Fundamental Food Microbiology, Third Edition - Fuad Fathir

SPOILAGE OF SPECIFIC FOOD GROUPS 285
buchneri). They can also produce cloudiness and mousy odor. Some Leuconostoc
spp. can produce sliminess and cloudiness. Oenococcus oenos can convert malic
acid to lactic acid and CO 2 and reduce the acidity of a wine. Sometimes, this
malolactic fermentation is used advantageously to reduce the sourness of wine.
Beer spoilage can be caused by some lactic acid bacteria and yeasts. Growth of
Pediococcus spp. causes an increase in acidity and cloudiness. Some Lactobacillus spp.
can also multiply and cause turbidity. Acetobacter and Gluconobacter, in the presence
of air, can produce cloudiness and sliminess as well as make the beer sour. Wild yeasts
(yeasts other than those used in fermentation) can grow in beer, causing off-flavor.
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XII. CANNED FOODS
Canned foods are heat treated to kill microorganisms present, and the extent of heat
treatment is predominantly dependent on the pH of a food (Chapter 4). High-pH
(pH 4.6 and above; also called low-acid) foods are heated to destroy most heatresistant
spores of the pathogenic bacteria, Clo. botulinum, to ensure that a product
is free of any pathogen. 1,2,10 However, spores of some spoilage bacteria, which have
greater heat resistance than do spores of Clo. botulinum, can survive. Thus, these
products are called commercially sterile (instead of sterile, which means free of any
living organism) foods. The spores that survive the heat treatment designed to destroy
Clo. botulinum spores are thermophilic spores and can germinate at 43�C and above.
However, once germinated, some can outgrow at temperatures as low as 30�C. The
other food group, designated as low-pH or high-acid food with pH below 4.6, is
heat treated to kill all vegetative cells and some spores. Although low pH inhibits
germination of spores and subsequent growth of Clo. botulinum, spores of some
aciduric thermophilic spoilage bacteria can germinate and grow when the products
are stored at higher temperatures, even for a short time, which induces germination.
Some spores of thermoduric mesophilic spoilage bacteria (including pathogenic)
can also survive heating in these products, but they are inhibited to germinate by
the low pH.
Canned food spoilage is both due to nonmicrobial (chemical and enzymatic
reactions) and microbial reasons. Production of hydrogen (hydrogen swell), CO 2,
browning, corrosion of cans due to chemical reactions, and liquification, gelation,
and discoloration due to enzymatic reactions are some examples of nonmicrobial
spoilage. Microbial spoilage is due to three main reasons: (1) inadequate cooling
after heating or high-temperature storage, allowing germination and growth of thermophilic
sporeformers; (2) inadequate heating, resulting in survival and growth of
mesophilic microorganisms (vegetative cells and spores); and (3) leakage (can be
microscopic) in the cans, allowing microbial contamination from outside following
heat treatment and their growth.
A. Thermophilic Sporeformers
Thermophilic sporeformers can cause three types of spoilage of low-acid (high-pH)
foods (such as corn, beans, peas) when the cans are temperature abused at 43�C and
above, even for a short duration.