The science that deals with the microorganisms involved in the spoilage, contamination, and preservation of food.
2.Explain how food serves as a substrate for micro organisms. What are the intrinsic factors of food that influence microbial growth?
Food and microorganisms
RELEVANCE OF MICROBIOLOGY
Food serves as a interacting medium between various living species because, it is a source of nutrients for humans, animals as well as microorganisms. Food fit for human consumption is also a medium for the growth and activity of microorganisms. Hence human food is always associated with a variety of microorganisms. Since the primary function of microorganisms is self-perpetuation, they use the human or animal food as a source of nutrients for their own growth and activity. Microbial activity in a food can be beneficial in certain cases it leads to deterioration of the food and renders it unfit for human consumption. Four aspects of microbial activity are of relevance to processing and preservation of food.
Food chemicals from microorganisms
Food poisoning & food borne diseases &
Microorganisms can be used as processing aids in the production of fermented foods. New and modified foods with better shelf stability; palatability, flavour and organoleptic properties are produced by fermentation using specific microorganisms under controlled conditions.
Food chemicals from microorganisms:
A variety of food chemicals and additives produced by fermentation involving select species of microorganisms In addition, microorganisms themselves may be used as food. The biomass produced by fermentation can be harvested & used as a protein rich raw material for the formulation of foods.
Food poisoning & food borne diseases:
Pathogenic microorganisms grow in the food utilizing the nutrients in the food & produce toxins, which are detrimental to the health of the consumer when such food is consumed. Food also serves as a vector or medium for certain pathogens that cause food infections & diseases.
The metabolic activity of various microorganisms not only utilizes the nutrients in food but also causes the spoilage of food through undesirable enzymatic changes affecting the quality of the food. The enzymatic changes include the formation of products, which contribute off-flavours & affect the organoleptic, textural and keeping qualities of food.
BACTERIA, YEAST AND MOULD:
Thousands of genera and species of microorganisms have been identified and classified. Several hundreds of these are associated in one way or other with food products. Many of them are of industrial importance as they find use in the production of new foods and food chemicals by fermentation and also in the preservation of food products. Microorganisms that are of importance in food microbiology include bacteria, yeast and molds.
Bacteria are unicellular organisms of aerobic or anaerobic nature and exhibit many morphological forms. Three principal shapes have been well recognized, namely, spherical shapes of cocci, rod shape of bacilli and spiral form of Spirilla and Vibrios.
All bacteria associated with food are small in size, typically a few micrometers long and smaller in diameter. Bacteria form spores, which are seed like and far more resistant to heat, presence of inhibitory chemicals and other adverse conditions during food processing. Most bacteria multiply best at temperatures between 16 and 38C and are termed mesophilic. Psychrotropic or psychrophilic bacteria can grow at low temperatures while thermophilic ones can grow at higher temperatures.
Morphological characteristics important in food bacteriology:
One of the first steps n the identification of bacteria in food is microscopic examination to ascertain the shape, size, aggregation, structure and staining reactions of the bacteria present. The following characteristics may be of special significance.
The presence of capsules or slime may account for sliminess of ropiness of a food. In addition, capsules serve to increase the resistance of bacteria to adverse conditions such as heat or chemicals. To the organism they may serve as a source of reserved nutrient.
FORMATION OF ENDOSPORES:
Bacteria of the genera Bacillus, Clostridium, Desulfotomaculum, Sporolactobacillus and Sporosarcina share the ability to form endospores. Of primary interest to the food microbiologists are the spore forming species of the genera Bacillus and Clostridium. Endospores are formed at an intracellular site, are very refractile, and are resistant to heat, ultraviolet light and desiccation.
Sporulation usually appears in the late logarithmic phase of growth, possibly because of nutrient depletion or product accumulation. During this transition of vegetative cell to spore, the spore become refractile, there is a massive uptake of Ca ions, and synthesis of dipicolinic acid(DPA) occurs, a compound absent from vegetative cells. The acquisition of heat resistance by the forming spore is closely correlated to the formation of DPA and the Ca 2+ uptake.
FORMATION OF CELL AGGREGATES:
It is characteristic of some bacteria to form long chains and of others to clump under certain conditions. It is more difficult to kill all bacteria in intertwined chains or sizable clumps than to destroy separate cells.
Cultural characteristics important in food bacteriology:
Bacterial growth in and on foods often is extensive enough to make the food unattractive in appearance or otherwise objectionable. Pigmented bacteria cause discolorations on the surface of liquids, growth may make surfaces slimy, or growth throughout the liquids may result in undesirable cloudiness or sediment.
Physiological characteristics important in food bacteriology:
The bacteriologist is concerned with the growth and activity of bacteria and other organisms in food and with the accompanying chemical changes. These changes include hydrolysis of complex carbohydrates to simple ones, hydrolysis of proteins to polypeptides, amino acids and ammonia or amines and hydrolysis of fats to glycerol and fatty acids.
O.R rxns, which are utilized by the bacteria to obtain energy from foods, yield such products as organic acids, alcohols, aldehydes, ketones and gases. A knowledge of the factors that favour or inhibit the growth and activity of bacteria is essential to an understanding of the principles of food preservation and spoilage.
Genera of Bacteria important in food Bacteriology:
Bacteria that play significant roles in foods are often grouped on the basis of their activity in foods without regard to their systemic classification.
Lactic acid bacteria ferment sugars to lactic acid and include species belonging to genera of Leuconostoc, lactobacillus, streptococcus and pediococcus. Their activity is desirable in a variety of foods such as sauerkraut and other pickled vegetables and dairy products for the production of flavour. They cause spoilage of wines.
Acetic acid bacteria oxidize ethanol to acetic acid. Species of genera Acetobacter and Gluconobacter are the most common. They are useful in vinegar manufacture but ate undesirable in alcoholic beverages.
Butyric acid bacteria are mostly the spore forming anaerobes of the genus clostridium. They produce butyric acid by fermenting sugars. Propionic acid bacteria produce propionic acid andbelong the genus propioni bacterium.
Proteolytic bacteria include a heterogenus group of bacteria, which produce extracellular proteases. Most species belonging to the genera of clostridium, Bacillus, pseudomonas and proteus.
Lipolytic bacteria are also a heterogeneous group of bacteria, which produce lipase. Organisms of the genera pseudomones, Alcaligens, Staphylococcus, Serratia and Micrococcus are lipolytic.
Saccharolytic bacteria ydrilyze disaccharides and polysaccharides to sampler sygas. Ex: Bacillus subtilis and Clostridium butyrium, which are also amylolytic.
Pectinolytic bacteria produce pectinases responsible for softening of plant tissues of loss of getting power in various plant foods. Ex: Bacillus, Achromobacter, Aeromonas, Arthrobacter and Flavobacterium are pectinolytic.
Thermiphylic and thermoduric bacteria are resistant to high temperature. Thermophilic bacteria are resistant. Cause spoilage of low acid canned foods. Important species include Bacillus steresthermiphilus and with thermosaccharoluticum. Thermiduric organisms survive heat treatment such as pasteurization. Ex: Bacillus, clostridium,Micrococcus streptococcus Lactobacillus and Mycobacterium are found in foods.
Psychrotrophic bacteria are able to survive and grow at refrigeration temperatures through their optimum temperatures of growth in around 20 to 30oc. Ex: Pseudomonal, Achromobacter, Alcaligenes and Flavobacterium are psychrotophic.
Halophilic bacteria include species of the genera Bacillus, Micrococcus, Vibrio, Moraxella, Halobacterium organisms require certain minimal concentrations of dissolved Nacl for their growth and survive at higher concentration of salt.
Osmophilic or saccharophilic bacteria grow in high concentrations of sugar. Ex: Leuconostoc species.
Pigmented bacteria produce colours during their growth in foods. Ex: Flavobacterium (yellow to orange), Serratia(red),Halococcus (red to orange) and Halobacterium (pink, red and orange), Lactobacillus plantarum produces rust colour pigment discolouring cheese. Flavobacterium species causes discolouration on the surface of meat and spoilage of Shell fish, poultry, eggs, butter and milk.
Slime or rope forming bacteria include Alcaligenes viscolactis, Enterobacter aerogenes and Klebsiella oxytoca and some species of streptococcus and Lactobacillug plantarum causes ropiness or ropiness in milk.
Gas forming bacteria include species of the genera of Leuconostic, Lactobacillus and propionibacterium which produce carbon-di-oxide species of Eschericbis, Enterobacter proteus, bacillus and Clostridium produce both carbon-di-oxide and hydrogen.
Off-flavour forming bacteria include those of genus streptomyces which produc undesirable flavours and musty of earthy odour and taste. Manu species of pseudomonas produce a variety of metabolites that affect the flavour of foods deleteriously.
Coliform bacteria ex. Escherichia coli and Enterobacter aerogenes. They cause spoilage of a variety of foods producing off-flavours and sliminess.
Some of the important disease causing bacteria include the following:
Which batulinum produces a neutotoxin in canned meat products and causes the fatal disease botulism.
Corynebacterium species includes the diphtheria organism which diptheriae.
Erwinia species are plant pathogens and damage plants and plant products causing bacterial soft rot.
E. C oli species includes some serotypes which are pathogenic to humans.
Myco bacterium species includes the tubercle bacilli M. tuberculosis that causes tuberculosis especially through raw milk from infected cows.
Salmonella species are enteric pathogens that grow in foods and cause food infection.
Shigella species is transported by foods and causes bacittary dysentery.
Staphylococcus species includes the important S.aureus which produces an enterotoxin causing food poisoning.
Streptococcus species includes pathogenic S.agalartide which causes mastitis in cows and S.pyogenes which causes septic sore throat, Scarlet fever and other diseases in humans.
Vibrio species is pathogenic to humans. 3.EXPLAIN ABOUT THE GENERAL CHARACTERISTICS, CLASSIFICATION AND IMPORTANCE OF MOULDS, YEAST AND BACTERIA
GENERAL CHARACTERISTICS OF MOULDS IN FOOD MICROBIOLOGY. Moulds are multicellular , filamentous fungi whose growth is recognized by its fuzzy or cottony appearance.
They may be of white, coloured or dark or smoky.
The thallus or vegetative body is characteristic feature of moulds.
HYPHAE AND MYCELIUM:
Hyphae à tubular, filamentous structure
Mycelium à interwined hyphae
Submerged hyphae à growing within the food
Aerial hyphae à growing into air above the food
Sclerotia à tightly packed masses of modified hyphae, often thick-walled. à More resistant to heat