1. Crystalline and Granular Sweeteners and Bulking Agents
Sucrose, dextrose, and fructose are available in both crystalline and syrup forms. Few microorganisms are contained in crystalline sweeteners. Maltodextrins, polydextrose, and corn syrup solids are available in granular form. Some of these materials may carry viable microorganisms, usually yeasts. Bottler's standards for 10-g samples of granulated sugars are less than 200 mesophils, 10 yeasts, and 10 molds (National Soft Drink Association, 1975).
In addition to sucrose, dextrose, and fructose, corn sweeteners are available as syrups. Because syrups contain water and provide energy, they may support growth of osmophilic fungi. These microorganisms, usually being highly aerobic, grow on surfaces. They can be killed by exposure to ultraviolet light and their growth can be inhibited by sealing full containers in which they are packed. This is not practicable when the container is a tank into which air must be admitted to displace syrup as it is drawn out during use. For them to flow steadily, syrups must be kept warm in pipelines that are used to transfer the sweetener to the batching tank for making mixes. Therefore, it is critical that the concentration of solids in the syrup be so high as to inhibit growth of the most osmophilic yeasts that might be contained. The usual solids concentration of these syrups is 71-82%, making the water activity (aw) approximately 0.80. Syrups with a high dextrose equivalent (DE) are significantly more microbiologically stable than those with a low DE; for example, 62 DE versus 36 DE. The sugar concentration, measured in Brix, ranges from 67 to 86°, depending on the sweetener. Smaller sugar molecules exert greater osmotic pressure than larger ones given the same weight concentration. Therefore, concentrations of glucose, fructose, sucrose, and maltose necessary to limit microbial growth are lower than for corn syrups, which contain polymers of glucose that are products of incomplete hydrolysis of starch. High-fructose corn syrups of 42 and 55% have aw values of 0.75 and 0.68, respectively (L. True, personal communication, 1997).
Osmotolerant yeasts can grow at aw of less than 0.85. Even syrups with an aw as low as 0.65 have been found to support growth of osmophilic yeasts (Troller, 1979). Most of these are in the genus Zygosaccharomyces (Walker and Ayres, 1970). Other genera of yeasts reportedly found are Candida, Pichia, Schizosac-charomyces, and Torula.
Condensate formation in syrup storage tanks raises the aw and gives fungi opportunities to grow. Condensate accumulation can be prevented by forcing filtered and ultraviolet-treated air over the surface of the syrup.
In the preparation of corn syrups, the steps of steeping, wet milling, washing, purifying, and drying have a potential effect on microbial growth and survival. During steeping, corn is soaked in water at 45-50°C for 48 h at a pH of approximately 4 (Whistler and Paschall, 1967). During this period, the mixture is susceptible to growth of microorganisms that produce alcohols and butyric acid. A common microbial inhibitor added during steeping is sulfur dioxide (0.10.2%).
Typical manufacturer's maximal standards for microorganisms in syrups follow: aerobic plate count, 100/g; yeasts, 20/g; molds 20/g; E. coli, none in 30 g; and Salmonella, none in 100 g.
Honey is sometimes used in frozen desserts in the dual role of sweetener and flavoring agent. A typical concentration of honey in honey-flavored ice cream is 9%. Yeasts are likely contaminants of honey, because flowers from which the nectar is derived are the habitat of yeasts. Several species of Zygosaccharomyces have been isolated from defect-free and fermented honeys (Walker and Ayres, 1970). Because of its high hygroscopicity and viscosity, unprotected honey tends to develop areas (gradients) in which the aw is high enough to permit yeast growth.
Pure synthetic or natural flavorings vary widely in content of microorganisms. Flavorings that are heat sensitive cannot be given a lethal heat treatment. Those that are low in viscosity and contain no suspended matter can be filter-sterilized. Some are naturally antagonistic to microbial growth, especially those that have an alcohol base. Most are used in such small quantities that their contribution to the bacterial load is insignificant. Most are added after pasteurization, making it critical that they contain no pathogens.
Alcohol is used to extract flavorful substances, such as vanilla, that are used to add flavor to frozen desserts. Pure vanilla is required to contain at least 35% ethanol to be labeled vanilla extract. This concentration of alcohol is sufficient to dehydrate and destroy most vegetative microbial cells. Other extractants include ethylene and propylene glycols.
Cacao beans are fermented before being ground and pressed to separate some of the cocoa butter from the cocoa. Grinding alone produces chocolate liquor, whereas pressing and grinding yields cocoa and cocoa butter. The latter contains only minor flavor notes, whereas the chocolate flavor is carried in the cocoa. Cocoa powders contain from 10 to 24% cocoa butter (fat) unless they have been extracted with a solvent. The microflora of uncontaminated cocoa and chocolate liquor consists nearly exclusively of bacterial spores and numbers are usually less than 100/g.
Fruit ice creams represent approximately 15% of the total market. 1. Fresh and Frozen
Frozen fruits, especially berries, have been widely used in the frozen desserts industry for many years. Freezing tends to disrupt the structure and destroy the turgidity of fruits. On thawing, fruits become soft, juices escape from the cells, and color fades.
Because of the relatively low pH of fruits, the microflora of fresh and frozen fruits is dominated by yeasts, including the genera Saccharomyces and Crypto-coccus, and by molds, including species of Alternaria, Aspergillus, Botrytis, Fu-sarium, Geotrichum, Mucor, Penicillium, and Rhizopus. Small numbers of soil-borne bacteria are present also, including species of Bacillus, Pseudomonas, and Achromobacter. These bacteria do not compete well with the fungi in the pH range common to fruits. However, some lactic acid bacteria as well as species of Acetobacter, Gluconobacter, and Zymomonas may develop in the acidic environment of the fruit processing plant.
A principal source of pathogens in fresh and frozen fruits is persons who pick and handle them. Insects also may contaminate fruits. Peeling, washing, and blanching are processes that lower numbers of microorganisms on raw fruits. A major recall of frozen strawberries was initiated in March 1997 when they were associated with an outbreak of hepatitis A in Michigan. The recall was extended to frozen strawberry fruit bars and to strawberry ice cream containing the same pack of berries that originated in California (FDA Enforcement Reports, 1997).
Freezing kills some microorganisms on fruits but is not a dependable lethal process. Furthermore, it is not feasible from a quality viewpoint to blanch most fruits (except peaches) to destroy microorganisms. However, bactericidal chemicals, such as hypochlorite, may be added to wash water to reduce numbers of microorganisms on surfaces. Antioxidant dips are frequently applied to minimize browning. These include ascorbic acid, sulfur dioxide, and sugar syrup. Sulfur dioxide has some antimicrobial effect, and syrups may kill organisms that are susceptible to high osmotic pressures.
With the advent of highly effective heating and aseptic packaging processes, mostly aseptically processed fruits are used. In general, steam under pressure is not needed to destroy the microflora of fruits, because they are acidic, and heating at 100°C or less is adequate. The more acidic the fruit, the lower the heat treatment required to preserve it. Among the fruits often used in frozen desserts, peaches and apricots fall within the ''acid foods'' range of pH 3.7-4.5, whereas berries have a pH less than 3.7, placing them in the ''high-acid foods'' group.
Fruits that are aseptically processed can be stored at room temperature for several months with no microbial spoilage. Processors frequently use swept-sur-face heat exchangers that heat the mixture of fruit, sugar, acid, and stabilizer to 88-121°C, depending on the fruit. After holding the mixture for approximately 3 min at the maximal temperature, it is cooled to approximately 27°C and pumped directly to an aseptic filling machine. It is filled into sterile containers that are usually made of laminated polyethylene and foil.
In an alternative processing system (Fig. 1), fruit is pumped through coils that heat, hold, and cool the product. The coils cause the fruit to mix well in the
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