Ice cream is a frozen foam. The continuous phase is a viscous syrup that makes up 18-20% of the volume at 0°C. The suspended phase consists of tiny air cells, ice crystals, fat globules, and colloidal substances (principally casein and stabilizing gums). These components occupy about 45, 25, 5, and 3% of the volume, respectively, when the overrun is 90%. Microorganisms are also suspended in the continuous phase. Their viability is mainly affected by the pH, osmotic pressure, and their abilities to withstand high concentrations of salts plus the physical forces of ice crystals.
Freezing results in concentration of dissolved substances in the syrup. Substances detrimental to microorganisms include acids, salts, and, for some bacteria, sugars. In general, the order of survival of microorganisms in frozen desserts, ranked from highest to lowest survivability, is (a) bacterial spores, (b) spores of molds and yeasts, (c) gram-positive bacteria, (d) vegetative cells of molds and yeasts, and (e) gram-negative bacteria. Microbial toxins are resistant to freezing.
Ice cream contains from approximately 34 to 44% total solids. The most abundant component is carbohydrates, especially sugars. A typical full-fat formula may include 12% sucrose and 6% lactose as well as approximately 2% glucose and maltose from corn sweeteners. (These monosaccharides and disac-charides are listed as sugars in current nutritional labeling practice.) Additionally, such a formula includes approximately 4% higher saccharides from hydrolyzed corn starch. These carbohydrates lower the freezing point of the mix to about -3°C (26.6°F). The characteristic mix also contains approximately 1% ash, which is made up of minerals, especially calcium, magnesium, and phosphorus.
As ice is frozen out of the continuous phase, dissolved substances become increasingly concentrated, and the freezing point of this phase decreases. As the amount of available water decreases, pH also decreases, especially in the highly acidic products, frozen yogurt, sherbet, and sorbet, and osmotic pressure and viscosity increase. If heat is steadily and continuously removed, the cryohydric point of the least-soluble substance is reached ultimately. At this temperature, this substance starts to precipitate, and latent heat of fusion is released. Therefore, the rate of decline in temperature is slowed until that substance is precipitated. There is a large number of substances in ice cream that may precipitate; therefore, during freezing, rates of temperature decline are not expected to be constant once eutectic points begin to be reached.
An unstable rate of decrease in temperature of ice cream being frozen is not expected to be a factor in survival of microorganisms, but formation of crystals and increasing concentrations of salts are likely to be detrimental. Salts tend to destabilize proteins and lipoproteins, and renaturation of them on thawing does not always occur. This is especially important for permeases that are located on the exterior of the cell. Sugars, however, may protect microorganisms from injury by freezing. Luyet (1962) suggested that microorganisms that best survive freezing are those that are able to dehydrate themselves most rapidly. Such cells are able to reduce the number of intracellular ice crystals that form, crystals that may puncture the cytoplasmic membrane.
Others have shown that cold-shock proteins are produced by some bacteria and that these have a protective effect against freezing. The temperature that stimulated production of the proteins varied with the bacterium: 4°C with psych-rotrophic Pseudomonas fluorescens KUIN-1 (Obata et al., 1998); 10°C with Lactococcus lactis ssp. cremoris (Broadbent and Lin, 1999) and with Salmonella Enteritidis (Jeffreys et al., 1998); 20°C with Streptococcus thermophilus CNRZ302 (100-fold increase in survival after four freeze-thaw cycles compared to mid-exponential phase cells grown at 42°C) (Wouters et al., 1999); 25°C with Lactobacillus acidophilus (Lorca, 1998).
L. monocytogenes, which is notably resistant to cold temperatures, contains an unusually high proportion of branched chain fatty acids (>85%). Furthermore, cells grown at 6°C contained about one-third more total lipid than did those grown at 30°C. Ratios of neutral lipids to phospholipids and of anteiso-15 to anteiso-17 fatty acids were considerably higher in the cells grown at the lower temperature (Mastronicolis, 1998).
Enzymes of bacteria able to grow in the cold have a relatively high turnover number and catalytic efficiency, but they suffer high thermosensitiviy. Their highly flexible structure enables them to undergo conformational changes during catalysis. The weak interactions involved in protein stability are either reduced in number or modified to provide this high flexibility (Feller and Gerday, 1997; Gerday et al., 1997). These characteristics make the enzymes more susceptible to heat denaturation, which is one of the reasons that psychrotrophic bacteria are readily destroyed by pasteurization.
The quantity of milk fat in ice cream ranges from less than 0.5% to more than 16%. Milk fat is an insulator in that it slows the rate of heat transfer through the frozen foam. Air cells, which may constitute up to one-half of the volume of ice cream, are also insulators. Both fat globules and air cells restrict growth of ice crystals. In so doing they reduce the amount of damage done to microbial cells by extracellular ice.
Colloidal substances that associate with water through hydration reduce the amount of water to be frozen, thus reducing the size and number of extracellular ice crystals. It is expected, therefore, that chances of survival of microorganisms are enhanced as increasing concentrations of colloidal substances are included and free water content is decreased in ice cream mixes. Furthermore, freezing causes cells to dehydrate, thus decreasing the amount of available water to form intracellular ice. Gases dissolved in the cytoplasm are lost. These events cause the viscosity of cellular matter to increase, thus slowing molecular interactions.
In frozen yogurt, the concentration of lactic acid is expected to vary from 0.1 to 0.2% of the total weight of the mix. As a percentage of the weight of the unfrozen aqueous phase at the temperature of storage of ice cream, -20°C, lactic acid may constitute 1-2%. Depending on the buffering capacity of the mix constituents, the pH in the microenvironment of the microbial cells of the ice cream may be detrimental to viability of the cells.
Raw milk and cream are likely to contain the following pathogens sporadically but consistently when milk is assembled from numerous farms to a single large facility: Campylobacter jejuni (and other campylobacteria), Salmonella Dublin
(and other salmonellae), Escherichia coli (at times including pathogenic strains), and L. monocytogenes. Animals used for food production are infrequent carriers of these bacterial pathogens and a few others.
Ryser and Marth (1991) summarized results of tests of raw milk in the United States, Canada, and Europe, finding 3.1, 2.7, and 4.1%, respectively, of the samples contaminated with L. monocytogenes. However, numbers commonly found in raw milk are seldom more than 10/mL. Sources of Listeria in raw milk include infected mammary glands, poorly fermented silage, and soil. This bacterium is generally considered to be transmitted by nonzoonotic means (Kozak et al., 1995).
Raw fluid milk and cream spoil relatively rapidly. In general, raw milk is delivered from producing farms to processors within 40-72 h of production and is not permitted to be held for more than 72 h in the receiving dairy before processing. Most manufacturers process raw milk much sooner than the maximal time the system permits. It is important to do so to minimize risks of spoilage by psychrotrophic bacteria, especially members of the genus Pseudomonas. These bacteria are prolific producers of hydrolytic enzymes, including proteinases (Mayerhofer et al., 1973), lipases (Christen and Marshall, 1983), phospholipases (Fox et al., 1976), and glycosidases (Marin and Marshall, 1983). Many of the proteinases, phospholipases, and lipases retain their activity after pasteurization. Some can be inactivated at the relatively low temperatures of 40-60°C (Marshall and Marstiller, 1981; Christen and Marshall, 1985).
Concentrated milks, commonly known as condensed milk and condensed skim milk, are widely used as ice cream ingredients. Concentrated milk products are almost always pasteurized before or during the concentration operation. Therefore, the incidence of microbial pathogens in these products is practically nil, and they have the microbiological keeping quality of pasteurized milk. Concentrated whey has similar characteristics. Bulk sweetened condensed milk and skim milk are prepared with sufficient sugar (approximately 42%) to prevent outgrowth of most spoilage bacteria. Furthermore, the evaporative process by which they are concentrated uses heat sufficient to destroy most vegetative forms of microorganisms. Therefore, they can be shipped and stored for limited periods without refrigeration.
Dry dairy ingredients include nonfat dry milk, dry buttermilk, dry whey, and whey protein concentrate. Processing commonly involves pasteurization, concentration, and drying. The heat of these processes kills most of the vegetative microorganisms; therefore, viable bacteria recoverable from them usually are mostly spore formers. Major advantages to the use of dried dairy ingredients are their storability and low weight per unit of solids. The latter factor reduces the cost of transportation, whereas the former provides maximal flexibility in use and helps balance supply with demand.
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