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Chapter 10

FOOD2010 - Unit 5 Chapter 10.docx

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University of Guelph
Food Science
FOOD 2010
Massimo Marcone

UNIT 5: Chapter 10 – Food Microbiology and Fermentation 10.1 What are Microorganisms? - Microorganisms are living entities that are too small to be seen with the naked eye - Consists of bacteria, viruses, protozoa, and fungi such as yeasts and molds - Primary function of microorganisms is self-perpetuation - Many microorganisms utilize organic matter (CHO, proteins, lipids, etc) to form inorganic compounds (nitrates, sulfates, etc) - Parasites and viruses depend on a living host for nutrients and to carry out metabolic reactions required for growth - All living organisms are classified as either procaryotes or eucaryotes o Procaryotes = no nucleus (includes bacteria’s) o Eucaryotes = contain a nucleus (includes fungi, protozoa, plants and animals) - VIRUSES ARE NEITHER because they are noncellular (they are considered a life form) - Microorganisms have scientific names consisting of a genus name and a species name - Organisms belonging to the same genus share one or more prominent phenotypic, or characteristic - Organisms belonging to the same species share many phenotypic characteristics as well as being genetically very similar (with at least 70% similarity in their nucleic acid material, i.e. RNAor DNA) - Bacteria are unicellular organisms - Found just about everywhere in nature (soil, air, water and the intestinal tract of mucous membranes of animals and humans) - Divided into gram-positive and gram-negative cells - Gram staining – classifying cells based on whether they retain crystal violet in their cell membrane or not - Gram-negative bacteria have thin cell walls and an outer membrane - Gram-positive bacteria have thick cell walls and no outer membrane - Bacteria are also classified according to their shape: the spherical coccus, rod- shaped bacillus and the cell with twists- the spirillum - Coccus shaped organisms can also occur in the diplococcus arrangement, where two cells remain attached after cell division - Rod-shaped organisms include most disease-causing bacteria - Some bacteria are able to develop into spores by coating their membrane and cell wall with extra layers of material in a process called sporulation - Spore development often occurs as a response to unfavourable growth conditions, such as lack of nutrients or lack of water - In this state, bacteria are able to significantly increase their survival to processing treatments such as heating, drying and irradiation - They are in a pseudodormant condition, unable to grow or divide - Once conditions become favourable again, or if exposed to a short heat treatment, the spores germinate into vegetative cells, resuming growth and metabolic activity - Fungi include two types of microorganisms o Molds – are multi- or unicellular and found in decaying matter  Grow in the form of tangled mass called mycelium  Molds of importance in foods multiply by spores known as conidia o Yeasts are unicellular and can grow over a wide range of conditions - Protozoa are single-celled eucaryotes o Protozoa of interest are parasites o Do not grow in food but require at least one animal host to carry out their life cycle o Most are phagotrophic- able to ingest particulate food o One cyst yields several trophozoites – motile parasites that penetrate the small intestine of the infected animal - Viruses are obligate parasites and host-specific - Ones associated with food are typically RNA-containing viruses - Attach to the host cell by receptors and then either inject their nuclear material into the host or become engulfed by the host - Once inside the host, the virus nucleic acid is replicated using the host’s enzymes and virus particles are synthesized - Most food viruses are considered temperate viruses which insert their nucleic acid into the host’s DNA, leaving the host cell intact 10.2 FactorsAffecting Microbial Growth Nutrient Availability - Most nonparasitic organisms can be classified as chemotrophs or phototrophs - Chemotrophic – organisms require chemicals for metabolism - Chemotrophs can be subdivided into: o Lithotrophic – require inorganic compounds such as minerals o Organotrophic – require organic compounds such as carbohydrates - Phototrophic – require energy in the form of light to live - Nutrient needs depend on the organism and on other factors such as temperature WaterActivity - Bacteria have stricter requirements than other organisms - Need quite a bit of water to survive, as measured in terms of water activity – the amount of water available for microbial growth - Most bacteria require a minimum water activity of 0.90, yeasts = 0.80, and mold = 0.70 - At low water activities, microorganisms die because water inside the cell diffuses out in an effort to balance the osmotic pressure - This migration results in cell death due to dehydration shrinkage Acidity/Alkalinity - Many bacteria cannot survive at pH values less than 4.5 with a preference of values near neutrality - Yeasts can live at pH 3.0 with a maximum of 8.5 - This broad range enables yeasts to survive in environments not suitable for most bacteria - Molds can tolerate even more extreme pH values than yeasts with tolerance to pH between 2.0 to 11.0 - The pH alters a microorganism’s ability to transport molecules in or out of the cell through the cell membrane - In an acidic environment of low pH, protons saturate the membrane making it difficult for cations to move in or out - At high pH, hydroxyl ions saturate the membrane, preventing the movement of anions in or out of the cell - In addition, proteins which are very sensitive to pH changes are denatured and precipitate out of solution Oxygen - Amount of oxygen in the environment is also a crucial survival/growth factor for microorganisms - This is talking about the oxidation-reduction potential of that medium - Redox potential – depends on the ratio of total oxidizing (electron-accepting) molecules to the total reducing (electron-donating) molecules in the medium - Oxidized environment means that the molecules have a relatively high affinity for electrons - Reduced environment means that the molecules have a low affinity for electrons - Molds are aerobes requiring oxygen to be present - Some bacteria, notably the ones that cause food spoilage, are also strict aerobes - Most bacteria that cause disease are facultative anaerobes – they prefer aerobic but have the capability of growing even if oxygen is not present - Microaerophilic – require some oxygen to be present but cannot tolerate the levels present in aerobic environments, usually 21% oxygen - Anaerobes cannot tolerate any oxygen, requiring the environment to be completely reduced - Aerobic microorganisms die due to lack of oxygen because of their inability to produce adenosine triphosphate (ATP) - Anaerobic microorganisms die due to too much oxygen because of their inability to remove toxic oxygen-derived radicals such as superoxide radical from the cell Temperature - Today we classify microorganisms into five categories according to their ability to tolerate specific temperatures - Maximum growth temperature: the temperature that causes inactivation of an organism’s enzymes and structure damage to the extent that these outbalance the enhanced ability to synthesize new cell material - Optimum growth temperature: the temperature that corresponds to the shortest generation time (time it takes for the cells to divide), usually a matter of minutes - Minimum growth temperature: is the temperature corresponding to the longest generation time, usually exceeding one thousand minutes - Psychrophiles: are organisms that prefer low temperature - Psychroptrophs: organisms that prefer high temperatures but can grow at low temperatures - Thermotrophs: tolerate high temperatures - Thermophiles: prefer high temperatures - Mesophiles: a classification to which most disease-causing microorganisms belong, cannot tolerate extremes of temperature, preferring the levels found in the tissues of humans and animals - Cell death due to low temperatures occurs because of slowing down of reaction rates during metabolism and because of a decrease in cell membrane fluidity, which slows down transport of nutrients into the cell - Death due to high temperatures occurs because of inactivation of enzymes, as well as denaturation of cell structural components Food Effects - Foods that are high in protein can exert a buffering effect, such that microorganisms are able to live and grow in the food even if the pH is below the minimum levels necessary for survival - Certain components and characteristics of foods can prevent a change in the redox potential of the food in spite of the oxygen content of the atmosphere in which it is packaged o This is called the poising effect and it depends on the presence of reducing compounds such as some sugars, the ability of food tissue such as fruit and vegetables to use oxygen and the pH of the food - The more alkali the pH, the more negative is the redox potential, thus the more reduced and anaerobic the food - Some microorganisms can inherently survive extremes of temperatures through special abilities Using the Hurdle Concept - Nonlethal levels of the various factors discussed above can be used in combination to inhibit or reduce microorganisms in foods - For example, combining the absence of oxygen, such as in vacuum packaging, with refrigeration can inhibit the growth of aerobic bacteria commonly involved in spoilage of fresh meats - In this example, lack of oxygen and low-temperature storage act as two hurdles to impede bacterial growth - The hurdle concept is more effective when several hurdles are applied 10.3 Foodborne Microorganisms Sources of Microorganisms - Gusts of wind pick up organisms from the soil and m
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