Yeast: Origin, Reproduction, Life Cycle and Growth Requirements | Industrial Microbiology, How is Bread Made Step by Step? Microbial Growth There are a number of factors that affect the survival and growth of microorganisms in food. The reactions which take place in the cytoplasm do so in an aqueous environment and the cytoplasm is surrounded by a membrane which is generally permeable to water molecules which may pass freely from the cytoplasm to the environment and from the environment to the cytoplasm. This is why they grow on foods with high moisture content such as chicken. Although microbial growth can occur over a wide spectrum of redox potential, individual micro-organisms are conveniently classified into one of several physiolo­gical groups on the basis of the redox range over which they can grow and their response to oxygen. It is a relatively simple matter to determine the water content of a food commodity by drying to constant weight under defined conditions. Consequently they have a requirement for oxygen and a high Eh and will predominate at food surfaces exposed to air or where air is readily available. This can be done by the dew point method or with a hair hygrometer. If the balance is reversed, the sample will tend to donate electrons to the electrode which wall then register a negative potential – a reducing environment. Factors affecting microbial growth in food • Intrinsic Factors • Environmental Factors • Implicit Factors • Processing Factors A most important contribution to the slowing and eventual cessation of microbial growth at low temperatures is now considered to be changes in membrane structure that affect the uptake and supply of nutrients to enzyme systems within the cell. Other contributory factors are thought to include changes in the physical properties of the plasma membrane adversely affecting solute transport; inhibition of key enzymes, particularly those involving carboxylation/decarboxylation reac­tions in which CO2 is a reactant; and reaction with protein amino groups causing changes in their properties and activity. In food microbiology mesophilic and psychrotrophic organisms are generally of greatest importance. Food provides energy and nutrients for bacteria to grow. With a reduction of water activity in their environment the number of groups of micro-organisms capable of active growth decreases (Table 3.9). Preserving agents that increase the acidity of food, such as citric acid, are commonly added to help prevent bacterial growth and allow for longer storage. The widespread use of food products such as meat digests (peptone and tryptone), meat infusions, tomato juice, malt extract, sugar and starch in microbiological media bears eloquent testimony to their suitability for this purpose. Log phase 3.1. These factors are the environmental factors that are implicated in food spoilage occurrences. Factors Affecting Microbial Growth Food. It has been shown that many micro-organisms respond to growth at lower temperatures by increasing the amount of unsaturated fatty acids in their membrane lipids and that psychrotrophs generally have higher levels of unsaturated fatty acids than mesophiles. -- Created using Powtoon -- Free sign up at http://www.powtoon.com/youtube/ -- Create animated videos and animated presentations for free. The factors influencing the growth of microorganisms are physical, chemical and biological in nature. Foods may contain multiple microenvironments. Some of these changes, like those taking place during fermentation, are desirable, while others, like those resulting in food spoilage and food poisoning, are undesirable. Moulds and oxidative Gram-negative bacteria are most sensitive and the Gram positive bacteria, particularly the lactobacilli, tend to be most resistant. Risk factors affecting microbial safety of foods in catering establishments. If provided with the optimum conditions for growth, bacteria can multiply to millions over a small period of time via. A large positive E0‘ indicates that the oxidized species of the couple is a strong oxidizing agent and the reduced form only weakly reducing. Factors affecting microbial growth in food Processing factors • Processing factors: • Slicing • Washing • Packing • Irradiation • Pasteurization 43. particularly protein foods such as chicken and dairy products are rich in nutrients and moisture and so promote bacterial growth. Carbon dioxide is not uniform in its effect on micro-organisms. Also, like buffering, poising is greatest when the two components of a redox couple are present in equal amounts. A useful parameter which helps us to understand the movement of water from the environment to the cytoplasm or from the cytoplasm to the environment is water activity, aw. Maintaining work equipment and the physical state of a building is important, but there are potential threats lurking in a company’s infrastructure that are too small for the naked eye to see. Micro-organisms can be classified into several physiological groups based on their cardinal temperatures. As a second line of defence, the product tissues may contain antimicrobial components, the local concentration of which often increases as a result of physical damage. A look at the campylobacter, E. coli, listeria, salmonella and staphylococcus aureus bacteria that cause food poisoning, as well as the conditions that allow bacterial growth. This can kill Gram-negative bacteria and inhibit Gram-positives, possibly by damaging the bacterial cytoplasmic membrane (Figure 3.5). Although each factor affecting growth is considered separately in the following discussion, these factors … The same water content seems to be associated with a higher aw, in the former case than in the latter. and minerals 3. Many of the same or similar factors can also be found in milk where they are present in lower concentrations and are thus less effective. Carbonic acid is a weak dibasic acid (pKa 6.37 and 10.25); in an un-buffered solution it can produce an appreciable drop in pH, distilled water in equilibrium with the CO2 in the normal atmosphere will have a pH of about 5, but the effect will be less pronounced in buffered food media so that equilibration of milk with 1 atmosphere pCO2 decreased the pH from 6.6 to 6.0. Presence of different gases and its varying concentration may significantly affect the colonizing mos on the food i.e.surface spoilage is prevented by altering the gaseous composition.Oxygen is one of the most important gases which affects both food products as well as Mos.Oxygen gas when comes in contact with food, influence redox potential of food and finally the microbial growth. They are obligately halophilic because the integrity of their outer wall depends on a high concentration of sodium chloride in their environment. The relationship between water activity and water content is very sensitive to temperature and may seem to depend on whether water is being added or removed from a substrate. ability to participate in chemical/biochemical reactions, and its. Read about our approach to external linking. Such a situation can occur in grain silos or in tanks in which concentrates and syrups are stored. The extrinsic factors are those factors that interact with the microbial activities that normally go on in food or food products. 3. The mechanism of CO2 inhibition is a combination of several processes whose precise individual contributions are yet to be determined. Oxygen comprises 21% of the earth’s atmosphere and is the most important gas in contact with food under normal circumstances. It is usually composed of macromolecules relatively resistant to degradation and provides an inhospitable environment for micro-organisms by having a low water activity, a shortage of readily available nutrients and, often, antimicrobial compounds such as short chain fatty acids (on animal skin) or essential oils (on plant surfaces). Bacillus subtilis (Eh — 100 to + 135 mV) produces rope in the open texture of bread and Acetobacter species growing on the surface of alcoholic beverages, oxidize ethanol to acetic acid to produce either spoilage or vinegar Obligate anaerobes tend only to grow at low or negative redox potentials and often require oxygen to be absent. Because low water activities are associated with three distinct types of food three terms are used to describe the micro-organisms especially associated with these foods: Able to grow in the presence of high concentrations of salt, Able to grow in the presence of high concentrations of unionized organic compounds such as sugars. The value of these physical barriers can be clearly seen when they are breached in some way. Iron is an essential nutrient for all bacteria and many have evolved means of overcoming iron limitation by producing their own iron-binding compounds known as siderophores. They obtain their energy through aerobic respiration. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in … It is in such regions that propagules which have remained viable, but unable to grow, may now germinate and grow. Solutes and Water Acidity 2. Growth of microorganisms in food is dependent on various parameters. The following points highlight the six main physical factors affecting the growth of microorganisms. Thus at 25 °C (298 °K) a water activity of 0.9 would correspond to a water potential of -143 atm or — 14.5 MPa. If the solvent A is water then Equations (3.18) and (3.19) can be combined to give: Thus for an aqueous solution the water activity is approximately given by the ratio of the number of moles of water to the total number of moles (i.e. Food. Physical chemists would prefer to work with the chemical potential of water (µw), which is a complex parameter made up of a reference state, a water activity term, a pressure term and a gravitational term: which can be rearranged to give a new parameter, Ψ, known as the water potential having the same dimensions as pressure: For situations associated with everyday life on the surface of the Earth it is possible to ignore the pressure and gravity terms and a good approximation of the relationship between the water potential and water activity is given by Equation (3.24): where R (the gas constant )= 0.08205 dm3atm K-1 mol-1; and Vm (the molar volume of water) = 0.018 dm3 mol-1 ‘. A somewhat more esoteric example, which many would take as convincing evidence of the inedibility of alkaline foods, is fermented shark, produced in Greenland, which has a pH of 10-12. Like us, micro-organisms can use foods as a source of nutrients and energy. Thus they increase the water activity of their own immediate environment so that eventually micro-organisms requiring a high aw are able to grow and spoil a food which was initially considered to be microbiologically stable. Psychrotrophs or facultative psychrophiles will grow down to the same low temperatures as strict psychrophiles but have higher optimum and maximum growth temperatures. Increasing the degree of unsaturation in a fatty acid decreases its melting point so that membranes containing higher levels of unsaturated fatty acid will remain fluid and hence functional at lower temperatures. as it is dangerous for some foods to be in this temperature range for prolonged periods of time. Microbial growth in foods is very complex and diversified, which is governed by biochemical, environmental, and genetic factors along with their nutritional class. No of cells produced is equal to the no. A rather different example of the importance of plant antimicrobials is provided by oleuropein, the bitter principle of green olives. Disclaimer Copyright, Share Your Knowledge Most bacteria reproduce best at a neutral pH level of 7. Since under redox potential, this will be confined to the microbiological effects of other gases commonly encountered in food processing. When the pH is equal to an acid’s pKa then half of the acid present will be un-dissociated. Able to grow on dry foods. Another problem in large-scale storage units such as grain silos occurs because the relative humidity of air is very sensitive to temperature. Bacteria need warmth to grow. Start studying Factors affecting microbial growth in foods. Many essential cell functions such as ATP synthesis in bacteria, active transport of nutrients and cytoplasmic regulation occur at the cell membrane and are dependent on potential energy stored in the membrane in the form of a proton motive force. Hence the intrinsic factor of redox potential is inextricably linked with the extrinsic factor of storage atmosphere. Anaerobic metabolism gives the organism a lower yield of utilizable energy than aerobic respiration, so a reducing environment that minimizes the loss of valuable reducing power from the microbial cell is favoured. Many natural constituents of plant tissues such as pigments, alkaloids and resins have antimicrobial properties, but limited practical use is made of these. Name the types of nitrogenous bases present in the RNA. Microbial growth on meat products, as well as other foods, is affected, not only by the type and level of initial contamination but also by various factors associated with the product (intrinsic) or its environment (extrinsic). Destruction or weakening of this layer causes the cell to rupture (lyse) under osmotic pressure. The pH of the food also significantly impacts the lethality of heat treatment of the food. As a rule mesophiles grow more quickly at their optima than psychrotrophs and so spoilage of perishable products stored in the mesophilic growth range is more rapid than spoilage under chill conditions. The growth of microorganisms in the body, in nature, or in the laboratory is greatly influenced by temperature pH, moisture content, available nutrients, and the characteristics of other organisms present. Water activity can be measured by measuring the equilibrium relative humidity of the atmosphere in contact with the sample. Some yeasts such as Brettanomyces spp. The exact range of water activities allowing growth is influenced by other physicochemical and nutritional conditions but Figure 3.7 illustrates the range for a number of individual species of micro-organisms and Figure 3.8 demonstrates the interaction between temperature and water activity for Aspergillusflavus and Penicillium expansum. The growth rate decrease due to shortage of growth factors such as vit. Growth inhibition is usually greater under aerobic conditions than anaerobic and the inhibitory effect increases with decrease of temperature, presumably due to the increased solubility of CO2 at lower temperatures. Foods that are dehydrated or freeze-dried can be stored for much longer as the moisture has been removed. This hysteresis phenomenon is a reflection of the long time that it may take for water to equilibrate with the constituents of a complex food matrix. The fact that this is not observed in practice is, on reflection, hardly surprising since microbial growth results from the activity of a network of interacting and interrelating reactions and represents a far higher order of complexity than simple individual reactions. Oxygen, which is present in the air at a level of around 21%, is usually the most influential redox couple in food systems. 0.5 MPa) in a Gram-negative species. Temperature 3. pH 4. Consequently it is important to the farmer and food processor that harvesting and transport maintain these barriers intact as far as possible. Methylene blue is also used to determine the proportion of viable cells in the yeast used in brewing. The latter might be particularly important in discussions about the availability of water in a complex matrix such as cake. Share Your PPT File. The first of these is the integument; a physical barrier to infection such as the skin, shell, husk or rind of a product. Analysis of their volatile flavour and odour fractions, known as essential oils, has frequently identified compounds such as allicin in garlic, eugenol from allspice (pimento), cloves and cinnamon, thymol from thyme and oregano, and cinnamic aldehyde from cinnamon and cassia which have significant antimicrobial activity (Figure 3.4). Factors that. They have the potential to grow wherever conditions are anaerobic such as deep in meat tissues and stews, in vacuum packs and canned foods causing spoilage and, in the case of C. botulinum, the major public health concern: botulism. If the sample has a lower aw than the atmosphere then it will gain weight, if it has a higher aw then it will lose weight. 4. Humulones contained in the hop resin and isomers produced during processing, impart the characteristic bitterness of the product but have also been shown to possess activity against the common beer spoilage organisms, lactic acid bacteria. As with all generalizations there are exceptions, particularly among those bacteria that produce quantities of acids as a result of their energy-yielding metabolism. They include: pH, water activity, oxidation reduction potential, nutrient content, antimicrobial contents, biological structure (b) Extrinsic factors: Are factors … The pH-sensitive genus Shewanella (formerly Alteromonas) plays a significant role in fish spoilage but has not been reported in normal meat (pH<6.0). But, of course, the freezing point of water can be depressed by the presence of solutes and there are a number of micro-organisms which can actively grow at subzero temperatures because their cytoplasm contains one or more com­pounds, such as a polyol, which act as an antifreeze. TOS4. Obligate or strict aerobes are those organisms that are respiratory, generating most of their energy from oxidative phosphorylation using oxygen as the terminal electron acceptor in the process. They lack the enzymes catalase and superoxide dismutase, which catalyse the breakdown of these species as outlined below. If the external pH is sufficiently low and the extracellular concentration of acid high, the burden on the cell becomes too great, the cytoplasmic pH drops to a level where growth is no longer possible and the cell eventually dies (Figure 3.2). Most fresh foods, such as fresh meat, vegetables, and fruits, have a. w values that are close to the optimum growth level of most. The optimum temperature range for bacterial growth is between 5-63℃. Both products contain the enzyme lysozyme which catalyses the hydrolysis of glycosidic linkages in peptidoglycan, the structural polymer responsible for the strength and rigidity of the bacterial cell wall. availability to facilitate growth of microorganisms. Carbon dioxide dissolves in water to produce carbonic acid which partially dissociates into bicarbonate anions and protons. Other components limit microbial growth by restricting the availability of key nutrients. Radiance and radiation Electromagnetic waves of different lengths display different effect on microorganisms • Infrared – no direct lethal effect (heat!) Microbial Growth. Probably the supreme example of this is the white or albumen of the hen’s egg which possesses a whole battery of inhibitory components. From the Nernst equation, it is clear that the hydrogen ion concentration will affect the Eh, and for every unit decrease in the pH the Eh increases by 58 mV. More acidic foods can typically be stored longer without spoiling. The cell will try to maintain its internal pH by expulsion of the protons leaking in but this will slow growth as it diverts energy from growth-related functions. This is a useful, if rather arbitrary, convention, since the distribution of micro-organisms through the growth temperature range is continuous. Explain its significance. Answer Now and help others. This is well illustrated by the food poisoning outbreaks in aerobic foods caused by the strict anaerobe This can be seen by comparing the values recorded for raw meat and minced meat, and for whole grain and ground grain in Table 3.5. Intrinsic factors include those that are internal to the food product itself, such as nutrient content, pH levels, water activity, redox potential, and other antimicrobial components acting as defense mechanisms against microbes. Another feature evident from Figure 3.13 is that the curve is asymmetric – growth declines more rapidly above the optimum temperature than below it. For example, it has been observed that Clostridium acetobutylicum can grow at an Eh as high as +370 mV maintained by ferricyanide, but would not grow at +110mV in an aerated culture. The most important requirement is that water should be present in the liquid state and thus available to support growth. Oxygen depletion appears to be the principal mechanism; as the oxygen content of the medium decreases, so the redox potential declines from a value of around 400 mV at air saturation by about 60 mV for each tenfold reduction in the partial pressure of oxygen. Unlike protons and other charged molecules, un-dissociated lipophilic acid molecules can pass freely through the membrane; in doing so they pass from an external environment of low pH where the equilibrium favours the un-dissociated molecule to the high pH of the cytoplasm (around 7.5 in neutrophils). The ability of low pH to restrict microbial growth has been deliberately employed since the earliest times in the preservation of foods with acetic and lactic acids. In the production of Spanish-style green olives, it is removed by an alkali extraction process, primarily for reasons of flavour. Antimicrobial Barriers and Constituents: The best answers are voted up and rise to the top. As the temperature increases above the optimum, the growth rate declines much more sharply as a result of the irreversible denaturation of proteins and the thermal breakdown of the cell’s plasma membrane. The pH of post-rigor mammalian muscle, around 5.6, is lower than that of fish (6.2-6.5) and this contributes to the longer storage life of meat. This relationship for a single couple is expressed by the Nernst equation: where Eh and E0’ are both measured at pH 7; R is the gas constant; T, the absolute temperature; n, the number of electrons transferred in the process and F is the Faraday constant. Intrinsic and extrinsic factors play very important roles to maintain a microbiologically safe food system. This is because microbial inhibition by weak acids is not solely due to the creation of a high extracellular proton concentration, but is also directly related to the concentration of un-dissociated acid. Non-viable cells fail to reduce the dye and appear blue. Temperature Plays a Key Role in Food Safety Worker Careers. If one side of a silo heats up during the day due to exposure to the sun then the relative humidity on that side is reduced and there is a net migration of water molecules from the cooler side to re-equilibrate the relative humidity. of cells dying 2. In this example the material has been allowed to equilibrate effectively at a known water activity before measuring the water content but Figure 3.11 demonstrates the differences which may be observed depending on whether a given water content is achieved by adding water to a dry commodity or removing it from a wet commodity. As measured with the glass electrode, pH is equal to the negative logarithm of the hydrogen ion activity. Thus a compound like sodium chloride, which dissociates into two ions in solution, is more effective at reducing the water activity than a compound like sucrose on a mole-to-mole basis. Examples important in food microbiology are the lactobacilli and acetic acid bacteria with optima usually between pH 5.0 and 6.0. This is numerically equal to the equilibrium relative humidity (ERH) expressed as a fraction rather than as a percentage: This has important implications for the storage of low aw foods. In this article we will discuss about the extrinsic as well as intrinsic factors affecting the growth of micro-organisms in food. The high positive Eh values registered by fruit juices (see Table 3.5) are largely a reflection of their low pH. where n is the number of electrons, e, transferred. Water is a remarkable compound. At a water activity of 0.6, corresponding to a water potential of — 68 MPa, the cytoplasm would need to contain very high concentrations of an appropriate compatible solute and it is probable that macromolecules such as DNA would no longer function properly and active growth must cease. Factors affecting microbial growth in food (a) Intrinsic factors: These are inherent in the food. It has, for example, been claimed that inclusion of cinnamon in raisin bread retards mould spoilage. When microorganisms grow in food, they cause varying degrees of change in the food's characteristics as a result of metabolic activity. One factor often identified is the effect of CO2 on pH. 6.1.1. The most important factors that affect microbial growth in foods can be summarized in the following categories: (i) factors related to the food itself, the “intrinsic factors,” which include nutrient content, water activity, pH value, redox potential, and the presence of antimicrobial substances and mechanical barriers to microbial invasion; (ii) factors related to the environment in which the food is … If it cannot cope with this it will increase in size and burst. Activity is proportional to concentration and the proportion­ality constant, the activity coefficient, approaches unity as the solution becomes more dilute. The acidity of food is also an important factor affecting bacterial growth 1. Under these circumstances the temperature of liquid water may be well above 100 °C and the relatively recent exploration of submarine volcanic vents has uncovered some remarkable bacteria which can indeed grow at such high temperatures. Redox dyes such as methylene blue or resazurin are sometimes used to indicate changes in Eh which are correlated with microbial levels. Intrinsic and extrinsic factors that affect growth of microorganism and help us understand how to preserve food There are a number of instruments which measure relative humidity through its effect on the electrical properties, such as conductivity or resistivity, of materials. Animal products too, have a range of non-specific antimicrobial constituents. As redox conditions change there will be some resistance to change in a food’s redox potential, known as poising. Some redox couples typically encountered in food materials and their E0’ values are shown in Table 3.4. Redox potential exerts an important elective effect on the microflora of a food. What are the characters Mendel selected for his experiments on pea plant? The presence or absence of oxygen can naturally affect this, but for many anaerobes, oxygen exerts a specific toxic effect of its own. The measured Eh will also be influenced by the relative proportions of oxidized and reduced species present. In our everyday lives we think of water as existing in its liquid state between its freezing point (0°C) and boiling point (100 °C) and we might expect that this would limit the minimum and maximum temperatures at which growth could possibly occur. This knowledge will help you develop strategies to control them. This force is an electrochemical potential produced by the active translocation of protons from the cell interior to the external environment. In 1886 Francois Marie Raoult described the behaviour of an ideal solution by an equation which has since then been known as Raoult’s law: where PA is the partial vapour pressure of A above a solution, in which XA is the mole fraction of the solvent A, and PA0 is the vapour pressure of pure liquid A at the same temperature. This effect is linked to the inability of obligate or aero-intolerant anaerobes to scavenge and destroy toxic products of molecular oxygen such as hydrogen peroxide and, more importantly, the superoxide anion radical (02–) produced by a one- electron reduction of molecular oxygen. With the exception of those soft drinks that contain phosphoric acid, most foods owe their acidity to the presence of weak organic acids. or even days 4. The storage of fresh fruit and vegetables requires very careful control of relative humidity. The water content, however, may not give a good indication of how available that water is, i.e. Which organelle is known as “power house” of the cell? The decrease in Eh as a result of microbial activity is the basis of some long- established rapid tests applied to foods, particularly dairy products. It stands for food, acidity, time, temperature, oxygen, and moisture. If it is too low then many vegetables will lose water and become flaccid. An example of a water sorption isotherm is shown in Figure 3.10. Usually, however, their role in preservation is likely to be minor and, in some cases, they can be a source of microbial contamination leading to spoilage or public health problems. Physical damage to the integument allows microbial invasion of the underlying nutrient-rich tissues and it is a commonplace empirical observation that damaged fruits and vegetables deteriorate more rapidly than entire products, and that this process is initiated at the site of injury. If the pH is increased then dissociation of the acid will increase as well, so that when pH = pKa+1 there will be 10 times as much dissociated acid as un-dissociated. These do not dissociate completely into protons and conjugate base in solution but establish an equilibrium: Equation (3.13) is known as the Henderson-Hasselbach equation and describes the relationship between the pH of a solution, the strength of the acid present and its degree of dissociation. Microbial growth can occur over a temperature range from about — 8 °C up to 100oC at atmospheric pressure. The active translocation of protons from the cell similarly, exclusion of air is very to. Moisture factors affecting the growth of microorganisms in food materials and their of! The liquid state of Spanish-style green olives, it can not cope with it! Rupture ( lyse ) under osmotic pressure every 20 minutes of redox potential, this be. 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