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

Lesson 21


21.1 Introduction

Food spoilage is an enormous economic problem worldwide. Through microbial activity alone, approximately one-fourth of the world’s food supply is lost. Milk is a highly nutritious food that serves as an excellent growth medium for a wide range of microorganisms. The microbiological quality of milk and dairy products is influenced by the initial flora of raw milk, the processing conditions, and post-heat treatment contamination. Undesirable microbes that can cause spoilage of dairy products include Gram-negative psychrotrophs, coliforms, lactic acid bacteria, yeasts, and moulds. In addition, various bacteria of public health concern such as Salmonella spp., Listeria monocytogenes, Campylobacter jejuni, Yersinia enterocolitica, pathogenic strains of Escherichia coli and enterotoxigenic strains of Staphylococcus aureus may also be found in milk and dairy products. For this reason, increased emphasis should be placed on the microbiological examination of milk and dairy foods. Microbiological analysis are critical for the assessment of quality and safety, conformation with standards and specifications, and regulatory compliance

The causative agent of an illness is usually determined through epidemiological studies, but confirming the identity of a key ingredient or the original source of product contamination, or critical factors contributing to their occurrence is problematic. This inability to attribute cases of food-borne illness to causal vehicles is a major issue internationally and is especially difficult where illness is linked to foods with multiple ingredients. Critical in this process is the capacity to link epidemiological data to animal and food monitoring data. The development of public health interventions requires accurate data defining the source from which humans are acquiring pathogens and how specific foods contribute to the total burden of food-borne illness. However, outbreak data represents only a small component of actual cases of food-borne illness, as many outbreaks go unrecognized. People do not always seek medical attention for mild forms of gastroenteritis and not all food-borne illnesses require notification to health authorities.

Following criteria in food matrix may be considered while characterizing the risk:
  1. Intrinsic properties of the product (i.e. the impact of aw, pH, salt concentration, and their effect on the growth of contaminating microorganism)
  2. Extent to which food is exposed to factory environment or handling after heat treatment
  3. Hygiene and control during distribution and retail sale
  4. Degree of reheating or cooking before consumption {many dairy products are RTE (Ready to Eat), so this is rarely a factor}.
21.2 Ice-Cream

Ice-cream is a frozen aerated emulsion made from cream or milk products or both and other food components. Manufacture of ice-cream involves the preparation of an ingredient mix comprising milk fat, milk solids, sweetener, water and other ingredients which are pasteurized and homogenized, aged, then whipped to incorporated air while being frozen. The final product is then packaging and hardened during frozen storage prior to distribution. Other types of ice-cream are available in many forms, flavors and packages. Different products prepared both from edible fats and milk or milk products include gelatin, soft serve, stick ice creams and confections, etc.

21.2.1 Microbial pathogens of major concern

While pathogenic bacteria will not grow in ice-cream, some pathogens, if present, may survive long periods of frozen storage. Therefore, any pathogens present in ice-cream as a result of post-process contamination may pose a potential hazard to consumers. The major microbial pathogens of concern associated with ice-cream are L. monocytogenes, Salmonella spp. and S. aureus. Reported outbreaks of food-borne illness attributed to ice-cream have typically involved home-made ice-cream where raw milk and/or raw egg were used and the heat treatment was inadequate. Outbreaks involving consumption of commercially manufactured ice-cream have been found to be the result of post-processing contamination. In the 1994 outbreak of S. enteritidis food- borne illness a transport tanker previously used to transport unpasteurized raw egg was used to transport ice-cream mix that was not subsequently re-pasteurized. Several recalls of ice-cream due to contamination of L. monocytogenes have occurred in the US since 1985. However no direct link to listeriosis has been documented. Chocolate-coated ice-creams were recalled in 1995 because of L. monocytogenes contamination. An outbreak of S. orangeburg associated with gelatin was reported in 1998. Contamination of the gelatin most likely resulted from equipment contaminated with the Salmonella. Gelatin differs from ice-cream in that it has a very low (dairy) fat content (varying from 1.4 -8%). Milk based gelatin also has less air incorporated (approximately 35-40%) compared to ice cream which is approximately 50%. Aeromonas spp. and B. abortus have been detected in surveys of ice cream and ice cream products overseas, while L. monocytogenes has been detected in surveys of ice cream in Australia.

21.3 Dairy Desserts

In recent years there has been rapid proliferation in the range of dairy-based desserts available in the marketplace. These are typically branded, ready-to-eat products that are sold through retail outlets such as from supermarket cabinets with products ranging from medium to long shelf-life. These products often include probiotic bacteria, fiber, vitamins, minerals, and include flavors and colors that appeal to children and adults. Dairy-based desserts include acidified and non-acidified products. Examples of these types of products include custards, puddings, sachet desserts etc. Difficulties with differentiation are increasingly common with a blurring of the lines which differentiate yoghurt (fermented) products, from cream desserts, and products containing probiotics. Dairy-based desserts can be based on fresh milk (skim or full-fat), milk powder (skim or whole) or on milk protein concentrates. Flavors, colours and sweeteners may be added, along with a wide variety of hydrocolloid thickening agents to improve texture, of which, starches and carragenans are most common. Other additives used include emulsifiers and binding agents.

21.3.1 Microflora of major concern

The microbiological profile of these products is extremely varied reflecting the nature of the ingredients incorporated into these products and variations in the preservation and processing operations employed in their manufacture. Published microbiological data on these types of products is limited. Survey data typically indicates that these products conform to national regulations and are of acceptable microbiological quality. In determining the potential pathogens associated with these products the typical microflora associated with milk and creams are combined with microflora originating from ingredients that vary from fruit to flavorings. Of particular concern is the survival of spores from B. cereus in the milk or presence there in ingredients such as thickeners.

The major risk in chilled dairy desserts is that they will become contaminated with pathogens which could grow during the products’ shelf-life. Components of this type of product such as cream and custards are by formulation (pH and aw) and method of manufacture (i.e. exposed to the factory environment) high risk. Custard and cream rely on proper heat treatment to eliminate pathogens that may be present in the raw materials used. Where these products are heat treated non-spore forming vegetative cells will be destroyed whereas spores of B. cereus may survive and become activated. The rapid cooling of products that have a heating step will help prevent growth of these spores. Another major public health concern can arise from post-pasteurization contamination, particularly from heat labile ingredients and during filling and packaging. Points in the process where product can become re-contaminated are during assembly of the final product. Items such as roasted nuts added as decorative toppings to desserts can also be a route of contamination. In addition, great care must be taken to avoid the addition of psychrotrophic bacteria such as L. monocytogenes which may grow during prolonged refrigerated storage. Another concern relates to spores of psychrotrophic B. cereus that may survive pasteurization and grow and elaborate toxin during the extended storage of some types of dairy desserts. Risks to the consumer from these products, if contaminated, are temperature abused and consumed at or beyond end of normal shelf-life.

Table 21.1 Microbiological criteria for ice cream, frozen dessert

t 21.2

21.4 Dairy-Based Dips

As with dairy desserts there has been an increase in the number of dairy-based dips in the marketplace. These products are very diverse and typically ready-to-eat commodities and are sold from cabinets in retail outlets. Dairy-based dips range from processed cheese-type products and starch-thickened bases flavoured with cheese solids to sour cream or yoghurt based and flavoured dips. A wide range of condiments can be added to the dairy dip base including herbs and spices, dehydrated vegetables and flavouring agents. These products range from medium to long shelf-life.

21.4.1 Microflora of major concern

The microbiological profile of these types of products is extremely varied reflecting the nature of the various components incorporated into these products and variations in the preservation and processing operations employed in their manufacture. Cold filling is frequently practiced and careful management is essential to avoid contamination. This is especially important where heat labile ingredients are added to the product after a terminal heat process. Published microbiological data on these types of products is limited. Survey data typically indicates these products conform to national regulations and is of acceptable microbiological quality. In determining the potential pathogens associated with these products the typical microflora associated with milk and cream is combined with microflora originating from ingredients that vary from vegetables and fruit to flavourings, herbs and spices. Where heat labile ingredients are added after a heat treatment steps, great care must be taken to avoid the addition of psychrotrophic bacteria such as L. monocytogenes.

21.5 Cultured and Fermented Milk Products

Yoghurt and fermented milk products are prepared by fermentation of milk or milk products using specific micro-organisms that reduce the pH and coagulate milk proteins. Yoghurt is characterized by fermentation with thermoduric S. salivariu subsp. thermophilus and L. delbrueckii subsp. bulgaricus with or without other lactic acid producing bacteria. Fermented milk products include yoghurt, cultured buttermilk, cream (sour cream) and acidophilus milk.

21.5.1 Microbial pathogens of major concern

Fermented products are rarely associated with food-borne disease as their pH is too low and the lactic acid concentration too high to permit growth of vegetative pathogens and death of non-growing cells is likely to be rapid. However, consumption of yoghurt containing large numbers of yeasts can lead to digestive disturbances. The limited outbreaks of food-borne illness that have been reported typically have involved S. aureus, C. botulinum and E. coli 0157:H7. Slow growth by the starter culture provides an opportunity for growth of pathogens that contaminate the milk or ingredients, for example, staphylococcal toxin may accumulate in the ingredients where too much sugar inhibited the growth of starters but not the growth of S. aureus, resulting in illness. In yoghurt outbreak, under processing of canned hazel-nut puree used to flavor the yoghurt caused growth and toxigenesis of C. botulinum spores in the puree. In addition the sugar in the ingredients was replaced by aspartame, leading to an increase in water activity to a level allowing growth of the pathogen. From a number of microbiological surveys of cultured and fermented milk products identified in the literature, only one reported the positive identification of a pathogen (Y. enterocolitica in fermented cow’s milk)

Table 21.2 Microbiological criteria for fermented milks


21.6 Cheese

The term cheese covers over 1,000 varieties of fermented dairy products with significant variations in their flavors, texture and appearance. The process of converting liquid milk into cheese involves a series of steps that are modified to produce a cheese of the desired characteristics. Starter culture and rennet are added to milk resulting in the production of a cheese curd through a process of coagulation and acidification. The curds are usually cut and with mild (38-43°C) heating there is separation of the whey which is drained from the curds. The curds are salted before they are pressed into moulds and then stored under controlled conditions to ripen the cheese. Cheese may be grouped according to manufacturing or processing procedures, consistency or rheology, country of origin, general appearance, sources of milk and chemical analysis.

21.6.1 Microbial pathogens of major concern in cheeses

Cheese has been the vehicle in a number of outbreaks of food-borne illness, involving pathogenic micro-organisms such as Bacillus spp., Brucella spp., C. botulinum, E. coli, L. monocytogenes, Salmonella, Shigella and S. aureus. Evidence from outbreak investigations suggests that illness resulting from consumption of cheese is often the result of faulty controls in cheese production; use of contaminated starter cultures or contaminated ingredients; post-pasteurization contamination; or mishandling during transportation and/or distribution. In microbiological surveys conducted overseas and a number of potential pathogens have been detected in cheeses made from pasteurized milk, namely L. monocytogenes and S. aureus. Additional pathogens have been detected in raw milk cheese (B, cereus, Brucella spp., pathogenic E. coli and Y. enterocolitica). Bacillus spp. and L. monocytogenes have been detected in pasteurized milk cheeses.

Table 21.3 Microbiological criteria for all other cheeses


21.7 Indigenous Dairy Products

21.7.1 Khoa

Khoa a partially dehydrated milk product is prepared from whole milk by continuous heating in an iron cauldron (‘karahi’) over a direct fire or in a steam kettle. It is consistently stirred and scraped with a spatula during heating till a semi-solid (doughy) consistency is obtained. The desirable consistency is achieved when the product shows sign of leaving the bottom and sides of karahi. After removing from the fire, the contents are worked up in a solid mass which is now called khoa or mawa. Depending on the method of preparation three varieties of khoa are available, viz. 1) pindi, 2) dhap and 3) danedar. These varieties differ in composition and texture and are made use of in preparing different sweets, viz., burfi and peda (from pindi); gulabjamun and pantoa (from dhap) and kalakand (from danedar). Microbiological quality of khoa

Khoa, like other indigenous products such as chhana, kheer, dahi, etc., can serve as a favourable medium for the growth of a variety of microorganisms because of high moisture content and good nutritive value. The market khoa keeps well for 48 hours under usual Indian conditions of handling and storage. However, storage beyond this period often results into deterioration due to microbial action. These microorganisms gain access into this product as contaminants from different sources. Various groups of bacteria (acid producers, proteolytic, chromogenic, lipolytic, aerobic sporeformers, psychrotrophs, thermophiles and pathogens), yeast and moulds grow profusely on khoa. The rapid spoilage of khoa is attributed to contamination with moulds from external sources. Pathogens of major concern

A number of pathogens like E. coli, S. typhi, S. dysenteriae and V. cholerae are able to survive for long periods during storage of khoa. Subsequently, a number of related studies have revealed the occurrence of staphylococci, especially those of coagulase positive types in khoa. The staphylococcus has been known to produce heat stable enterotoxin in this product which causes food poisoning. Since the product is manufactured by traditional method without any regard to quality of raw material used and hygienic storage, the shelf life of the product is adversely affected by the thermoduric organisms and organisms acquired during storage. High nutritional value and high water activity (aw = 0.96) of khoa is conducive to the growth of bacteria. Microbial content of heat dried dairy product is temperature dependent and time of preheating evaporation process is also an predisposing factor contamination and growth during storage also affect microbial population of heat dried dairy products. Psychrotrophic bacteria may also affect quality and flavour of heat-treated product, some heat tolerant enzymes produced by some psychrotrophic species cause spoilage both before and after heating. Microbes produce undesirable effects like change in odour, colour, taste and texture of food. Besides, this contamination of products with pathogenic bacteria can result into outbreaks of gastrointestinal infection and thus threat to consumer. A plethora of studies carried out in different part of India evidenced that pathogenic organism as B. cereus, S. aureus often contaminate khoa. Probably the microbe’s access to khoa is mainly by improper handling of workers and contaminated utensils used during processing. In a study total fifty samples of khoa were brought from different localities of Chambal region at random and processed. Bacterial colony counts were also performed. Staphylococcus species and Streptococcus species were the predominant isolates. The viable counts obtained ranged from 1.3×104 to 2.1×106 CFU/g. Contamination of khoa by pathogenic bacteria could be an important factor of gastrointestinal infections including food poisoning and food borne illness. E.coli was isolated from milk products like mawa/ khoa, cream, dahi, cheese, butter and gulabjamun.

In 2002, Soomro et al. observed that 12 (60%) of mawa/ khoa samples were contaminated with E. coli in contrast to 11 (55%) of dahi followed by 8 (40%) gulabjamun samples. Jtakar et al. (1982) studied on microbiological quality of market milk sweets in twin cities of Hyderabad and Secunderabad and observed 90% of peda, 75% of kalakand and 100% of rasagolla samples were contaminated with yeasts and moulds. Garg and Mandokhot (1984) observed that peda in general had more bacterial contamination than burfi contaminated with E. coli, P. aerogenes, S. flexneri, S. schottmuelleri and hemolytic streptococci. Tamberkar and Butda (2010) investigated 50 samples of peda randomly collected from various shops of Amravati city and analysed for bacteriological quality. Out of 92 strains of bacteria identified, the prominent were P. aerogenosa (23.91%), S. aureus (17.39%), S. typhi (16.30%), E. coli (14.13%), E. aerogenes (11.9%), Shigella flexneri (8.69%), Proteus vulgaris (7.6%), etc.

Table 21.4 Microbiological criteria for khoa


21.7.2 Paneer

Paneer is an important indigenous coagulated milk product prepared by the combined action of acid coagulation and heat treatment of cow or buffalo milk or a combination thereof (milk solids and suitably processed may be used). The phenomenon of precipitation involves the formation of large structural aggregates of proteins in which milk fat and other colloidal and soluble solids are entrapped along with whey. According to PFA Rules, 1955 paneer is a product obtained from cow or buffalo milk or a combination thereof by precipitation with sour milk, lactic acid or citric acid. It shall not contain more than 70% moisture and the milk fat content shall not be less than 50% of the dry matter. Skim milk is also recommended for the preparation of paneer where cow or buffalo milk is precipitated with sour milk, lactic acid or citric acid. The product shall not contain more than 70% moisture and milk fat content shall not be more than 13% of the dry matter. Microbial pathogens of major concern

Microbiological quality of Paneer, like other indigenous milk products, chiefly depends on the condition of manufacture, subsequent handling, storage and sale of the product. The possible sources of contamination might be air, water, utensils, cutting knife, muslin cloth as well as persons handling the product. Hence the number and types of microorganisms and their distribution in the product may vary depending on the location of the sweetmeat maker (halwai) shop, extent of exposure of the product to the atmosphere, temperature and period of storage, etc. Despite a higher final temperature (62°C) the duration of heating employed in the manufacture of Indian cheese (Paneer) was not sufficient to inactivate E. coli O157:H7. In a study 60 market dairy food samples, one sample each of raw milk, paneer and ice cream were found to be positive for E. coli O157:H7 with respective RT-PCR counts of 6.7, 6.2 and 5.9 log CFU respectively. Paneer is used in the preparation of certain curries and about 5% of the milk produced is converted to paneer. It may contain as high as 70% moisture which is conducive to microbial growth. Studies carried out on microbial quality of paneer have indicated that it is often contaminated with S. aureus and coliforms. The HACCP has been applied to identify the Critical Control Point for coliforms and Staphylococcus contamination.

21.7.3 Chhana

Chhana is one of the two chief bases (the other being khoa) for preparing a variety of indigenous sweetmeats. Chhana is also called paneer in certain parts of the country. However, the only difference between paneer and chhana preparation is that pressing is avoided in the latter which gives a loose texture to chhana unlike paneer. The PFA and ISI definitions of paneer also apply to chhana. Microflora of major concern

Chhana samples showed an average bacterial count of 1.6 ×104 per gram. However, during storage at 37°C, the count increased to 31×106 and 110 × 106 at the end of 24 and 48 hours, respectively. The spoilage of product was chiefly due to thermoduric bacteria. Among the bacterial types isolated from chhana micrococci predominated and constituted 45% of the total microflora followed by sporeformers (34%).

21.8 Casein, Whey Products and Other Functional Milk Derivatives

An increasing awareness of the nutritional and health benefits of dairy products has driven the development of markets for a wide array of functional and nutritional ingredients derived from milk. Improvements in fractionation technologies have allowed the manufacture of these on a commercial basis from surplus milk and other dairy by-products.

21.8.1 Microflora of major concern

Pathogens of concern in the production of skim milk and skim milk powder include Salmonella, L. monocytogenes, B. cereus, C. perfringens and E. sakazakii. Casein and whey products produced from skim milk might contain spores of bacilli and clostridia and cells of other pathogens might survive extended periods in the dried products if present, although growth will not occur. A microbiological survey of dairy products conducted overseas detected B. cereus in whey powder. Products formed from severely temperature-abused milk might contain S. aureus enterotoxin, which is exceptionally heat stable, but this is unlikely to occur in a well-regulated processing environment. Fat-enriched milk fractions e.g. anhydrous milk-fat (AMF) may protect pathogenic microorganisms such as E. coli, Salmonella or Listeria, if present. However this is unlikely given the low moisture content of the product.

Table 21.5 Microbiological criteria for paneer/ chhana


Last modified: Wednesday, 7 November 2012, 4:16 AM