| Applications of biotechnology to traditional fermented foods |
|V. Animal derivatives|
Fermentation traditionally offers an easy and low-energy preservation method for meats that results in distinctive products that have an important part in the diet of people making them. Such fermented meats contribute both nutritional value and pleasure to meals. However, products are not the same from time to time. Indeed, the product may spoil, cause illness due to pathogenic microorganisms or their toxins, and even become lethal due to botulinum toxin production if the normal beneficial microbial flora do not multiply as usual. To prevent these problems, the use of starter cultures has become commonplace in many countries, including developing countries.
One example of such fermented meat is nham, a traditional Thai sausage. Nham is made by mixing salt (3 percent by weight) and garlic with ground lean pork. Nitrate and nitrite salts also are added in commercial production. The mixture is then wrapped in a banana leaf or stuffed in cellulose tubing. Fermentation is at ambient temperature (about 30°C in Thailand) for 3 to 4 days, after which it remains in good condition for only 1 to 2 days without refrigeration. Since nham is frequently eaten raw, it is important that pathogenic bacteria be killed as well as that botulinum toxin and staphylococcal enterotoxins are not produced. Since hogs are frequently infested with Trichinella spiralis, these larvae should not be viable.
A study was conducted on nham made with and without the addition of one of two levels of a commercially available dry starter culture preparation (Griffith Laboratories, Ltd., Thailand) (1). Portions in polyethylene film bags were inoculated, sealed, and incubated at 30°C. The inoculum was S. aureus (a mixture of three enterotoxin-producing strains) and E. cold (three strains). Microbial numbers, pH, and titrable acidity were determined at intervals during the fermentation. The meat used was from two hogs that had been experimentally infected with trichinae at weaning; viable trichinae were determined at 24-hour intervals.
S. aureus was able to multiply (10x) and remain viable only in the control inoculated samples. E. cold was not detected at 96 hours in the sausage made with the higher level of starter (1.5 percent by weight) and had decreased greatly in products made with the 0.75 percent level. The use of the higher level of starter preparation resulted in loss of infectivity of the trichinae larvae, although further research is necessary to confirm this effect. The addition of starter culture resulted in more rapid acid production and slightly lower end-point pH.
It is important to keep in mind that natural fermentations are difficult to replicate in other settings. For example, the meat mixture for nham is traditionally wrapped in small banana leaf packets. The leaves contribute to the surface flora of the sausage, which no doubt changes the fermentation pattern. Flora of work surfaces and of the pork itself may be different.
Drying often follows fermentation of similar meat products to provide for long-term preservation. Dendeng ailing, Indonesian seasoned beef that has only a traditional short fermentation period before drying, was found to have a lower pH and total gram-negative bacteria, staphylococci, and E. cold counts when prepared with a starter culture of Lactobacillus plantarum than in the traditional manner. Those with a starter culture dried more rapidly at 50°C and had lower water activities (2).
The effectiveness of lactic acid bacteria in suppressing the multiplication of undesirable microorganisms is largely attributed to the production of organic acid. However, additional factors include the production of bacteriocins and hydrogen peroxide. More general effects include competition for essential nutrients.
To maximize the quality, reproducibility, and safety of the product, strains of bacteria are selected based largely on the qualities of self stability and viability as used, rapid acid production, and desirable product qualities. As in the starter culture preparation used for nham, strains of Lactobacillus and Pedicoccus are the most common (3,4). The compatibility of strains is important, which includes resistance to or lack of production of bacteriocins. In addition to tolerance to the salt and nitrite levels of the mixture, the culture must be active in the temperature range used for the fermentation. The product must have the expected palatability characteristics. No harmful compounds may be produced. These same attributes can be more efficiently arrived at through the application of the techniques of molecular biology.
The success of traditional fermentations depends on the complex interaction of the food components, the natural flora of the ingredients, and the surfaces in contact with the food, atmosphere, and ambient temperature. Our knowledge of these conditions is still limited for many of the fermented meats. Alaskan outbreaks of botulism from native sea and land mammal products may have increased as plastic bags became the common container and the fermentation rate was speeded by placing the container near the stove (5). Thus, as transitions occur from traditional fermentations to new adaptations, knowledge of the basic processes becomes essential.
1. Petchsing, U., and M. J. Woodburn. 1990. Staphylococcus aureus, and Escherichia coli in nham (Thai-style fermented pork sausage) International Journal of Food Microbiology 10: 183-192.
2. Darmadji, P., M. Izumimoto, T. Miyamoto, and K. Katoaka 1990. Lactic fermentation effects on preservative qualities of dendeng, giling. Journal of Food Science 55:1523-1527.
3. Smith, J. L., and S. A. Palumbo. 1983. Use of starter cultures i' meats. Journal of Food Protection 46:997-1006.
4. Bacus, J. N., and W. L. Brown. 1985. The lactobacilli: Mea products. The pediococci: Meat products. Pp.55-72, 85-96 in Bacterial Starter Cultures for Foods. S.E. Gilliand, Ed. Boca Raton, Fla.: CRC Press.
5. Wainwright, R. B., W. L.Heyward, J. P. Middagh, C. H. Hatheway, A. P. Harpster, and T. R. Bender. 1988. Food-born' botulism in Alaska, 1947-1985: Epidemiology and clinical findings Journal of Infectious Diseases 157:1158-1162.