Bifidobacterium are morphologically similar to some Lactobacillus spp. and were previously included in the genus Lactobacillus. The cells are Gram-positive, rods of various shapes and sizes, present as single cells or in chains of different sizes. They are non-spore forming, non-motile, and anaerobic, although some can tolerate O₂ in the presence of CO₂.
The species grow optimally at 37 to 41°C, with a growth temperature range of 25 to 45°C. They usually do not grow at a pH above 8.0 or below 4.5. They ferment glucose to produce lactic and acetic acid in a 2:3 molar ratio without producing CO₂, and also ferment lactose, galactose, and some pentoses. They have been isolated from feces of humans, animals, and birds and are considered beneficial for the normal health of the digestive tract.
They are present in large numbers in the feces of infants within 2 to 3 days after birth, and usually present in high numbers in breast-fed babies. They are usually found in the large intestine, and many species of Bifidobacterium have been isolated from the feces of humans and animals. Some of these include Bifidobacterium bifidum, B. longum, B. brevis, B. infantis, and B. adolescentis.
All have been isolated in humans; however, some species are more prevalent in infants than in adults. Some of these species have been added to dairy products to supply live cells in high numbers to restore and maintain intestinal health in humans.
Characteristics of Propionibacterium
The genus includes species in the classical or dairy Propionibacterium group and the cutaneous or acne Propionibacterium group. The cells are Gram-positive, pleomorphic thick rods 1 to 1.5 µm in length, and occur in single cells, pairs, or short chains with different configurations.
They are non-motile, non-sporulating, anaerobic (can also tolerate air), catalase positive, and ferment glucose to produce large amounts of propionic acid and acetic acid. Some species ferment lactose, sucrose, fructose, galactose, and some pentoses. They grow optimally at 30 to 37°C, and some species form pigments.
They have been isolated from raw milk, some types of cheeses, dairy products, and silage. Four species of dairy Propionibacterium are included in the genus: Propionibacterium freudenreichii, P. jensenii, P. thoenii, and P. acidipropionici. All four species are associated with natural fermentation of Swiss-type cheeses, but P. freudenreichii has been used as a starter culture in controlled fermentation.
Characteristics of Brevibacterium
The genus contains a mixture of coryniform bacterial species, some of which have important applications in cheese production and other industrial fermentations. Brevibacterium linens is used in cheese ripening as it has extracellular proteases. The cells are non-motile, Gram-positive, and capable of growing in high salt and wide pH ranges.
Characteristics of Acetobacter
A species in this genus, A. aceti, is used to produce acetic acid from alcohol. The cells are Gram-negative; aerobic; rods (0.5 to 1.5 µm); occurring as single cells, pairs, or chains; and can be motile or non-motile.
They are obligate aerobes, catalase positive, and oxidize ethanol to acetic acid and lactic acid to CO₂ and H₂O. They grow well from 25 to 30°C. They are found naturally in fruits, sake, palm wine, cider, beer, sugar cane juice, tea fungus, and soil. Some species synthesize large amounts of cellulose.
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Role of Yeasts and Molds in Food
Many yeasts and molds are important in food, but most are involved with the spoilage of food and mycotoxin production. Several are used in food bioprocessing; however, genetic improvements are being made to improve their desirable characteristics.
Characteristics of Yeasts
Among the many types of yeasts, only a few have been associated with fermentation of foods and alcohol, production of enzymes for use in food, production of SCPs, and as additives to impart desirable flavor in some foods. The most important genus and species used is Saccharomyces cerevisiae.
It has been used to leaven bread and produce beer, wine, distilled liquors, and industrial alcohol; produce invertase (enzyme); and flavor some foods (soups). Many strains have been developed to suit specific needs. The cells are round, oval, or elongated. They multiply by multipolar budding or by conjugation and formation of ascospores.
The strains are generally grouped as bottom yeasts or top yeasts. Top yeasts grow very rapidly at 20°C, producing alcohol and CO₂. They also form clumps that, because of rapid CO₂ production, float at the surface. In contrast, bottom yeasts grow better at 10 to 15°C, grow slowly and produce CO₂ slowly, do not clump, and thus settle at the bottom.
Top yeasts and bottom yeasts are used according to the need of a particular fermentation process. Candida utilis has been used to produce SCPs. It is false yeast (Fungi imperfecti) and reproduces by budding (not by conjugation).
The cells are oval to elongated and form hyphae with large numbers of budding cells. They are also involved in food spoilage. Kluyveromyces marxianus and K. marxianus var. lactis can hydrolyze lactose and have been associated with natural fermentation, along with other yeasts and lactic acid bacteria, of alcoholic dairy products such as kefir.
They have also been associated with spoilage of some dairy products. They are used to produce β-galactosidase (lactase) for use commercially to hydrolyze lactose. The enzyme is now used to produce low-lactose milk.
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Characteristics of Molds
Although most molds are associated with food spoilage and many form mycotoxins while growing in foods, other species and strains are used in processing of foods and to produce additives and enzymes for use in foods. Molds are multicellular, filamentous fungi. The filaments (hyphae) can be septate or nonseptate and have nuclei.
They divide by elongation at the tip of a hypha or by forming sexual or asexual spores on a spore-bearing body. Among many genera, several species from the genera Aspergillus and Penicillium, and a few from Rhizopus and Mucor, have been used for beneficial purposes in food to identify a non-mycotoxin-producer strain in the case of natural fermentation, but should be an important consideration in the selection of strains for use in controlled fermentation.
Aspergillus oryzae is used in fermentation of several oriental foods, such as sake, soy sauce, and miso. It is also used as a source of some food enzymes. A. niger is used to produce citric acid and gluconic acid from sucrose. It is also used as a source of the enzymes pectinase and amylase. Penicillium roquefortii is used for ripening of Roquefort, Gorgonzola, and blue cheeses.
Some strains can produce the neurotoxin roquefortin. P. camembertii is used in Camembert cheese and P. caseicolum is used in Brie cheese. They are also used to produce the enzyme glucose oxidase.
Frequently Asked Questions
- What are the morphological characteristics of Bifidobacterium?
Bifidobacterium are Gram-positive rods of various shapes and sizes, present as single cells or in chains, non-spore forming, non-motile, and anaerobic, with some tolerating O₂ in the presence of CO₂. - How do Bifidobacterium contribute to digestive health?
Bifidobacterium are considered beneficial for the normal health of the digestive tract, present in large numbers in the feces of infants, especially breast-fed babies, and are usually found in the large intestine. - What are the key species of dairy Propionibacterium?
The four species of dairy Propionibacterium are Propionibacterium freudenreichii, P. jensenii, P. thoenii, and P. acidipropionici, associated with natural fermentation of Swiss-type cheeses. - What role does Brevibacterium linens play in cheese production?
Brevibacterium linens is used in cheese ripening as it has extracellular proteases and is capable of growing in high salt and wide pH ranges. - How is Acetobacter used in food production?
Acetobacter aceti is used to produce acetic acid from alcohol, oxidizing ethanol to acetic acid and lactic acid to CO₂ and H₂O. - What distinguishes top yeasts from bottom yeasts in Saccharomyces cerevisiae?
Top yeasts grow rapidly at 20°C, form clumps, and float due to rapid CO₂ production, while bottom yeasts grow better at 10 to 15°C, grow slowly, do not clump, and settle at the bottom. - Which molds are used in cheese ripening and what are their specific applications?
Penicillium roquefortii is used for ripening Roquefort, Gorgonzola, and blue cheeses; P. camembertii for Camembert cheese; and P. caseicolum for Brie cheese. - What beneficial products are derived from Aspergillus species?
Aspergillus oryzae is used in fermentation of oriental foods like sake, soy sauce, and miso, and as a source of food enzymes; A. niger produces citric acid, gluconic acid, pectinase, and amylase.
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