MILK AND MILK products

PASTEURIZED MILK

Pasteurized milk has an initial microflora that consists primarily of thermoduric bacteria and spores such as Bacillus , Micrococcus, Lactobacillus, Microbacterium, Corynebacterium, Streptococcus, Enterococcus, and Arthrobacter spp. Large numbers of these microorganisms in the raw milk supply can contribute significantly to the SPC of pasteurized products. Most thermoduric bacteria grow slowly in refrigerated milk and are outgrown by gram-negative psychrotrophic bacteria (Pseudomonas, Flavobacterium, and Alcaligenes spp. as well as some members of the coliform group) that contaminate pasteurized milk after pasteurization. In the absence of psychrotrophic bacteria or if large number of thermoduric bacteria survive pasteurization, certain thermodurics, particularly psychrotrophic sporeforming Bacillus spp., can grow and cause spoilage (e.g. sweet-curdling). Spoilage by gram-negative bacteria results in fruity, rancid, bitter, and unclean flavors. Generally, populations in excess of 106 per mL are required before flavor defects are detectable. For maximum shelf life, refrigerated storage should be below 4ºC.

Bacteria related to foodborne illness are destroyed by proper pastaurization. Recent outbreaks of salmonellosis and listeriosis in pasteurized milk have been linked to post-pasteurization contamination. Post-pasteurization contamination with L. monocytogenes and Yersinia enterocolitica is of major concern to the dairy industry since these organisms grow at refrigeration temperatures.

Recommended tests

• Aerobic plate count

• Total coliform count

Additional tests

• Psychrotrophic count

• Moseley Keeping Quality test

• Salmonella

• Listeria

• Campylobacter

• Yersinia

DRIED PRODUCTS

Dried products are often used as ingredients of other foods and are subject to further processing. Yet, dried milks are considered sensitive products from a public health aspect because they are often consumed after reconstitution without additional heating. It is well known that dried milk can be a source of foodborne illness because of contamination with Salmonella (environmental contamination) and Staphylococcus intoxication (due to growth of this microorganism and toxin production prior to the heating processing). Because of their low water activity, dried products rarely spoil or deteriorate because of microbial growth.Contamination of the powder can occur at many places — in the spray drier, during fluid bed drying, and during packaging. The species of contaminating bacteria can vary widely, but it usually concerns species that can grow in wet remnants of milk powder in the drier or in the surroundings of the manufacturing line. Contamination via (in) direct human contacts should also be considered (e.g., S . aureus ). Bacteria can easily survive in dry powder, and undesirable bacteria can start to grow if the water content increases to over 20%. The supply of cooling air into the spray drier and into the fluid bed drier can be a source of direct contamination. It may also be responsible for indirect contamination because it gives, at certain sites, better conditions for survival and growth of bacteria in remnants of not fully dried powder. Special precautions are needed if the drier and its accessories have been wet-cleaned. To restrict such incidental contamination, the plant and its surroundings should be rigorously freed of remnants of (wet) powder. Improper storage of dry milk will promote the growth of molds. Molds can cause discoloration, musty flavors, breakdown of milk components resulting in off-flavors, and possible production of mycotoxins.

Recommended tests

• Aerobic plate count.

• Total coliform count. east and mold count.

• Thermoduric count

• Psychrotrophic count

• Salmonella

• Staphylococal enterotoxins

• Aerobic spore count

butter

Butter must contain at least 80% fat. It can be salted (1.75-2% salt that can result in a salt-in-water concentration as high as 16%) or unsalted, it may or may not contain added starter cultures for additional flavor. The microbiota of butter reflects the quality of the cream,the sanitary conditions of the equipment used to manufacture the butter, and the environmental and sanitary conditions during packaging and handling. Pasteurization of cream causes a significant reduction in the number of all but the most heat-resistant bacteria. Palatability problems caused by bacteria include rancid, putrid, malty, and fishy tastes. Yeast and molds can grow on the surface causing discolorations and flavor problems. Butter has been implicated in one outbreak caused by Staphylococcus.

recommended tests

• Aerobic plate count

• Total coliform count

• Lipolytic bacteria count

• Proteolytic bacteria count

• Yeast and mold count

• Psychrotrophic count

• Staphylococcus aureus count

ice cream and other frozen dairy products

The microbial quality of frozen dairy products largely reflect the quality of the ingredients. Milk, cream and the soluble components are usually pasteurized. Sporeformers and some hardier thermoduric bacteria are the only survivors. Flavors, coloring agents and ingredients such as fruits, nuts, and chocolate chips added to the mix after pasteurization can be a source of contamination. In addition, post-pasteurization contamination can occur from poorly cleaned equipment, air, product rework, the processing environment and personnel. Although there is no growth in frozen dairy products, many bacteria (including pathogens such as Listeria monocytogenes, if present) can survive. Salmonella contamination can occur at the retail level by the improper handling and storage of the utensils, Staphylococcal intoxications have also been reported.

recommended tests

• Aerobic plate count

• Total coliform count

• Thermoduric count

• Yeast and molds counts

• ingredient testing

• pathogen testing (Salmonella, L. monocytogenes, Campylobacter, and Yersinia).

concentrated dairy products

Concentrated milk products include evaporated milk, concentrated milk, sweetened condensed milk, and, to a limited extent, condensed sour products. These products are pasteurized and concentrated and are shelf stable at room temperature. They are concentrated at a low temperature and pressure. The reduced temperature can provide an opportunity for the growth of thermoduric and thermophilic bacteria unless the pH is too low. Mold can grow and increase the pH to allow bacteria to grow and spoil the product. Sweetened milk has 28% milk solids and 8.5% milkfat. The presence of a high amount of sugar in this product (42-43% sucrose) prevents bacteria spoilage. It has a low water activity of around 0.83. The main spoilage type of this product is caused by the osmophilic (xerophilic) sucrose-fermenting yeasts such as Starmerella lactis-condensi (syn. Torula lactis-condensi) and some molds especially Aspergillus species with a eurotium-morph such as Aspergillus repens and A. glaucus growing on the surface of the product (mold button formation). At BCN Labs, we have isolated the yeast Starmerella lactis-condensi from liquid sucrose on a regular basis. It ferments sucrose and produces gas. The packaging machine should fill the cans very accurately with a safety margin of 1 g. Too little condensed milk in the cans means that more air is left, which increases the chance of growth of molds and micrococci (with time, they can produce bacterial age thickening). If the cans are overfilled, the milk may spill over the side and encourage growth of osmophilic yeasts.

recommended tests

• Aerobic plate count

• Total coliform count

• Yeast and mold count

• Osmophilic (xerophilic) mold count

• Thermoduric count

• Thermophilic count

cheese

Cheeses can be fresh or ripened. Ripened are classified by their moisture content as soft surface-ripened, semi-soft, hard or hard-grating cheese. The microbiota of the raw or heated milk is rapidly overshadowed by the activity of the starter microorganism(s). The primary action of the starter culture is to produce lactic acid, which promotes curd formation in conjunction with coagulating enzymes such as rennet, lowering the redox potential and reduction or prevention of the growth of pathogens and spoilage microorganisms. They also contribute to flavor of the product. Ripening under controlled conditions of temperature and humidity determines the final flavor and texture of the product. Microbial spoilage is limited because of the combined effect of acid and salt, and it is less likely in the lower moisture cheeses. Ripened cheese is prone to surface growth of yeasts and molds, particularly if exposed to atmospheric oxygen.A slow starter culture can allor growth of bacteria related to foodborne illnesses such as Staphylococcus, Salmonella, Listeria and enteropathogenic E. coli.

recommended tests

• Yeast and mold count

• Total coliforms count

• Psychrotrophic counts — refrigerated cheeses such as cottage or ricotta cheese

• Staphylococcus enterotoxin/thermonuclease

• Pathogens: Salmonella, Campylobacter, Listeria monocytogenes, and Yersinia.

yogurt and fermented milks

Yogurt, fermented milk (buttermilk, kefir). and cultured cream (sour cream) are unripened, cultured dairy products. The low pH (< 4.5) prevents the growth of most spoilage and pathogenic organisms. Microorganisms that cause deterioration of these products can enter the product through poor sanitation or can be introduced by the addition of flavoring materials such as fruits, nuts, cereals, and other flavoring materials. Yeast and molds are the most predominant microorganisms involved in their spoilage. The spoilage of yogurts by yeasts is generally recognized by the development of yeasty off-flavors, loss of texture quality due to gas production, and the swelling and eventual blowing off of the product container. When produced by "good manufacturing practice," yogurts should contain no greater than 1 yeast cell per g and, if correctly stored under refrigeration (5WC), a product shelf life of 3 to 4 weeks may be expected (Davis, 1970). The introduction of sugar and fruit into yogurts makes yogurts a less selective growth environment, and such yogurts are likely to support the growth of a wider variety of yeast species. Inadequate refrigeration leads to rapid growth of yeasts in yogurts (Suriyarachchi and Fleet, 1981). Routine monitoring of yeast levels in the fruit ingredients and the final product is an important aspect of yogurt quality control. Coliforms if present, decline rapidly in yogurt but can survive in cultured buttermilk and sour cream. Enterococci are probably better indicators of sanitation than coliforms because coliforms are very sensitive to the high acid conditions in fermented products and may not be recovered on selective media due to acid injury. Since they are cultured products, a total aerobic count is insignificant except in products that are heated to inactivate added cultures.

recommended tests

• Aerobic plate count — only in heated products.

• Yeast and mold count

• Total coliforms count or enterococci counts

• Psychrotrophic counts

ADAPTED FROM

Richter, R. L. and Vedamuthu, E.R. 2015. Milk and Milk Products, chapter 47, p. 483. In F. P. Downes and K. Ito (eds.), Compendium of Methods for the Microbiological Examination of Foods. American Public Health Association, Washington, DC.

other references

Davis, J. G. 1970. Laboratory control of yogurt. Dairy Ind. 35:139-144.

Suriyarachchi, V. R. and Fleet, G. H. 1981. Occurrence and Growth of Yeasts in Yogurts, Appl. Environ. Microbiol. 42:574-579.