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. Some redox couples typically encountered in food materials and their E0’ values are shown in Table 3.4. This is achieved by the production of increasing concentrations of solutes which must not interfere with cytoplasmic function. Foods that are dehydrated or freeze-dried can be stored for much longer as the moisture has been removed. Home Economics: Food and Nutrition (CCEA). Redox dyes such as methylene blue or resazurin are sometimes used to indicate changes in Eh which are correlated with microbial levels. where n is the number of electrons, e, transferred. As with all generalizations there are exceptions, particularly among those bacteria that produce quantities of acids as a result of their energy-yielding metabolism. 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. Most foods are at least slightly acidic, since materials with an alkaline pH generally have a rather unpleasant taste (Table 3.2). and minerals 3. 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. When food commodities having a low water activity are stored in an atmosphere of high relative humidity water will transfer from the gas phase to the food. 6.1.1. As a rule, fish spoil more rapidly than meat under chill conditions. Foods may contain multiple microenvironments. As a consequence, herbs and spices may contribute to the microbiological stability of foods in which they are used. Increasing the access of air to a food material by chopping, grinding, or mincing will increase its Eh. or even days 4. The mechanism of CO2 inhibition is a combination of several processes whose precise individual contributions are yet to be determined. Answer Now and help others. However oleuropein and its aglycone are also inhibitory to lactic acid bacteria; if not removed at this early stage, they would prevent the necessary fermentation occurring subsequently. Factors Affecting. A cell suspension stained with methylene blue is examined under the microscope and viable cells with a reducing cytoplasm appear colourless. Similarly the boiling point of water can be elevated by increased hydrostatic pressure and, in nature, very high pressures exist at the bottom of the deep oceans. The following points highlight the six main physical factors affecting the growth of microorganisms. At temperatures above the maximum for growth, these changes are sufficient to kill the organism – the rate at which this occurs increasing with increasing temperature. 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). They include: pH, water activity, oxidation reduction potential, nutrient content, antimicrobial contents, biological structure (b) Extrinsic factors: Are factors … Factors that. Some of the important factors affecting bacterial growth are: … Egg white, where the pH increases to around 9.2 as CO2 is lost from the egg after laying, is a commonplace exception to this. It is in such regions that propagules which have remained viable, but unable to grow, may now germinate and grow. glutathione and cysteine in meats, and to a lesser extent, ascorbic acid and reducing sugars in plant products, would on their own tend to establish reducing conditions. 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. The inability of an organism to utilize a major component of a food material will limit its growth and put it at a competitive disadvantage compared with those that can. This tolerance of a wider range of temperature means that psychrotrophs are found in a more diverse range of habitats and consequently are of greater importance in the spoilage of chilled foods. 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. 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. Temperature Plays a Key Role in Food Safety Worker Careers. Before sharing your knowledge on this site, please read the following pages: 1. 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. These factors are the environmental factors that are implicated in food spoilage occurrences. Microbial Growth FATTOM Food Acid Temperature Time Oxygen Moisture Factors Affecting Microbial Growth. From them, they derive the chemical elements that constitute microbial biomass, those molecules essential for growth that the organism cannot synthesize, and a substrate that can be used as an energy source. also show considerable -tolerance of high CO2 levels and dominate the spoilage microflora of carbonated beverages. Microaerophils are organisms that require a low concentration of oxygen (2% to 10%) for growth, but higher concentrations are inhibitory. Another problem in large-scale storage units such as grain silos occurs because the relative humidity of air is very sensitive to temperature. Probably of more importance than its effect on the growth menstruum is the ability of CO2 to act in the same way as weak organic acids, penetrating the plasma membrane and acidifying the cell’s interior. Also, like buffering, poising is greatest when the two components of a redox couple are present in equal amounts. Heterogeneity and gradients of pH, oxygen, nutrients, etc., are key ecological factors in foods. Cytoplasm is an aqueous solution and so must have a lower water activity than pure water; thus a micro-organism in an environment of pure water will experience a net flow of water molecules into the cytoplasm. Microbial Growth There are a number of factors that affect the survival and growth of microorganisms in food. 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). They lack the enzymes catalase and superoxide dismutase, which catalyse the breakdown of these species as outlined below. Outbreaks of botulism associated with crushed garlic in oil and home canned peppers demonstrate that even in relatively high concentrations plant antimicrobials are not a complete guarantee of safety. 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. Many natural constituents of plant tissues such as pigments, alkaloids and resins have antimicrobial properties, but limited practical use is made of these. 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. Bacteria need the following conditions to grow: Our tips from experts and exam survivors will help you through. Obligate anaerobes, such as Clostridia, are of great importance in food microbiology. Bacteria need warmth to grow. It is a relatively simple matter to determine the water content of a food commodity by drying to constant weight under defined conditions. The presence or absence of oxygen can naturally affect this, but for many anaerobes, oxygen exerts a specific toxic effect of its own. Start studying Factors affecting microbial growth in foods. 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. The optimum temperature range for bacterial growth is between 5-63℃. If it cannot cope with this it will increase in size and burst. Thus the resistance of lithium chloride, or the capacitance of anodized aluminium, changes with changes of relative humidity. Some micro-organisms are killed by prolonged exposure to CO2 but usually its effect is bacteriostatic. An easy way to remember the key conditions that affects the growth of microbes is the acronym FAT-TOM. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. 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. A class of antimicrobials known collectively as phytoalexins are produced by many plants in response to microbial invasion, for example the antifungal compound phaseollin produced in green beans. Among the organisms capable of growth at low temperatures, two groups can be distinguished: the true or strict psychrophiles (‘cold-loving’) have optima of 12- 15°C and will not grow above about 20 °C. The factors are: 1. An oxidation-reduction (redox) reaction occurs as the result of a transfer of electrons between atoms or molecules. 3 MPa) in a Gram-positive bacterium or as little as 5 atm (ca. Nutrients needed by microorganisms include: Carbon carbon containing compounds are needed as an energy source (ex. Considered as a hydride of oxygen (H2O) it has quite exceptional properties when compared with the hydrides of neighbouring elements in the periodic table such as ammonia (NH3), methane (CH4), hydrogen sulfide (H2S), and hydrofluoric acid (HF), see Table 3.8. Acidic foods with a pH below 7, or alkaline foods with a pH above 7, may stop or slow down the rate of bacterial growth. FOOD SPOILAGE AND FOOD PRESERVATION Intrinsic Factors Affecting Microbial Growth • pH • Moisture Content • Water They obtain their energy through aerobic respiration. The latter might be particularly important in discussions about the availability of water in a complex matrix such as cake. When first introduced into brewing, hops probably contributed to microbiological stability, but this is less likely nowadays with the relatively low hopping rates used. Lag phase 5. As the temperature is decreased from the optimum the growth rate slows, partly as a result of the slowing of enzymic reactions within the cell. Another feature evident from Figure 3.13 is that the curve is asymmetric – growth declines more rapidly above the optimum temperature than below it. However, it is important to note that, even if active growth is impossible, survival may still occur and many micro-organisms can survive at very low water activities and are frequently stored in culture collections in this form. Bacteria, fungi and algae cope by having a rigid strong wall capable of withstanding the osmotic pressure of the cytoplasm which may be as high as 30 atm (ca. These are known as cardinal temperatures and are, to a large extent, characteristic of an organism, although they are influenced by other environmental factors such as nutrient availability, pH and aw. As water activity is decreased, or osmotic pressure is increased, in the environment it is essential that the water activity of the cytoplasm is even lower, or its osmotic pressure even higher. 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. Psychrotrophs or facultative psychrophiles will grow down to the same low temperatures as strict psychrophiles but have higher optimum and maximum growth temperatures. -- Created using Powtoon -- Free sign up at http://www.powtoon.com/youtube/ -- Create animated videos and animated presentations for free. 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. Consequently it is important to the farmer and food processor that harvesting and transport maintain these barriers intact as far as possible. Antimicrobial components differ in their spectrum of activity and potency, they are present at varying concentrations in the natural product, and are frequently at levels too low to have any effect. Because of the different groups of organisms involved, it can also be different in character. Food Microbiology, Micro-Organisms in Food. TOS4. This is a useful, if rather arbitrary, convention, since the distribution of micro-organisms through the growth temperature range is continuous. If it is too high then condensation may occur and microbial spoilage may be initiated. Privacy Policy3. Since under redox potential, this will be confined to the microbiological effects of other gases commonly encountered in food processing. What are the characters Mendel selected for his experiments on pea plant? This partial dissociation of weak acids, such as acetic acid, plays an important part in their ability to inhibit microbial growth. Oxygen, which is present in the air at a level of around 21%, is usually the most influential redox couple in food systems. water + solute), i.e. The milk enzyme lacto-peroxidase will catalyse the oxidation of thiocyanate by hydrogen peroxide to produce inter alia hypo-thiocyanate. Hygienic food handlers` practices. A large negative E0‘ indicates the reverse. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. 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. Probably the supreme example of this is the white or albumen of the hen’s egg which possesses a whole battery of inhibitory components. 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. Factors affecting microbial growth in food • Intrinsic Factors • Environmental Factors • Implicit Factors • Processing Factors Most fresh foods, such as fresh meat, vegetables, and fruits, have a. w values that are close to the optimum growth level of most. Values for Ks have been measured experimentally for a range of organisms and nutrients; generally they are extremely low, of the order of 10-5M for carbon and energy sources, suggesting that in most cases, nutrient scarcity is unlikely to be rate-limiting. The temperature a food is stored, prepared and cooked at is crucial. As our foods are of plant and animal origin, it is worthwhile to consider those characteristics of plant and animal tissues that affect the growth of microorganisms. The concentration of key nutrients can, to some extent, determine the rate of microbial growth. Content Guidelines 2. 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. Microbial growth •The microbial growth is the increase in number of cells rather than in size of individual cells. Inhibition of Microbial Growth. of cells dying 2. Destruction or weakening of this layer causes the cell to rupture (lyse) under osmotic pressure. 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. This knowledge will help you develop strategies to control them. Water potential may contain both an osmotic component, associated with the effect of solutes in solution, and a matric component, associated with the interaction of water molecules with surfaces, which can be clearly demonstrated by the rise of water in a capillary tube. 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. Carbon dioxide is not uniform in its effect on micro-organisms. : It should be noted that water activity is a colligative property, that is to say it depends on the number of molecules or ions present in solution, rather than their size. Thermophiles are generally of far less importance in food microbiology, although thermophilic spore formers such as certain Bacillus and Clostridium species do pose problems in a restricted number of situations. Why mitochondria is called as the power house of the cell? 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. Although the cytoplasm must be in the liquid phase for active growth (and it is important not to confuse growth and survival, for many micro-organisms can survive but not grow when their cytoplasm has been completely dried) water in the environment of the living organism may be present, not only in the liquid phase as pure water or a solution, but also in the atmosphere in the gaseous phase, or associated with what would be described macroscopically as the solid phase (Figure 3.6). A rather different example of the importance of plant antimicrobials is provided by oleuropein, the bitter principle of green olives. The inhibitory effect of carbon dioxide (CO2) on microbial growth is applied in modified-atmosphere packing of food and is an advantageous consequence of its use at elevated pressures (hyperbaric) in carbonated mineral waters and soft drinks. ability to participate in chemical/biochemical reactions, and its. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in … Methylene blue is also used to determine the proportion of viable cells in the yeast used in brewing. 0.5 MPa) in a Gram-negative species. Factors affecting microbial growth in food (a) Intrinsic factors: These are inherent in the food. Conditions needed for bacterial growth. 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. Antimicrobial Barriers and Constituents: The best answers are voted up and rise to the top. Aero-tolerant anaerobes are incapable of aerobic respiration, but can nevertheless grow in the presence of air. 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. Therefore, it’s important to have an understanding of the conditions that aid microbial growth. 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. One of the most important factors to consider with microbial growth is temperature. The pH of the food also significantly impacts the lethality of heat treatment of the food. The growth rate decrease due to shortage of growth factors such as vit. Bacteria need moisture in order to grow. Mesophiles, with temperature optima around 37 °C, are frequently of human or animal origin and include many of the more common foodborne pathogens such as Salmonella, Staphylococcus aureus and Clostridium perfringens. The value of these physical barriers can be clearly seen when they are breached in some way. Non-viable cells fail to reduce the dye and appear blue. This is why they grow on foods with high moisture content such as chicken. 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. How the vascular cambium is responsible for secondary growth? In general, bacteria grow fastest in the pH range 6.0-8.0, yeasts 4.5-6.0 and filamentous fungi 3.5-4.0. Life as we know it is totally dependent on the presence of water in its liquid state. Freshwater protozoa, on the other hand, cope with the net flow of water into the cell by actively excreting it out again with a contractile vacuole. In this article we will discuss about the extrinsic as well as intrinsic factors affecting the growth of micro-organisms in food. Similarly, exclusion of air as in modified vacuum packing or canning will reduce the Eh. Lysozyme is most active against Gram-positive bacteria, where the peptidoglycan is more readily accessible, but it can also kill Gram-negatives if their protective outer membrane is damaged in some way. Microbial Growth. The tendency of an atom or molecule to accept or donate electrons is expressed as its standard redox potential, E0‘. The relationship between the two, known as the Monod equation, is mathematically identical to the Michaelis-Menten equation of enzyme kinetics, reflecting the dependence of microbial growth on rate-limiting enzyme reactions: where µ is the specific growth rate; µm the maximum specific growth rate; S the concentration of limiting nutrient; and Ks the saturation constant. The same water content seems to be associated with a higher aw, in the former case than in the latter. The high positive Eh values registered by fruit juices (see Table 3.5) are largely a reflection of their low pH. In fact the ability of lactic acid bacteria to acquire resistance to hop resins means that the brewery environment probably acts as a very efficient natural enrichment culture for humulone-tolerant bacteria, thus negating any beneficial effects. A large positive E0‘ indicates that the oxidized species of the couple is a strong oxidizing agent and the reduced form only weakly reducing. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. 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. parameters in the food to inhibit growth. Figure 3.12 shows the result of such an experiment with samples of madeira cake and Table 3.10 shows the water activities of a variety of saturated salt solutions at 25 °C. The latter occurs in plants such as mustard, horseradish, watercress, cabbage and other brassicas to produce antimicrobial isothiocyanates (mustard oils) and in Allium species (garlic, onions and leeks) to produce thiosulfinates such as allicin (Figure 3.3). . Relative humidity and water activity are interrelated, thus relative humidity is essentially a measure of the water activity of the gas phase. 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Can be generally classified as intrinsic factors affecting the growth of microorganisms in food stored... Retards mould spoilage result of their outer wall depends on a high concentration of sodium chloride their... With temperature illustrates several important features of this relationship ( Figure 3.5 ) are largely a reflection their! The enzymes catalase and superoxide dismutase, which catalyse the oxidation of thiocyanate by hydrogen.! Classified into several physiological groups based on their cardinal temperatures: carbon carbon containing are. Of time useful, if rather arbitrary, convention, since the distribution of micro-organisms capable of growth! In Eh which are correlated with microbial growth is between 5-63℃ the FAT-TOM! Fresh fruit and vegetables requires very careful control of relative humidity is essentially a measure of the to! Spices to flavour food ( Table 3.6 ) effects of other gases commonly encountered in.! 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As the result of a food material by chopping, grinding, or mincing will increase its.!: physical and nutritional a lower pH prevents bacterial growth and spoilage is dominated by and... As vit occurs as the power house of the water content seems to be most resistant values associated a! Soft drinks factors affecting microbial growth in food contain phosphoric acid, Plays an important elective effect on micro-organisms the no with!

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