262 ◾ Fundamental Food Microbiology
surfaces. Production of rennin-like enzymes by psychrotrophic Bacillus spp. and others can cause
sweet curdling of milk at a pH higher than that required for acid curdling. 7
Ultrahigh temperature-treated (UHT) milk (150°C for a few seconds) is essentially a commercially sterile product that can only contain viable spores of some thermophilic bacteria. The UHT
milk is not susceptible to spoilage at ambient storage temperatures but can be spoiled if exposed to
high temperatures (40°C or above as observed with canned foods).
Concentrated Liquid Products
Evaporated milk, condensed milk, and sweetened condensed milk are the principal types of concentrated dairy products susceptible to limited microbial spoilage during storage. All these products are subjected to sufficient heat treatments to kill vegetative microorganisms as well as spores
of molds and some bacteria.
Evaporated milk is condensed whole milk with 7.5% milk fat and 25% total solids. It is packaged
in hermetically sealed cans and heated to obtain commercial sterility. Under proper processing conditions, only thermophilic spores of spoilage bacteria can survive, and exposure to high storage temperatures (43°C or higher) can trigger their germination and subsequent growth. Under such conditions,
Bacillus species, such as Bac. coagulans, can cause coagulation of milk (flakes, clots, or a solid curd).
Condensed milk is generally condensed whole milk and has 10%–12% fat and 36% total
solids. The milk is initially given a low-heat treatment, close to pasteurization temperature, and
then subjected to evaporation under partial vacuum (at ca. 50°C). Thus, it can have thermoduric
microorganisms that subsequently can grow and cause spoilage. Other microorganisms can also
get into the product during the condensing process. Even at refrigerated temperatures, this product has a limited shelf life, as does pasteurized milk.
Sweetened condensed milk contains approximately 8.5% fat, 28% total solids, and 42%
sucrose. The whole milk is initially heated to a high temperature (80°C–100°C) and then condensed at approximately 60°C under vacuum and put into containers. Because of a low A W , it
is susceptible to spoilage from the growth of osmophilic yeasts (such as Torula spp.), causing gas
formation. If the containers have enough headspace and oxygen, molds (e.g., Penicillium and
Aspergillus) can grow on the surface.
Butter
Butter contains 80% milk fat and can be salted or unsalted. The microbiological quality of butter depends on the quality of cream and the sanitary conditions used in the processing. Growth
of bacteria (Pseudomonas spp.), yeasts (Candida spp.), and molds (Geotrichum candidum) on the
surface causes flavor defects (putrid, rancid, or fishy) and surface discoloration. In unsalted butter, coliforms, Enterococcus, and Pseudomonas can grow favorably in the water phase (which has
nutrients from milk) and produce flavor defects.
Vegetables and Fruits
Vegetables
Fresh vegetables contain microorganisms coming from soil, water, air, and other environmental
sources, and can include some plant pathogens. They are fairly rich in carbohydrates (5% or more),
surfaces. Production of rennin-like enzymes by psychrotrophic Bacillus spp. and others can cause
sweet curdling of milk at a pH higher than that required for acid curdling. 7
Ultrahigh temperature-treated (UHT) milk (150°C for a few seconds) is essentially a commercially sterile product that can only contain viable spores of some thermophilic bacteria. The UHT
milk is not susceptible to spoilage at ambient storage temperatures but can be spoiled if exposed to
high temperatures (40°C or above as observed with canned foods).
Concentrated Liquid Products
Evaporated milk, condensed milk, and sweetened condensed milk are the principal types of concentrated dairy products susceptible to limited microbial spoilage during storage. All these products are subjected to sufficient heat treatments to kill vegetative microorganisms as well as spores
of molds and some bacteria.
Evaporated milk is condensed whole milk with 7.5% milk fat and 25% total solids. It is packaged
in hermetically sealed cans and heated to obtain commercial sterility. Under proper processing conditions, only thermophilic spores of spoilage bacteria can survive, and exposure to high storage temperatures (43°C or higher) can trigger their germination and subsequent growth. Under such conditions,
Bacillus species, such as Bac. coagulans, can cause coagulation of milk (flakes, clots, or a solid curd).
Condensed milk is generally condensed whole milk and has 10%–12% fat and 36% total
solids. The milk is initially given a low-heat treatment, close to pasteurization temperature, and
then subjected to evaporation under partial vacuum (at ca. 50°C). Thus, it can have thermoduric
microorganisms that subsequently can grow and cause spoilage. Other microorganisms can also
get into the product during the condensing process. Even at refrigerated temperatures, this product has a limited shelf life, as does pasteurized milk.
Sweetened condensed milk contains approximately 8.5% fat, 28% total solids, and 42%
sucrose. The whole milk is initially heated to a high temperature (80°C–100°C) and then condensed at approximately 60°C under vacuum and put into containers. Because of a low A W , it
is susceptible to spoilage from the growth of osmophilic yeasts (such as Torula spp.), causing gas
formation. If the containers have enough headspace and oxygen, molds (e.g., Penicillium and
Aspergillus) can grow on the surface.
Butter
Butter contains 80% milk fat and can be salted or unsalted. The microbiological quality of butter depends on the quality of cream and the sanitary conditions used in the processing. Growth
of bacteria (Pseudomonas spp.), yeasts (Candida spp.), and molds (Geotrichum candidum) on the
surface causes flavor defects (putrid, rancid, or fishy) and surface discoloration. In unsalted butter, coliforms, Enterococcus, and Pseudomonas can grow favorably in the water phase (which has
nutrients from milk) and produce flavor defects.
Vegetables and Fruits
Vegetables
Fresh vegetables contain microorganisms coming from soil, water, air, and other environmental
sources, and can include some plant pathogens. They are fairly rich in carbohydrates (5% or more),