Fundamental Food Microbiology, Third Edition - Fuad Fathir

262 FUNDAMENTAL FOOD MICROBIOLOGY
3. Some Important Thermoduric Psychrotrophs
They include facultative anaerobes, such as spores of Bacillus coagulans and Bac.
megaterium, some strains of Lab. viridescens; and anaerobes, such as spores of
Clostridium laramie, Clo. estertheticum, Clo. algidicarnis, Clo. putrefaciens, and
unidentified Clostridium spp. The spores survive low-heat treatment. Following
germination and outgrowth, the cells grow at low temperature.
When a food is temperature abused above 5�C (such as during transport or
display in stores), some true mesophiles (growth temperature range 15 to 45�C,
optimum 25 to 40�C) can also grow. However, at 10 to 15�C, psychrotrophs will
generally grow much faster than these mesophiles.
B. Thermophilic Bacteria
By definition, the bacteria in this group grow between 40 and 90�C, with optimum
growth at 55 to 65�C. Some high-heat-processed foods are kept warm between 50
and 60�C for a long period of time (at delis, fast-food establishments, and restaurants).
Spores of some thermophilic Bacillus and Clostridium spp. can be present
in these heat-treated foods, which at warm temperature germinate and multiply to
cause spoilage. In addition, some thermoduric vegetative bacteria surviving low-heat
processing (such as pasteurization) or thermophiles getting in food as postheat
contamination can also multiply in these warm foods, especially if the temperature
is close to 50�C. These include some lactic acid bacteria, such as Pediococcus
acidilactici and Streptococcus thermophilus, as well as some Bacillus and Clostridium
spp. They can also survive and cause spoilage of foods that are cooked at low
heat (60 to 65�C as for some processed meats) or kept warm for a long time.
C. Aciduric Bacteria
Bacteria that can grow relatively rapidly in food at pH 4.6 or below are generally
regarded as aciduric (or acidophilic). They are usually associated with spoilage of
acidic food products such as fruit juices, pickles, salsa, salad dressings, mayonnaise,
and fermented sausages. Heterofermentative lactic acid bacteria (such as Lab. fructivorans,
Lab. fermentum, and Leu. mesenteroides) and homofermentative lactic acid
bacteria (such as Lab. plantarum and Ped. acidilactici) have been associated with
such spoilage. (Yeasts and molds are aciduric and thus are also associated with
spoilage of such foods.)
A. Food Types
V. SIGNIFICANCE OF FOODS
Foods differ greatly in their susceptibility to spoilage by microorganisms. This is
mainly because of their differences in intrinsic factors (A w, pH, O–R potential,
nutrient content, antimicrobial substances, and protective structures). A food with a