Food Microbiology and Fermented Food PDF

Food Microbiology and Fermented Food 1. What is microorganisms? l Small living entities l Bacteria, viruses, protozoa and fungi l Utilize organic matter to form inorganic compounds to form inorganic compounds, not parasites and viruses l Eukaryotes vs prokaryotes: with vs without true nucleus l...

Food Microbiology and Fermented Food 1. What is microorganisms? l Small living entities l Bacteria, viruses, protozoa and fungi l Utilize organic matter to form inorganic compounds to form inorganic compounds, not parasites and viruses l Eukaryotes vs prokaryotes: with vs without true nucleus l Scientific name: genus and species names E.g. Escherichia coil 大腸桿菌 Bacteria l Unicellular organism l 1 μm in length l Found everywhere [ubiquitous] l Crystal violet retention: G+ or G- cells l Different shapes: coccus, bacillus, spirillum l Sporulation- Response to unfavorable conditions Factors affecting microbial growth l Nutrients è Chemotrophic [lithotrophic vs organotrophic] VS Phototrophic è Predominant organisms in a food imply ideal conditions for them to survive (pectinolytic organisms)d è Depend on other bacteria to survive l Water activity (aw) è Bacteria need more water than other organism è Water activity - A measure of the water availability for growth - Measure the unbound water >bacteria 0.9 >yeasts 0.8 >molds 0.7 è Death if aw too low l Acidity/Alkalinity è Medium needs to be at a pH range è Extreme survival pH for some - Bacteria 4.5-9.0; optimal 6.5-7.5  - Yeasts 3.0-8.5; optimal 4.0-6.5 - Molds 2.0-11.0; optimal 4.5-6.8 è pH - Affects molecule transportation in/out of cell membrane (Low pH, cell membrane positively charged, vice versa) - Affects protein -> denature/precipitate -> disrupt cell metabolism l Oxygen è Aerobes: requires oxygen to be present; require oxidized conditions E.g. molds è Facultative anaerobes: prefer aerobic environment, but can grow even without oxygen/reduced environment for disease-causing microbe E.g. Salmonella spp è Microaerophilic: require some oxygen, but not high level E.g. Campylobacter, 6% è Anaerobes l Temperature è Classes - Psychrophiles 10-15C - Mesophiles 30-37C - Thermophiles 50-80C è Cell death - Low temperature: metabolism, fluidity decrease - High temperature: enzyme inactivation, denaturation l Food Effects è Nature of food has effect on survival and growth of microbes è High protein food -> buffering effect - E.g. Temperature - High: shielding effect from high protein/fat food - Low: switch metabolism to produce more unsaturated and short-chain fatty acids l Hurdle Concept è Hurdles: refer to factors which interrupt with the growth of microorganism è E.g. oxygen, temperature, pH, etc. è More hurdles, more effective Sources of Microorganisms l Found everywhere in nature l Air near earth surface or at high altitude l Land, lakes and oceans l Shallow or deep water l In soil: less active, poor environment, mostly spore-former l Human colon Sources of Microbial Flora in Foods l Associated with raw food l Acquired during handling and processing of food l Survived the preservation and storage treatments applied to food l Types of microorganisms in food è Spoilage ones è Pathogenic ones è Useful ones 1. Muscle foods l Fresh cut: 10000 total organisms/g l Interior: sterile l Sources of contamination è Beef or pork - Spoilage: psychrotrophic aerobes - Pathogen: Salmonella è Seafood: - Cold waters, Pseudomonas spp.; - Warm waters, Micrococcus, Bacillus spp. - Pathogen: Vibrio; Aeromonas spp. 2. Fruits and Vegetables l Low pH l Acid-tolerate types: Lactorbacillus, Leuconostoc spp. l Predominant: lactic acid bacteria l Soil: spore-forming organisms, E.g. Bacillus and Clostridium spp. 3. Dairy l Lactic acid bacteria in unpasteurized milk l Pathogens: Mycobacterium, Coxiella, Listeria l Psychrotrophic organisms: Pseudomonas, etc. during packaging Food Spoilage- 1. Break carbohydrates [Complex] l Pectin (果膠) è Polysaccharide in many fruits è 1,4-linkages in galacturonic acid in fruits è Pectinase - Action against fruit, vegetable rot è In celery and carrot - Soft rot :Bacillus polymyxa, Erwinia carotovora è In citrus fruits - Penicillium citrinum l Cellulose è In vegetables è Glucose with β-1,4-linkages è Cellulase - Bacillus - Aspergillus - Penicillium spp. Break Carbohydrates [Simple] l Lactose -> Lactic acid -> sour odor è Lactobacillus l Simple sugars -> ethanol -> good for wine è Saccharomyces cerevisiae Synthesize Carbohydrate [Complex] l Form dextrans (glucose with α-1,6-linkages) è Leuconostoc mesenteroides è Bacillus megaterium l Form levans (fructose with β-2,6 bonds) è Bacillus megaterium l Ropy consistency for spoiled fruit juices l Slime layer on fruits and vegetables Food Spoilage- 2. Break protein l Protein in meat -> Amino acids è Clostridium Laramie Anaerobic bacterium è Causing sour odor and flavor l Amino acids -> foul-smelling compounds è Lysine -> cadaverine (屍胺) è Ornithine -> putrescine (腐胺) è Alanine -> pyruvate + ammonia è Tryptophan -> indole + ammonia è Cysteine -> hydrogen sulfide Microbial Fermentation Milk Fermentation (Cheese) l Add lactic acid bacteria l 20C, pH 4.6, Protein ppt -> uniform curd l Add enzyme chymosin (rennet) l Rennet coagulation: Casein -> gel (Calcium phosphate paracaseinate) l Combination of bacteria + rennet -> softer coagulate Yogurt l Whole milk l Heat at 85C, 30 mins; cool to 43C l Inoculate Lactobacillus delbrueckii + Streptococcus thermophilus (1:1) l Incubate at 43.3C, 6 hours l pH to 4.2; growth stops Cereal Grain Fermentation l Beer è Barley, rice, corn as raw materials è Germinated barley – activated enzyme è Cooked to form a mash and wort (38C-77C) è Hops added è Inoculated with yeast Saccharomyces cerevisiae è pH drops to 4.0 at 10C in 9 days è Products: ethanol and carbon dioxide Important messages Microorganisms such as bacteria, yeasts, molds can be found in many places, including air, soil, water, and the intestinal tract of humans and animals. Factors that affect microbial growth include nutrient availability, aw, pH, oxygen content, temperature, and the nature of the food they live in. Microorganisms found in foods originate in the raw product and are acquired through handling, processing, and storage. Carbohydrates are spoiled by lactic acid bacteria, which break them down into simple sugars and metabolize them to produce malodorous compounds and lower the pH. Proteins are spoiled anaerobically by Pseudomonas, Acinetobacter, and molds to produce off-odor compounds Fermentation is the biochemical process by which individual sugar molecules are metabolized by microorganisms to produce various by-products, including lactic acid. Lactic acid bacteria are often used in fermentation reactions to lower pH. Milk is fermented by bacteria to produce cheese and yogurt, among other products; and vegetables like cabbage are fermented to produce fermented foods such as sauerkraut. Yeasts are used to ferment grain products such as barley to produce beer by the formation of ethanol from glucose.