtain rennet from reliable source Drain whey off; shape or mould curds; salting or brining Recontamination; note that salt will assist in the suppression of bacterial growth Check pH to ensure that fermentation is proceeding normally; clean and disinfect thoroughly between batches; monitor environment and product Ripening e.g. Outgrowth of contaminants Clean and disinfect thoroughly between batches; monitor environment and product Table 9.7. Outline of production process for fromage frais, an unripened soft cheese. Process stage Notes Thermization Heat to 57–68°C pending processing; does not destroy pathogens Separation to obtain skimmed milk Cream is a by-product Pasteurization This step may not occur in some processes Addition of rennin and starter culture Ripening for 16–24 hours at 18–25°C Produces pH of 4.5–4.6 Heat at 65°C for 4–5 min Contracts the curd and gives a denser texture; insufficient to destroy pathogens Centrifugal separation to remove whey Product should attain 13–20% solids Addition of sweet cream Aim is to increase fat content to 40% Cooling to 2–6°C Packaging This product must be marketed within 2–4 days Over a third of milk produced in the developed world is made into butter. The raw material for butter production is fresh cream, separated from milk using centrifugation. Cream may be pasteurized prior to butter-making, but even so, it must be used fresh. Holding cream for any length of time, even under refrigeration, can allow microbial growth of organisms present in raw cream, or re-contaminants in pasteurized cream. Also, heat-stable proteolytic or lipolytic enzymes that occur in cream and survive pasteurization can quickly give rise to off-flavours in the cream. The quality of the cream is very important in butter-making: poorly handled cream quickly develops lactic acid as a result of microbial fermentation, which, if not excessive, may be neutralized using calcium hydroxide. The cream will be assessed for acidity, flavour, odour and the presence of foreign particles. Cream and butter are very susceptible to taint, and strong odours in the milking parlour and bulk tank room on a farm can render cream and butter inedible, as can the presence of certain weeds in the forage consumed by cattle. Salt is added to the cream at around 10–13% to improve keeping quality. This level of salt inhibits microbial growth, particularly of yeasts and moulds, and also inhibits proteolytic and lipolytic organisms and enzymes. A starter culture comprising Streptococcus lactis (to aid preservation), S. citrovorus and/or S. paracitrovorus (to promote flavour) may be added, in which case the cream must be allowed to ripen for 3–4 h at 3°C to allow the acidity to build up to a level of 0.3–0.4% lactic acid prior to churning. Churning inverts the emulsion of the cream from an oil-in-water emulsion to a water-in-oil emulsion, the butterfat particles coalesce to form butter grains and the buttermilk is drained off. The butter grains are washed in potable water, then further salt may be added before the grains are pressed into butter moulds. If spreadable butter is desired, vegetable oils are added to the butter grains, and this mixture whipped to the desired consistency for packaging and sale. Butter, on average, comprises 82% fat, 14% water, 1% curd and 3% salt, and its low moisture content would be expected to inhibit microbial growth. However, its structure, being a water-in-oil emulsion, means that there is sufficient moisture and soluble nutrients distributed evenly throughout the product for microbial growth to occur between the fat globules. An outline of process control for butter is given in Table 9.8. Table 9.8. Outline of production process for butter. Process stage Notes Mixing of cream with salt and colourants Cream quality is very important; addition of 10–13% salt improves the keeping quality; a starter culture may be used to promote certain flavours – if so, the cream must ripen for 3–4 h at 3°C to acidify Churning Fat droplets in the cream coalesce, and butter grains break away from the liquid Washing of butter grains Use potable water Pressing and moulding to form hard butter, or addition of vegetable oils and whipping to produce spreadable butter More salt may be added Yoghurt (also spelled yogurt or yoghourt) is fermented milk, and contains over 1,000,000 cells/ml of both Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaris. Both these organisms are natural inhabitants of milk and it is their interaction during growth that gives yoghurt its unique organoleptic properties. Other organisms may be added during the production of yoghurt, particularly those that may be probiotic in nature, for example Bifidobacterium spp. and other Lactobacilli. The first step in the process of yoghurt making is to increase the solids non-fat content of the milk, by condensing or fortifying the raw milk. Traditionally this would have been achieved by heating the milk slowly on an open pan and allowing water to evaporate, but in modern yoghurt processing the milk is fortified by the addition of skim milk powder, whey or buttermilk powder, or condensed by evaporation of water under vacuum or removal of water by membrane filtration (ultrafiltration or reverse osmosis). In membrane filtration the membrane is a molecular sieve, through which milk is forced under pressure. Cells and large particles cannot pass through the membrane, so these are retained and water passes through the membrane and is drawn off. The condensed or fortified milk is next homogenized and heat treated at 90–95°C with a 2–5 min holding period. This treatment is greater than the treatment used for pasteurization of raw milk due to the increased solids content of the condensed or fortified milk. As well as effecting pasteurization, these temperatures promote the desired texture of yoghurt by linking β-lactoglobulin to caseins within the milk, break down the whey proteins into simpler compounds that will assist in fermentation later in the process, and also remove oxygen from the milk, which is desirable as the starter cultures used contain organisms that are micro-aerophilic. After heat treatment, the starter culture is added and fermentation begins. When a set yoghurt product is being produced, the mix is packaged at this stage, and the fermentation occurs in the pot, but for stirred yoghurts, the fermentation of the ‘base’ occurs in a large vat. Fermentation is allowed to proceed until the pH reaches 4.2–4.3, at a concentration of 1.2–1.4 g/100 ml of lactic acid. This will take around 12–16 h, but fermentation in bio-yoghurts is stopped after 4–5 h by chilling, at around 1.0 g/100 ml lactic acid, and the product is marketed. Fermentation is allowed to continue in mesophilic yoghurts for a further 7 to 10 h, as acid production plateaus over time. The development of the acidity is carefully monitored, either manually by titration of samples, or more commonly by measuring the electrical conductivity of the base, which increases as pH falls (Fig. 9.3). Once acidity exceeds 1.0 g/100 ml lactic acid, the caseins in the base coagulate, and the yoghurt sets. Over-acidification gives the yoghurt a sour taste, and the protein gel shrinks, expelling whey. This is often noticed in set yoghurts after purchase, as the fermentation will continue within the pot, and a domestic refrigerator often does not achieve temperatures required to inhibit fermentation, so whey is seen to develop on the surface of the yoghurt. In stirred yoghurt products, the base is agitated from pH 4.6 to break down the gel and give the desired texture. Slight over-acidification of stirred yoghurts is not the major problem that it is in set yoghurt, as added flavourings and fruit will mask the sourness.
9.1 Hygiene of Milk and Dairy Products
Milk
Milk-borne Disease
The Dairy Production Chain
Primary production
Hygiene of premises
Health and hygiene of animals
Processing of milk for human consumption
Cream and Ice Cream
Prevent contact with raw product
Thorough cleaning and disinfection between batches
Further Processing of Milk
Cheese
Rennet
pH 5.2–5.3;
milling and salting;
pressing;
wrapping
between batches; monitor
environment and product
Microbiological hazards in cheese and dairy products
10–14 days at
12–15°C, relative
humidity 90–95%;
storage at 4°C
Butter
Yoghurt
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