Fundamental Food Microbiology, Third Edition - Fuad Fathir

280 FUNDAMENTAL FOOD MICROBIOLOGY
beverages, some yeast species from genera Torulopsis, Candida, Pichia, Hansenula,
and Saccharomyces can grow and make the products turbid. Some Lactobacillus
and Leuconostoc species can also grow to cause cloudiness and Leuconostoc can
cause ropiness (due to production of dextrans) of the products. Noncarbonated
beverages can be similarly spoiled by yeasts, Lactobacillus, and Leuconostoc spp.
In addition, if there is enough dissolved oxygen, molds (Penicillium, Aspergillus,
Mucor, and Fusarium) and Acetobacter can grow; the latter produces acetic acid to
give a vinegar-like flavor. Fruit juices are susceptible to spoilage by molds, yeasts,
Lactobacillus, Leuconostoc, and Acetobacter spp. However, a particular type of juice
may be susceptible to spoilage by one or another type of microorganism. Molds and
Acetobacter can grow if enough dissolved oxygen is available. Yeasts can cause both
oxidation (producing CO 2 and H 2O) and fermentation (producing alcohol and CO 2)
of the products. Acetobacter can use alcohol to produce acetic acid. Heterofermentative
Lab. fermentum and Leu. mesenteroides can ferment carbohydrates to lactate,
ethanol, acetate, CO 2, diacetyl, and acetoin. In addition, Leu. mesenteroides and
some strains of Lab. plantarum can produce slime due to production of dextran and
other exopolysaccharides. In fruit drinks, Lactobacillus and Leuconostoc spp. can
also convert citric and malic acids (additives) to lactic and acetic acids and reduce
the sour taste (flat flavor). Because concentrated fruit drinks and fruit preserves have
a low A w (0.9), only osmophilic yeasts can grow; molds also can grow if oxygen is
available. A new group of bacterial species, Alicyclobacillus (e.g., Alicyclobacillus
acidoterrestris), has been recognized recently to cause spoilage of low-pH fruits and
vegetable juices.
To prevent growth of these potential spoilage microorganisms, several additional
preservation methods are used for these products, which include heat treatment,
freezing, refrigeration, and addition of specific chemical preservatives.
Tomato juice has a pH of ca. 4.3. It is generally given high heat treatment to
kill vegetative microorganisms. However, bacterial spores can survive. Flat sour
spoilage of tomato juice due to germination and growth of Bac. coagulans is documented.
Most other vegetable juices have pH values between 5.0 and 5.8, and many
have growth factors for lactic acid bacteria. These products are susceptible to spoilage
because of the growth of many types of microorganisms. Effective preservation
methods are used to control their growth.
VIII. CEREALS AND THEIR PRODUCTS
Some of the products susceptible to microbial spoilage include high-moisture cereal
grains, germinated or sprouted seeds and grains, refrigerated dough, breads, soft
pastas, and pastries. 1,2
A. Grains and Seeds
Grains and seeds normally have 10 to 12% moisture, which lowers the A w to ca.
£0.6 and thus inhibits microbial growth. In cereal grains, during harvesting, processing,
and storage, if the A w increases above 0.6, some molds can grow. Some