MICROBIOLOGICAL QUALITY AND SAFETY IN DAIRY INDUSTRY

Module 4. Microbiological risk profile and safety criteria for dairy products

Lesson 20
RISK PROFILE AND CRITERIA FOR MILK, FAT-RICH PRODUCTS AND CONCENTRATED AND DRIED MILKS

20.1 Introduction

In recent development consumers’ protection from food borne hazards has become a compelling duty for policy makers across the globe. The recent occurrence of serious food scares and food contamination events-such as Salmonella contagion in peanut butter, E.coli O104:H4 through seed sprouts in Germany, Listeria monocytogenes outbreak through melons in Colorado farm, USA, milk contamination with melamine in China, aerated drinks contamination with pesticide in India has raised food safety concerns and its impact on health, marketing and foreign trade. New serious chemical hazards have emerged in the food chain, such as natural toxicants like mycotoxins and marine toxins, environmental contaminants, such as mercury lead, and naturally occurring substances in plants. Although, traditional approaches have proved largely successful, risk assessment now also needs to take account of susceptible populations, combined with low level of exposure to several chemicals and effects on development of the fatal neural system. Food borne diseases have a significant impact not only on health but also on development. Moreover, globalization of the food trade and development of international food standards have raised awareness of the interaction between food safety and export potential for developing countries. With India being a member of the CAC, the Ministry of Health and Family Welfare, has the primary responsibility for determination of Government policy relating to food standards and enforcement of food control including national position on various issues relating to Codex. With the global food industry looking towards India as a food hot-spot, it is about time the national food legislation is aligned with Codex, encouraging innovation and facilitating trade without compromising consumer safety. The introduction of integrated food law provides the much required” one law-one –regulator” platform for raising the food safety standards of India to much global standards. Its speedy and effective implementation is quickly warranted to put India onto the global map. This would require an enabling implementation environment focused on creation of transparency, awareness creation, capacity building, certification of raw material & traceability system , developing right infrastructure, extensive R&D capacity and compliance of milk and milk products for FSSAI standards for Microbiological criteria for hygiene and safety indicators and presence of non-microbial contaminants: antibiotic residues, aflatoxin M1, pesticides, heavy metals etc., so as to match the dynamically changing requirements of food safety and standards. The initiatives would also require a wide spread awareness and promotion campaign focused on changing the mindset of food producers so as to encourage adherence to food safety standards.

20.2 Risk Profile and Criteria for Milk and Milk Products

20.2.1 Milk and cream

Milk is defined as mammary secretion of milking animals, obtained from one or more milking for consumption as liquid milk or for further processing but excludes colostrums. Milk refers to the fluid form of milk derived from cows, sheep, goats, buffaloes, camels, mares and other mammalian animals and available for human consumption through retail sale. Milk may be sold in many forms, including whole milk, skim milk, low-fat milk, flavoured milk and in other modified forms. Some of these products require the removal of the fat portion as cream. Cream is defined as “a milk product comparatively rich in fat, in the form of an emulsion of fat-in-skim milk, which can be obtained by separation from milk”. Cream is produced from whole milk by skimming or other separation means.

20.2.1.1 Microbial flora of milk

The microbial status of raw milk is influenced by various factors associated with milk production on farm. These factors impact on both the numbers of micro-organisms present in raw milk and the type of bacterial flora. Generally, few bacteria are present in milk drawn from the udder of a healthy animal, but bacteria may enter milk if it is drawn from an infected animal or if it is contaminated by unhygienic milking practices and poor milk handling. Main groups of microorganisms comprising the microflora of raw milk are shown in Table 20.1.

Table 20.1 Major microflora of raw milk

Various pathogenic micro-organisms may also be associated with raw milk. These include organisms shed by an infected animal (pathogens will predominate in milk from mastitis cows) or organisms that enter the milk from contaminated equipment and poor milking hygiene. Surveys of raw cow’s milk, mainly conducted overseas, have detected Aeromonas spp., B. cereus, Brucella spp., Campylobacter spp., Coxiella burnetii, pathogenic E. coli, L. monocytogenes, Mycobacterium spp., Salmonella spp., S. aureus, Streptococcus spp. and Y. enterocolitica. Pasteurization involves heat treatment of milk with the aim of ensuring a microbiologically safe product as well as to extend the shelf-life during refrigerated storage. Milk pasteurization may be carried out either as a batch holding heat treatment or a high-temperature-short-time (HTST) heat treatment. The batch process involves low-temperature-holding for 30 minutes or longer at temperatures of approximately 63°C. This has been largely replaced by HTST treatment at temperatures of ≥72°C for at least 15 seconds. The Code states: Milk must be pasteurized by

1. Heating to a temperature of no less than 72°C and retaining at such temperature for no less than 15 seconds and immediately shock cooling to a temperature of 4.5°C; or
2. Heating using any other time and temperature combination of equal or greater lethal effect on bacteria; where dairy products contain elevated levels of fat or solids, the specified temperature is increased to compensate for the protective effect of these fat and solids on microorganisms.

These specifications are sufficient to reduce populations of vegetative bacterial pathogens to a level considered safe for public health. The pasteurization process used by processors of milk often employs temperatures and times in excess of 72°C for 15 seconds. This is to provide a higher margin of safety and to extend the shelf-life of liquid milk. Pasteurization processes for cream products utilize higher temperatures because of the protective effects of fat on micro-organisms. Internationally recognized heat treatments for pasteurization of cream is 65°C for 30 minutes for cream with 10-20% fat, and 80°C for 15 seconds for cream with >20% fat. Thickened cream has thickeners such as alginates and/or carragenans added before pasteurization. Pathogens such as Salmonella, Campylobacter, Staphylococcus, pathogenic E. coli (particularly enterohaemorrhagic E. coli), Y. enterocolitica and Listeria monocytogenes which may be present in raw milk are inactivated by pasteurization. However, pasteurization will not destroy heat stable enterotoxins such as those produced by Staphylococcus aureus, if the organism has grown and produced enterotoxin in raw milk prior to pasteurization. Inadequate chilling of raw milk is one of the key factors for the build-up of Staphylococcus enterotoxins. Pasteurization also has the advantage of destroying many of the spoilage micro-organisms present in raw milk, especially psychrotrophic bacteria which may proliferate during low temperature storage of liquid milk products. Pasteurization, however, cannot be relied upon to destroy some of the more heat resistant bacteria (thermoduric) or bacterial spores produced by members of the genera Bacillus and Clostridium. After pasteurization, milk and milk products still contain low numbers of thermoduric microorganisms such as Micrococcus and Enterococcus species and some lactic acid bacteria. For this reason, pasteurized milk and milk products have a limited shelf-life even when stored at refrigeration temperatures.

To minimize growth of the surviving microbe, and to minimize post-process recontamination the steps of cooling pasteurized milk, filling and packaging and refrigerated storage of pasteurized milk and cream must be well managed. Pasteurized milk is particularly vulnerable to post-pasteurization contamination and asepsis and good hygiene is essential for preventing contamination by pathogenic micro-organisms and for defending its shelf-life.
The shelf life of milk is influenced by the number of psychrotrophic bacteria that survive pasteurization or subsequently contaminate the pasteurized product and grow at low temperatures in the liquid during storage. Although these contaminants are initially present in low numbers they can, under certain conditions, grow quickly and produce enzymes that break down protein and fat and generate off flavors and odours. The typical shelf life for pasteurized milk is from 7-14 days, although there are seasonal and regional variations.

UHT processing of milk involves heating milk at a temperature higher than 130°C with a holding period of 1-10 seconds with subsequent aseptic packaging. Usually the temperature and time combination is 138-145°C for 3-5 seconds. Sterilization treatment of milk is similar to that of UHT but at a higher temperature and is usually applied to condensed milk. The term ‘sterilization’, as used here, refers to commercial sterility of the milk or milk product. Milk and milk products of commercial sterility are not absolutely sterile in microbiological terms. However, those micro-organisms and spores that may survive the sterilization treatment are incapable of development under normal conditions of storage. Temperature and time combinations for the sterilization of milk and milk products range from 105-120°C for 10-40 minutes. Both UHT treatment and sterilization destroy bacterial endospores. Milk and milk products after UHT treatment or sterilization can be stored without refrigeration for extended periods of time.

20.2.1.2 Microbial pathogens of major concern in milk and cream

Available epidemiological data indicates that illness resulting from the consumption of pasteurized milk and cream is rare although outbreaks involving Campylobacter spp., Salmonella spp., E. coli O157:H7, L. monocytogenes and Yersinia spp. have been linked to consumption of pasteurized milk. These outbreaks have usually been traced to inadequate pasteurization and/or post-pasteurization contamination and/or temperature abuse and not to any failure of the pasteurization process.

Table 20.2 Microbiological criteria for milk and cream

20.2.2 Butter and butter products

Butter is produced from cream by churning or an equivalent process. Butter spreads are based on vegetable fats, a blend of vegetable and butter fat, or butterfat alone (light butter).

20.2.2.1 Microbial pathogens of major concern

While butter represents a dairy product of low risk to public health there have been incidents of food-borne illness attributed to this product. Staphylococcal food poisoning has been traced to whipped butter in the United States although temperature abuse was a contributory factor. There have also been two outbreaks of listeriosis linked to the consumption of butter. L. monocytogenes was isolated from several points in a production facility packaging small butter packages in an outbreak in Finland in 1998-1999. More recently a cluster of listeriosis cases implicating butter occurred in England. Mishandling may have been a contributing factor in this outbreak. In addition in the US, there have been several recalls issued for L. monocytogenes contaminated butter. Butter does not appear to be a good growth medium for L. monocytogenes as salt added during manufacture and distributed in the water phase is at or close to the limit for growth at refrigeration temperatures. However, growth has been demonstrated experimentally in butter during storage and it appears that L. monocytogenes favours the water rather than the lipid phase during butter making. This is supported by the outbreaks and recalls that have been associated with L. monocytogenes in butter.

Table 20.3 Microbiological criteria for pasteurized butter

20.2.3 Concentrated milk products

Concentrated milk products have reduced water content and include evaporated milks and sweetened condensed milks. Sweetened condensed milk is characterized by its high sugar content, which varies from 61-64% calculated as sucrose/ (sucrose + water) in the product. Unlike evaporated milk, which is preserved by heat treatment (UHT treatment or sterilization), sweetened condensed milk is preserved by its sugar content.

20.2.3.1 Microbial pathogens of major concern

No reported cases of food-borne disease outbreak have been attributed to the consumption of sweetened condensed milk or evaporated milk. These products generally do not support the growth of micro-organisms and are shelf stable. The main microbiological concern with evaporated milk is primarily non-pathogenic thermophilic spore-forming bacteria such as Bacillus stearothermophilus, which spoil the product. The main concern for sweetened condensed milks is S. aureus. Sweetened condensed milk is not a sterile product; the low water activity (aw between 0.83-.85) makes it unlikely to support the growth of pathogenic bacteria. Likewise, spores of Clostridium and Bacillus spp. present in sweetened condensed milk will also not be able to grow. The exception is S. aureus, which can grow at a aw of around 0.85. However vegetative cells of the non-spore former S. aureus will not survive the pre-heat treatment given to sweetened condensed milk and growth and toxin production of any spores is severely limited because of the anaerobic environment of sweetened condensed milk.

Table 20.4 Microbiological criteria for sweetened condensed milk

20.2.4 Dried milks

Whole milk, skim milk, whey, buttermilk, cheese and cream may be dried into powders by the application of heat. The fluid is initially concentrated by evaporation, then spray dried to form a powder.

20.2.4.1 Microbial pathogens of major concern

Microbial pathogens of major concern in dried milk include Salmonella, L. monocytogenes, B. cereus, C. perfringens, S. aureus and, more recently, Enterobacter sakazakii. While these organisms will not grow in powders, they may remain viable for long periods of time and resume growth when the powder is reconstituted and stored at favourable temperatures. Surveys conducted overseas have shown the presence of B. cereus and E. sakazakii (New name Chronobacter sakazakii) in dried milk, while Australian surveys have detected Salmonella spp. and S. aureus. Dried milk has been implicated in a number of food-borne disease outbreaks involving Salmonella and C. perfringens. In an outbreak due to consumption of milk powder contaminated with S. aureus, it was considered likely that illness was a result of preformed S. aureus enterotoxin surviving the heating process. Illness has also been attributed to S. aureus contamination and abuse of reconstituted non-fat dried milk. More recently a large outbreak of illness from S. aureus in Japan caused more than 13,000 cases and was due to preformed staphylococcal enterotoxin in the milk powder. This was traced back to poor hygienic and manufacturing practices during processing of liquid milk, in particular the storage conditions. Outbreaks demonstrate that failures in preventive systems, such as presence of water allowing multiplication or the presence of zones difficult to maintain and to clean (isolation from a drying tower) were the origin of contamination. In other cases illness has been due to contamination and abuse or reconstituted products. S. aureus, if present, may grow and produce enterotoxin when dry milk is rehydrated, if it is subject to time/ temperature abuse. C. perfringens and B. cereus are able to produce spores that can survive pasteurization and survive the manufacture of powdered milk production. They represent a problem when powdered milks are reconstituted and stored for prolonged periods at incorrect temperatures. Most B. cereus strains isolated from dairy products are able to grow and produce toxins below 10°C.

Although there have been no outbreaks of listeriosis linked to dry dairy products, the persistence of Listeria spp. in the dairy plant environment and the association of listeriosis with other dairy products indicate the potential for Listeria contamination of dry dairy products. There is evidence that L. monocytogenes can survive a typical spray-drying process in the manufacture of dried milk powders. Although dried milk powders will not support microbial growth due to their low water activity, L. monocytogenes is one of the few food-borne pathogens that can grow at refrigeration temperatures and, if present in the dried milk powders, it could possibly multiply when made up and stored in the refrigerator for a long period. Outbreaks due to Salmonella usually share a common factor, the accumulation of contaminated dust and powder deposits in the factory environment which are eventually transferred to the product by mechanical fault. The most common hazard reported is the accumulation of powder deposits in the drier insulation, which having become contaminated by environmental salmonellae, gains access to the product via stress cracks in the inner skin of the dryer. The second most important hazard is due to contaminated air and may occur during the secondary drier stages, transport of powder to silos or during filling and packing operations.

Table 20.5 Microbiological criteria for dried milks

20.2.5 Infant formulae

Powdered infant formula belongs to a special sub-set of powdered milks. These products are formulated to be as similar to human milk as is possible then concentrated and spray/or roller dried. In some cases, specific heat-labile ingredients are added after drying. Typically, infant formulae contain milk, whey proteins or soy proteins, or protein hydrolysate together with those forms of fat, carbohydrate, vitamins and minerals that are bio-available to the infant.

20.2.5.1 Microbial pathogens of major concern

Microbial pathogens of concern with powdered infant formulae are similar to those for dried milk powders, mainly B. cereus, C. perfringens, E. sakazakii L. monocytogenes, Salmonella, Shigella and S. aureus. However, control over the microbiological status of these products is essential because of the vulnerable status of infants. Global surveys of infant formulas have indicated the presence of B. cereus and E. sakazakii. While Salmonella is rarely found in surveys of powdered infant formula, low-level contamination of powdered infant formula with Salmonella has been epidemiologically and microbiologically associated with infections in infants. Illness has also been attributed to S. aureus contamination and to abuse of reconstituted infant powdered milk.
More recently a growing number of reports has linked E. sakazakii infection in infants to powdered infant formula. In several investigations outbreaks of E. sakazakii infection has occurred among neonates in neonatal intensive care units. Mortality rates from E. sakazakii infection have been reported to be as high as 50% or more, but this figure has declined to <20% in recent years (Codex). An outbreak involving Salmonella bredeney was traced to contamination of powdered milk-based infant formulae. Liquid, ready-to-feed infant formulae are commercially sterile, generally do not support the growth of micro-organisms and are shelf stable.

20.2.6 Colostrums

Bovine colostrum is the initial mammary secretion after the birth of a calf. It is produced for about 1-2 days (depleted usually within 4-5 days or 8-10 milking) and provides the newborn animal with a concentrated source of factors that boost its immune status and support physical and physiological development. Immediately post-partum the colostrum obtained from cows is excluded from bulk milk collection and was normally fed to farm animals. Until recently, it has not been widely commercially exploited although the high concentration of bioactive substances in colostrums has attracted increasing interest in the last few years because of their potential pharmaceutical and dietary uses. The sports food market is rapidly expanding, due to the perceived benefits of colostrum in providing an immune and performance boost to athletes. The use of colostrum as passive immune protection for humans has been reviewed recently. Important biologically active substances contained in colostrums include immune-globulins, leucocytes, lactoferrin, lysozyme, cytokines (interleukin (IL)-1β, IL-6. IL-10, tumour necrosis factor-α & granulocyte-, macrophage- and granulocyte/ macrophage colony- stimulating factors) and other hormones/ growth factors (e.g. insulin-like growth factors I and II). Some of the bioactive substances found in bovine colostrum provide specific (antibody) or non-specific (e.g. lactoferrin and lactoperoxidase) defences against infectious agents and foreign antigens.

20.2.6.1 Microflora of major concern

The microflora in powdered bovine colostrum is similar to that in other milk powder products and includes Salmonella, L. monocytogenes, B. cereus, C. perfringens and S. aureus. Post-pasteurization, colostrum may contain viable spores. Micro-organisms present in the dried product will also arise from post-processing contamination, and vegetative cells of pathogens might survive extended periods in the dried product although growth will not occur.

The final microbiological quality of colostrum powder will be influenced by the microbial load of the colostrum after milking, processing and the maintenance of good hygiene post- processing. S. aureus if present in the raw colostrum may grow and produce enterotoxin if the colostrum is subjected to temperature abuse prior to pasteurization. The persistence of Listeria spp. in the dairy plant environment and the association of listeriosis with other dairy products indicate the potential for contamination of dry dairy products such as colostrum powder. Both C. perfringens and B. cereus are able to produce spores that can survive pasteurization and even ultra-high temperature. If colostrum is inadequately stored when made up, the spores of C. perfringens and B. cereus can germinate and rapidly multiply, creating a potential health risk. The toxin of S. aureus is heat stable and, if poor sanitary conditions allow the organisms to proliferate and produce toxin in the pre-pasteurization stage, the toxin will carry over to the final product.