156 Modern Food Microbiology by the acetic acid producing strains is shown in Figure 7–1(F) and (G) Asaia, on the other hand, produces little or no acetic acid from ethanol, and its species not grow in the presence of 0.35% acetic acid.79 The three recognized species oxidize acetate and lactate to CO2 and water DAIRY PRODUCTS Milk Milk is used throughout the world as a human food in at least one form, and from at least one of a number of different mammals Bovine milk is typical of other milk types and it is the basis of the discussion that follows Many of the aspects of milk microbiology not covered below have been presented or reviewed by Frank17 and Murphy and Boor.50 Composition From the general chemical composition of cow’s milk in Table 7–2, some differences between this product and red meats in Table 4–9 are readily evident The protein content of milk is considerably lower (3.5 vs 18.0%) while the carbohydrate content is considerably higher (14.9 vs ca 1.0%) The higher structural protein content of red meats enables these products to exist as solids Although the average water content near the surface of fresh meats of ca 75.5% is lower than the average of 87% for milk, the aw of both products is near 1.0 The milk of goats and sheep is similar in composition to that of cows Milk protein consists mainly of casein, and it exists in several classes: α, β, etc If milk pH falls below 4.6, the casein precipitates Although casein represents 80–85% of total milk protein, when precipitation occurs, the liquid portion is referred to as whey The remaining proteins are found in whey and they include serum albumin, immunoglobulins, α-lactalbumin, etc Milk carbohydrate is principally lactose and its content is fairly consistent among breeds of milk cows at around 5.0% Although lactose is the main sugar, smaller quantities of glucose and citric acid exist The fat content varies between ca 3.5 and 5.0% depending upon cattle breed, and it consists mainly of triglycerides composed of C14 , C16 , C18 , and C18:1 fatty acids Smaller quantities of diglycerides and phospholipids occur Milk lipids exist largely in the form of fat globules that are surrounded by a phospholipid layer The ash content of around 0.7% consists of a relatively high level of Ca2+ and a lower level of Fe2+ Overall, the nonfat solids in cow’s milk average ca 9.0% while total solids range between 12.5 and 14.5%, and average ca 12.9% depending upon breed Table 7–2 Average Chemical Composition (%) of Whole Bovine Milk (Summarized from the Literature) Water Protein Fat Carbohydrate Ash 87.0 3.5 3.9 4.9 0.7 Milk, Fermentation, and Fermented and Nonfermented Dairy Products 157 The pH of fresh whole milk is around 6.6 but it may reach ca 6.8 from a cow that has mastitis Mastitis is an infection of the udder that is most often caused by Streptococcus agalactiae and S uberis but sometimes by Staphylococcus aureus or Streptococcus dysgalactiae Fresh milk from a mastitic cow typically contains leucocytes (white blood cells) >106 /ml in contrast to nonmastitic milk that contains leucocytes around 70,000/ml Milk contains a very adequate supply of B vitamins with pantothenic acid and riboflavin being the two most abundant Vitamins A and D are added for human consumption, and their presence has no known effect on the activity of microorganisms Overall, the chemical composition of whole cow’s milk makes it an ideal growth medium for heterotrophic microorganisms, including the nutritionally fastidious Gram-positive lactic acid bacteria How the milk microbiota utilize these constituents and bring about its spoilage is covered below under spoilage Processing Milk is processed in a number of ways to produce a variety of products such as cream, cheese, and butter Whole fresh milk is processed to produce a number of fluid products Skim milk (0.5% fat) or reduced fat milk (up to 2.0% fat) is produced by high-speed centrifugation following heating to ca 100◦ F to remove butter fat as cream, or by use of skim milk to which the desired fat content is added The latter is pasteurized either at 150–155◦ F (65.5–68.3◦ C) for 30 minutes or at 166–175◦ F (74.4–79.4◦ C) for 15 sec prior to cooling to around 4◦ C.78 Evaporated milk is produced by the removal of about 60% water from whole milk which results in the lactose content being about 11.5% Sweetened condensed milk is produced by the addition of sucrose or glucose before evaporation This leads to a product with a sugar content of about 54% or >64% in solution In the United States, grade A raw milk that is to be pasteurized should not have an APC that exceeds 300,000 cfu/ml for commingled or blended milk, or should not exceed 100,000/ml for milk from an individual producer After pasteurization, the APC should not exceed 20,000 cfu/ml, and the coliform count should not exceed 10/ml.15 Raw milk should not be held longer than days at 40◦ F (4.4◦ C) prior to pasteurization Chocolate milk is processed at a slightly higher temperature than unflavored milk (75◦ C for 15 sec rather than 72◦ C) A study of chocolate milk from four plants revealed that the APC was higher at 14 days post-processing than unflavored milk even though the initial numbers for both types were essentially the same.13 On day 14, 76.1% of unflavored and 91.6% of chocolate milk had APCs >20,000 cfu/ml with 26.1% of the former and 53.7% of the latter products having APCs >106 cfu/ml These investigators suggested that the chocolate flavor powder contributed to increased growth.13 The higher numbers were not due to higher numbers in the chocolate powder per se Pasteurization The objective of milk pasteurization is the destruction of all disease-causing microorganisms Endospores of pathogens such as Clostridium botulinum and spoilage organisms such as Clostridium tyrobutyricum, C sporogenes, or Bacillus cereus are not destroyed Although pathogens can be destroyed by nonthermal means, milk pasteurization is achieved solely by heating 158 Modern Food Microbiology The low temperature-long time (LTLT) method consists of heating the coolest part to 145◦ F (63◦ C) for 30 minutes This is referred to as the batch method The other more widely used method is the high temperature-short time (HTST) method, and it consists of heating to 161◦ F (72◦ C) for 15 sec This is the flash method, and it is inherently less destructive than the batch method The basis for the heating time and temperature is the thermal death time (TDT) of the most heat-resistant non-sporeforming milk-borne pathogens Prior to 1950, the LTLT method involved heating at 143◦ F for 30 minutes, which was the TDT of Mycobacterium tuberculosis However, after the discovery of the Q fever agent (Coxiella burnetti) and the determination of its presence in bovine, goat, and sheep milk, the LTLT method was changed so that it involved heating at 145◦ F for 30 minutes to correspond to the TDT of this pathogen In properly pasteurized milk, the naturally occurring enzyme alkaline phosphatase is destroyed.76 UHT (ultra-high temperature) is another thermal treatment that destroys non-sporeforming pathogens in milk, but in addition some sporeformers are severally reduced in numbers The UHT treatment is achieved by heating at temperatures of 275–284◦ F (135–140◦ C) for a few sec (the minimum treatment is 130◦ C for sec) UHT-treated milk is commercially sterile with a shelf life of 40–45 days at 40◦ F when aseptically packaged in sterile containers.7 UHT-treated whole milk is said to be more flavorful, due apparently to formation of some Maillard products Although pasteurized milk is free of non-sporeforming pathogens, it is not sterile The efficacy of either LTLT or HTST to destroy the mycobacterial subspecies that is associated with Crohn’s disease in humans has been called in to question, and this is discussed further below under milk-borne diseases Most if not all Gram-negative bacteria (especially psychrotrophs) are destroyed along with many Gram positives Thermoduric Gram positives belonging to the genera Enterococcus, Streptococcus (especially Streptococcus salivarius subsp thermophilus), Microbacterium, Lactobacillus, Mycobacterium, Corynebacterium, and most if not all sporeformers survive Among the survivors are a number of psychrotrophic species of the genus Bacillus.45 General Microbiota of Milk Theoretically, milk that is secreted to the udder of a healthy cow should be free of microorganisms However, freshly drawn milk is generally not free of microorganisms Numbers of several hundred to several thousand cfu/ml are often found in freshly drawn milk, and they represent the movement up the teat canal of some and the presence of others at the lower ends of teats Although the APC of milk from healthy cows is generally