MICROBIAL SPOILAGE OF FOOD.
Food Borne Microorganisms. PRINTABLE PDF 📥
1. Food Borne Yeasts
Yeasts
have been associated with foods since earliest times, both as beneficial agents
and as major causes of spoilage and economic loss. Current losses to the food
and dairy industry caused by yeast spoilage are estimated at several billion
dollars. As new food ingredients and new food manufacturing technologies are
introduced, novel food spoilage yeasts are emerging to present additional
problems. To date over 70 biological species of yeasts have been described and
thousands of different varieties have been shown to exist in all kinds of
natural and artificial habitats.
Yeasts
may be viewed as being unicellular fungi in contrast to the molds, which are
multi-cellular. Yeasts can be differentiated from bacteria by their larger cell
size and their oval, elongate, elliptical, or spherical cell shapes. Typical
yeast cell s range from 5 to 8 um in diameter, with some being even larger.
Older yeast cultures tend to have smaller cells. Most of those of importance in
foods divide by budding or fission.
Yeasts
can grow in presence of various types of organic acids such as lactic, citric
and tartaric acid etc and also over a wide range of acid pH and in up to 18%
ethanol. Many grow in the presence of 55-60% sucrose. Many colours are produced
by yeasts, ranging from creamy to pink to red. The asco- and arthrospores of
some are quite heat resistant.
i. Candida
Members
of the Candida genus form shining white colonies and cells
contain no carotenoid pigments. Candida tropicalis is the most
prevalent in foods in general Some members are involved in the fermentation of
cocoa beans, as a component of kefir grains, and many other products, including
beers, and fruit juices.
ii. Debaromyces
Debaromyces is one of the most prevalent yeast genera in the dairy products.
It can grow in 24% NaCl and at an aw as low as 0.65.
iii. Kluyveromyces
Kluyveromyces spp. produces β-galctosidase and are vigorous fermenters of sugars
including lactose. K. marxianus is one of the two most
prevalent yeasts in dairy products, kefir grain and causes cheese spoilage.
iv. Rhodotorula
The
genus Rhodotorula contains many psychrotrophic species that
are found on fresh poultry, shrimp, fish and beef. Some grow on the surface of
butter.
v. Saccharomyces
Saccharomyces are ascosporogenous
yeasts that multiply by lateral budding and produce spherical spores in asci.
They are diploid and do not ferment lactose. All bakers’ brewers’, wine and
champagne yeasts are S. cerevisiae. They are found in Kefir
grains and can be isolated from wide range of foods. S. cerevisiae rarely
causes spoilage.
A)
B)
Fig.1 Food Borne Yeasts
(a) Saccharomyces spp. (b) Candida spp.
vi. Torulaspora
Torulaspora multiplies by lateral budding. They are strong fermenters of
sugars. Torula delbrueckii is the most prevalent species.
Fig:2 Torula delbrueckii
2 Food-Borne Molds
Molds
are filamentous fungi that grow in the form of tangled mass that spreads
rapidly and may cover several inches of area in a very short period. It is also
referred to as mycelial growth. Mycelium is composed of branches of filaments
referred to as hyphae. The molds of great importance in foods multiply by
ascospores or conidia. The ascospores of some of the mold genera are notable
for their extreme degrees of heat resistance.
i. Alternaria
Alternaria spp. form septate mycelia with conidiophores and large brown
conidia are produced. They cause brown to black rots of fruits, apples, and
figs. Some species produce mycotoxins.
ii. Aspergillus
The Aspergillus spp.
appear yellow to green to black on a large number of foods. Some species cause
spoilage of oils. A. niger produces β-galactosidase,
glucoamylase, invertase, lipase and pectinase. A. oryzae produces
a -amylase. Two species A. flavus and A.
parasiticus produce aflatoxins, and others produce ochratoxin A and
sterigmatocystin.
iii. Geotrichum
The
yeast like fungi, Geotrichum are also referred to as dairy
mold.
iv. Mucor and Rhizopus
Mucor species that produce non-septate hyphae are prominent food
spoilers. Similarly, Rhizopus spp. also produce non septate
hyphae but give rise to stolons and rhizoids. R. stolonifer is
by far the most common species in foods and is also referred to as “bread
mold”.
Other important genera of molds related to spoilage of foods are Neurospora, Thamnidium, Trichothecium, Penicillium and Cladosporium etc.
Fig. 3 Food borne molds (a) Aspergillus spp.
and (b) Rhizopus spp.
Fig. 4 Food borne molds (a) Penicillium spp.
3. Food Borne Viruses
Viruses are filterable,
ultra microscopic particles and can be cultivated only on live tissues. Viruses
consist of a core of nucleic acid (DNA or RNA) and a protein coat. It is
commonly believed that some of the viruses are responsible for food borne
diseases in humans, particularly some non bacterial gastroenteritis due to
enteroviruses. Contaminated water and food are important carriers of hepatitis
viruses. Foot and mouth disease (FMD) causing virus in cattle can be
transmitted to human beings through foods. Similarly, viral diseases of poultry
have also been source of ailments in humans.
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