MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.5. MICROBIOLOGY OF GRAIN AND FLOUR

15.5.1. Grain Microflora

Microorganisms found on grain are divided into saprophytic and phytopathogenic groups. The saprophytic group includes epiphytic microorganisms that colonize healthy plants during their growth and maturation.

Epiphytic microorganisms are those that live On the surface of plants and derive Nutrition from natural plant tissue secretions. Epiphytes do not invade plant Tissues or exert a detrimental effect on their development. The numerical and species composition of plant epiphytic microflora depends on ambient Temperature and humidity. In warm, humid weather, non-spore-forming rod-shaped Bacteria dominate among epiphytes, whereas in hot, dry weather, spore-forming rods such as B. subtilis, B. mycoides, B. megatherium, and others become predominant. The microbial load on grain is quite substantial, reaching up to 2 x 10⁷ CFU/g.

Typical epiphytes constantly present on The surface of green plants are bacteria of the genus Erwinia.

Erwinia herbicola is a small, non-spore-forming, Gram-negative, motile rod (possessing two polar flagella). It is a facultative anaerobe. On meat-peptone Agar, it forms smooth, golden-yellow colonies. Immediately after threshing, the count of Erwinia herbicola on grain reaches 90–96% of the total microbial population. The predominance of this bacterial species serves as an indicator of grain freshness and sound quality.

Micrococci, bacilli, and Yeasts are detected in significant quantities only on damp grain that has not been dried promptly after threshing and has been stored in bulk for 3–4 days at temperatures above 10 °C. Damaged grain contains bacteria of the genera Lactobacillus, Clostridium, Sarcina, and Proteus.

Yeasts of the genera Saccharomyces, Torulopsis, and Candida do not significantly affect the preservation and quality of cereals, but under elevated humidity conditions, they promote self-heating and The Development of a musty "bin" odor in the grain.

Cereal grains frequently harbor field Fungi of the genera Alternaria, Helmintosporium, Cladsporium, Ascochyta, and others. The mycelium of these fungi most commonly invades the embryo and subsequently the endosperm during the milk-ripening stage. As a result, defective grains are formed: shriveled, spotted, and having a black germ.

Grain preservation and quality are primarily affected by storage fungi belonging to the genera Aspergillus (A. niger, A. glaucus, A. fumigatus), Penicillium (P. glaucum), and Fusarium, and to a lesser extent Mucor (M. mucedo, M. racemosus), Trichoderma, Rhizopus, Trichothecium, and Thamnidium. These fungi are practically undetectable on freshly harvested grain, but they appear on stored grain, which can be one of the causes of its self-heating and spoilage.

The species composition of grain microflora is influenced by storage temperature and humidity. Mesophilic microorganisms residing on grain multiply at temperatures of 25–30 °C. At around 10 °C, the growth of most mesophilic microorganisms ceases, although they do not die off. Fungi of the genera Penicillium, Rhizopus, Thamnidium, and Fusarium are capable of multiplying at such temperatures. Grain stored under conditions of reduced moisture can support the proliferation of xerophilic Aspergillus species: A. glaucus and A. restrictus. High grain moisture promotes the development of more moisture-demanding Aspergilli and Penicilli.

The phytopathogenic group comprises parasitic species of bacteria and fungi that survive at the expense of the host plant. During the growth and maturation periods, they induce plant diseases such as bacterioses and mycoses. Bacterioses of wheat, rye, barley, corn, and rice are widespread, with certain Representatives of the bacterial genus Pseudomonas acting as causative agents. The disease manifests as spotting and blackening of the glumes, rachis, and upper stem. In severe cases of the infection, the grains darken, become shriveled, and lose up to 60–70% of their mass.

Among fungal diseases—mycoses of cereal crops—the most common are ergot, bunt (or covered/wet smut), and fusarium HEAD blight.

Ergot. The CAUSATIVE AGENT OF purple ergot is the fungus Claviceps purpurea, a representative of the higher ascomycetous fungi. The fungus primarily infects rye, and less frequently wheat and barley, during the flowering period. In the inflorescences of ergot-infected plants, dark violet, horn-shaped sclerotia are clearly noticeable. During harvesting, these sclerotia shed, fall into the soil, and overwinter there. In spring, the sclerotia swell and germinate, producing fruiting bodies shaped like globular heads on thread-like stalks. Several reddish stroma develop from each sclerotium. Inside the stroma head are cavities where the sexual process takes place, including The formation of the ascogonium and antheridium, followed by plasmogamy and karyogamy. The process culminates in the formation of asci containing 8 thread-like ascospores.

Mature ascospores are discharged onto cereal flowers during mass flowering and germinate there into sporulating mycelium. A few days after infection, the conidial stage of the fungus develops. "Honeydew," in which the conidia are embedded, plays a major role in the dissemination of fungal conidia by insects. By the time rye ripens in the infected Ovary, the mycelium becomes dense, and horns (sclerotia) are formed in place of the grains. Under high humidity, sclerotia form within a week after the appearance of the honeydew; in dry weather, this takes two weeks.

Ergot infection reduces grain yield. Furthermore, the presence of sclerotia in grain can cause severe poisoning in humans and animals—mycotoxicoses (see Section 11.4). The admixture of ergot sclerotia in grain must not exceed 0.05%.

Smut. This disease is caused by microorganisms belonging to the Class of higher fungi—Basidiomycetes. A distinction is made between covered, loose, and blister smut. Currently, covered (wet) smut of wheat, barley, and rye is widespread, caused by species such as Tilletia caries and Tilletia hordei. Smut spores germinate on sprouted seeds even before seedling emergence. The mycelium penetrates the grain seedling and grows alongside the plant. By the flowering stage, the fungal mycelium invades the ovary, and in the milk-ripening phase, the interior of the grain becomes filled with smut spores. Instead of a caryopsis, rounded sacs filled with a mass of black chlamydospores develop in the ear. The infected ear has a darkish hue and resembles charred kindling (hence the Russian name "golovnya" or smut). During harvesting and threshing, these sacs rupture, and spores contaminate the surface of healthy grains. The grain acquires a herring-brine odor due to the presence of trimethylamine in the spores. Covered smut spores are hygroscopic and easily smear when damp. Flour milled from grain affected by covered smut acquires a dark tint, an unpleasant odor and taste, and can cause irritation of the Salivary Glands and intestinal dysfunction.

Fusarium head blight is a cereal disease caused by fungi of the genus Fusarium. When grain is infected by the fungus species F. graminearum, mycotoxins accumulate within it.

Flour from wheat infected with fungi of the genus Fusarium yields a moist dough with a sticky surface. Bread baked from such flour has a dull yellow crumb and a dark-colored crust. A decrease in the quantity and deterioration in the quality of gluten is observed: it becomes runny, slimy, or conversely, insufficiently extensible, short-breaking, with a dark color and an unpleasant odor. Under the action of fusarial Proteolytic Enzymes, There is a sharp weakening of the dough's consistency and elasticity. A characteristic feature of fusarioses is that a specific disease is often caused not by a single fungal species of this genus, but simultaneously by several.

Grain contamination with fusarium occurs not only in the field but also during storage under high humidity conditions. To prevent the development of Fusarium during storage, it is essential that grain moisture does not exceed 13–14%.

During grain storage, the number of microorganisms present on it declines due to the die-off of Erwinia herbicola, as a result of which spore-forming bacteria of the genus Bacillus and certain xerophilic fungi of the genus Aspergillus become predominant.