Fundamental Food Microbiology, Third Edition - Fuad Fathir

180 FUNDAMENTAL FOOD MICROBIOLOGY
4. Control Methods
Following the discovery of bacteriophage-related starter failure in food fermentation,
steps were developed to reduce phage contamination with starter cultures. 7,9,10 These
include proper sanitation to reduce phage buildup in the processing facilities, both
during bulk starter preparation and product fermentation; use of phage-insensitive
media; rotation of strains; and use of mixed strains to reduce the buildup of a
particular phage to a level that causes starter failure. Subsequently, phage-resistant
starter strains were developed. Initially, by growing a sensitive bacterial strain in the
presence of a specific lytic phage, cells that were not killed by the phage were
isolated and supplied to processors. Recent studies have indicated that by genetic
techniques, a bacterial strain can be made insensitive to one or more phages. These
include modifying the genetic makeup of a starter strain to inhibit phage adsorption,
destroying phage DNA by restriction enzyme systems of cells, or aborting the phages
before lysis. By combining these traits through genetic manipulation, a strain can
be developed that is resistant to several phages. Current studies on genome analysis
of lactic acid bacteria and bacteriophages will help develop phage-resistant starter
strains (see Chapter 11).
V. YEAST AND MOLD CULTURES
Specific strains of yeast cultures (e.g., Saccharomyces cerevisiae) used to leaven
dough in bakery products, and to produce alcohol in beer, wine, or distilled liquor,
have been developed. These yeasts are produced by culture producers as well as by
processors. Culture producers grow the yeast in suitable media, concentrate the cells,
and supply in frozen or dried form.
Molds used as starter in some products are also available from culture producers.
The strains used should not produce mycotoxins. The molds are grown on the surface
of a liquid or solid (bread) media until they sporulate. The spores are collected, dried
in powder form, packaged, and supplied to processors.
VI. CONCLUSION
Isolation and identification of microorganisms associated with food fermentation
have helped the use of specific species and strains in pure culture for controlled
fermentation. These starter cultures are currently produced by commercial culture
producers for use by food-processing companies directly to start fermentation of
raw materials. This has also helped to reduce product loss associated with culture
failure, notably from phage attack. Development of phage-resistant starter cultures
has also helped overcome the problem. Current studies on genome sequence of lactic
acid bacteria and their bacteriophages will be useful in the future development of
bacterial strains for their efficient use in food fermentation. The use of starter culture
in food fermentation is covered in Chapter 14.