There is no federal standard for counts of bacteria in frozen desserts in the United States. However, most states enforce standards for coliform bacteria at less than or equal to 10/g and for standard plate count at 50,000/g. One state enforces a maximum standard plate count of 20,000/g. Approximately 14 states permit coliform counts of up to 20/g for bulky flavored ice creams. These are products to which large amounts of flavorings, fruits, and nuts are added. Because many of these items are added after freezing, the chances of contamination with coliform bacteria is considerably greater than with plain ice creams. With the recent knowledge that microbial environmental contaminants include Listeria, it is prudent for manufacturers to consider the presence of coliform bacteria in ice cream as indicative of unsanitary practices and to increase the intensity of hygienic activities when coliform bacteria are found in finished products.
Coliform bacteria belong to the larger group of gram-negative asporogen-ous facultatively anaerobic glucose-fermenting bacteria of the family Enterobac-teriaceae, all of which are killed by pasteurization of ice cream mixes. Testing for this group of bacteria, rather than for the coliform group only, increases the sensitivity of the test for postpasteurization contamination. The modification of the coliform test is simple: instead of lactose, 1% glucose is added to the Violet Red Bile Agar used to plate the sample. Appearance of typical colonies arising from plating of 1 g of sample is indicative of postpasteurization contamination and the possible entrance of pathogenic bacteria into the product. Therefore, the cause of the problem should be determined and corrected.
The FDA has tested finished ice cream products for pathogens, principally L. monocytogenes, and numerous recalls have ensued when samples have been positive (Anonymous, 1986a, 1986b, 1986c, 1986d, 1994). The U.S. Code of Federal Regulations, Title 21, Part 7.40 (21 CFR 7.40) provides recall policies, procedures, and industry responsibilities.
Recall is a voluntary act of manufacturers and distributors who seek to protect the health and welfare of consumers from products that may present a risk of injury or gross deception or are otherwise defective. Recall is an alternative to FDA-initiated court action to remove violative, distributed products. Recalls are assigned classes I, II, or III depending on the relative degree of health hazard with the greatest risk associated with class I recalls. The FDA may request a recall when a distributed product presents a risk of illness and the manufacturer or distributor has not initiated a recall. A recalling firm is expected to conduct checks of the effectiveness of the recall action.
A survey of 530 samples of ice cream mix (85), ice cream (394), and ice cream novelties (51) by Health and Welfare Canada revealed only two samples that contained L. monocytogenes (Farber et al., 1989). Furthermore, the WHO Working Group (1988) reported the incidence of L. monocytogenes in ice cream as varying from 0 to 5.5% with very low numbers (1-15 cfu/g) usually being observed.
The heat resistance of L. monocytogenes is higher than that of many vegetative bacteria (Doyle et al., 1987). Its heat resistance can be enhanced in milk and cream in which it is contained in white blood cells (leukocytes). As an agent of bovine mastitis (Gitter et al., 1980), L. monocytogenes is phagocytized by leukocytes of the mammary gland. If numbers of phagocytized L. monocytogenes are sufficiently high, the pathogen may survive minimal conditions of high-temperature, short-time pasteurization of milk (Garayzabel et al., 1985; Doyle et al., 1987). However, the incidence of mastitis caused by L. monocytogenes is quite low. Furthermore, leukocytes and, consequently, phagocytized bacteria are mostly removed by clarification and separation during the preparation of cream for the manufacture of ice cream. No evidence has been forthcoming that these bacteria survive pasteurization of ice cream mix.
An outbreak of gastrointestinal infections caused by S. Enteritidis in ice cream was observed beginning in September 1994. After a case-control study implicated a national brand of ice cream, much of the product was recalled by the manufacturer. Gastroenteritis developed in an estimated 224,000 persons (Hennessey et al., 1996), but fewer than 600 cases were reported to public health departments (Anonymous, 1996). The attack rate was estimated at 6.6% among consumers of the affected products. Salmonella was isolated from 8 of 266 ice cream products (3%) but not from environmental samples. The source of the pathogens was believed to be transport trucks used to haul both nonpasteurized liquid eggs and pasteurized ice cream mix. The mix was not repasteurized at the plant to which it was delivered in the tank trucks for freezing. The lesson learned was that repasteurization should be done when a mix is given any opportunity to be contaminated after pasteurization and especially when it is moved from one location to another in reusable containers. Such reusable containers should be dedicated to transport of mix only.
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