Why mitochondria is called as the power house of the cell? A method known as the Landrock-Proctor method depends on gravimetrically measuring changes in water content of samples of the material after equilibration with atmospheres of known relative humidity which can be obtained using saturated solutions of a number of inorganic salts. 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. Vinegar and lemon juice have a similar effect. 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. Obligate anaerobes, such as Clostridia, are of great importance in food microbiology. Share Your PDF File Radiance and radiation Electromagnetic waves of different lengths display different effect on microorganisms • Infrared – no direct lethal effect (heat!) Oxygen comprises 21% of the earth’s atmosphere and is the most important gas in contact with food under normal circumstances. The tendency of a medium to accept or donate electrons, to oxidize or reduce, is termed its redox potential (Eh) and is measured against an external reference by an inert metal electrode, usually platinum, immersed in the medium. The factors influencing the growth of microorganisms are physical, chemical and biological in nature. 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. 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. 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. 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. Thus: where (aH) is the hydrogen ion activity and [H+] the hydrogen ion concentration. Those fish that have a naturally low pH such as halibut (pH ≈ 5.6) have better keeping qualities than other fish. 4. Like us, micro-organisms can use foods as a source of nutrients and energy. 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. Start studying Factors affecting microbial growth in foods. 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. For example, pseudomonads, such as Pseudomonas fluorescens, which grows at an Eh of +100 to +500 mV, and other oxidative Gram-negative rods produce slime and off-odours at meat surfaces. 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. 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). Food inducers and Inhibitors– Food have some factors that either stimulate growth or adversely affect microorganism growth. Factors affecting microbial growth in food Processing factors • Processing factors: • Slicing • Washing • Packing • Irradiation • Pasteurization 43. The same water content seems to be associated with a higher aw, in the former case than in the latter. •The requirements for microbial growth can be divided into two main categories: physical and nutritional. 0.5 MPa) in a Gram-negative species. It is well established that, although addition of strong acids has a more profound effect on pH pro rata, they are less inhibitory than weak lipophilic acids at the same pH. In food microbiology mesophilic and psychrotrophic organisms are generally of greatest importance. In the equation below, this is represented in its most general form to include the many redox reactions which also involve protons and have the overall effect of transferring hydrogen atoms. Foods that are dehydrated or freeze-dried can be stored for much longer as the moisture has been removed. or even days 4. 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. A rather different example of the importance of plant antimicrobials is provided by oleuropein, the bitter principle of green olives. where n is the number of electrons, e, transferred. Redox potential exerts an important elective effect on the microflora of a food. what the water activity is, unless the relationship between these two properties has been established. One of the most important factors to consider with microbial growth is temperature. Bacteria need warmth to grow. 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. 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. 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. 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. 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. 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. Therefore, it’s important to have an understanding of the conditions that aid microbial growth. 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. If the balance of the various redox couples present favours the oxidized state then there will be a tendency to accept electrons from the electrode creating a positive potential which signifies an oxidizing environment. 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. Decline phase 2.1. Hops and their extracts are ubiquitous ingredients in beer. When the pH is equal to an acid’s pKa then half of the acid present will be un-dissociated. An easy way to remember the key conditions that affects the growth of microbes is the acronym FAT-TOM. Name the types of nitrogenous bases present in the RNA. 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. Factors affecting microbial growth in food Intrinsic factors: by Alanoud Ahmad 1. 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. In plants, injury can rupture storage cells containing essential oils or may bring together an enzyme and substrate which were separated in the intact tissue. The tendency of an atom or molecule to accept or donate electrons is expressed as its standard redox potential, E0‘. water + solute), i.e. Foods may contain multiple microenvironments. The latter might be particularly important in discussions about the availability of water in a complex matrix such as cake. Intrinsic and extrinsic factors play very important roles to maintain a microbiologically safe food system. The value of these physical barriers can be clearly seen when they are breached in some way. 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. When microorganisms grow in food, they cause varying degrees of change in the food's characteristics as a result of metabolic activity. Thus the resistance of lithium chloride, or the capacitance of anodized aluminium, changes with changes of relative humidity. Privacy Policy3. Stationary phase 1.1. One factor often identified is the effect of CO2 on pH. also show considerable -tolerance of high CO2 levels and dominate the spoilage microflora of carbonated beverages. 3. Aero-tolerant anaerobes are incapable of aerobic respiration, but can nevertheless grow in the presence of air. Moulds and oxidative Gram-negative bacteria are most sensitive and the Gram positive bacteria, particularly the lactobacilli, tend to be most resistant. No of cells produced is equal to the no. Animal products too, have a range of non-specific antimicrobial constituents. Share Your Word File This can kill Gram-negative bacteria and inhibit Gram-positives, possibly by damaging the bacterial cytoplasmic membrane (Figure 3.5). This is when a bacterium divides in two every 20 minutes. Once micro­organisms have started to grow and become physiologically active they usually produce water as an end product of respiration. 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. It is in such regions that propagules which have remained viable, but unable to grow, may now germinate and grow. Food. Before sharing your knowledge on this site, please read the following pages: 1. This is known as the. 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. 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. 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. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in … 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. An oxidation-reduction (redox) reaction occurs as the result of a transfer of electrons between atoms or molecules. The storage of fresh fruit and vegetables requires very careful control of relative humidity. To take account of this and the effect of other factors, it is more appropriate to define cardinal temperatures as ranges rather than single values (Table 3.11). Carbon dioxide is not uniform in its effect on micro-organisms. Ovotransferrin in egg white and lactoferrin in milk are proteins that scavenge iron from the medium. A large positive E0‘ indicates that the oxidized species of the couple is a strong oxidizing agent and the reduced form only weakly reducing. 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. A large negative E0‘ indicates the reverse. With a reduction of water activity in their environment the number of groups of micro-organisms capable of active growth decreases (Table 3.9). The temperature a food is stored, prepared and cooked at is crucial. J.N. 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. All foods were at “some stage part of living organisms and, as such, have been equipped through the course of evolution with ways in which potentially damaging microbial infections might be prevented or at least limited. In addition, egg white has powerful cofactor—binding proteins such as avidin and ovoflavoprotein which sequester biotin and riboflavin restricting the growth of those bacteria for which they are essential nutrients, see Table 3.7. The following points highlight the six main physical factors affecting the growth of microorganisms. 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. In living cells an ordered sequence of both electron and hydrogen transfer reactions is an essential feature of the electron transport chain and energy generation by oxidative phosphorylation. There are a number of instruments which measure relative humidity through its effect on the electrical properties, such as conductivity or resistivity, of materials. The limiting value of water activity for the growth of any micro-organism is about 0.6 and below this value the spoilage of foods is not microbiological but may be due to insect damage or chemical reactions such as oxidation. This partial dissociation of weak acids, such as acetic acid, plays an important part in their ability to inhibit microbial growth. 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. 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. These factors are the environmental factors that are implicated in food spoilage occurrences. This is achieved by the production of increasing concentrations of solutes which must not interfere with cytoplasmic function. A cell suspension stained with methylene blue is examined under the microscope and viable cells with a reducing cytoplasm appear colourless. 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. It is worth remembering that since pH is a logarithmic scale differences in pH of 1, 2 and 3 units correspond to 10-, 100- and 1000-fold differences in the hydrogen ion concentration. 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 FATTOM Food Acid Temperature Time Oxygen Moisture Factors Affecting Microbial Growth. 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. Food Microbiology, Micro-Organisms in Food. 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). 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. Some yeasts such as Brettanomyces spp. Growth of microorganisms in food is dependent on various parameters. They are usually associated with salt lakes or salt pans where solar salt is being made and may cause the proteolytic spoilage of dried, salted fish. It has, for example, been claimed that inclusion of cinnamon in raisin bread retards mould spoilage. Temperature Plays a Key Role in Food Safety Worker Careers. What are antibiotics? This is analogous to buffering of a medium against pH changes and is, like buffering, a ‘capacity’ effect dependent on, and increasing with, the concentration of the couple. Yeast: Origin, Reproduction, Life Cycle and Growth Requirements | Industrial Microbiology, How is Bread Made Step by Step? glucose) for building blocks. Microbial growth in a food reduces its Eh. Conditions needed for bacterial growth. Some redox couples typically encountered in food materials and their E0’ values are shown in Table 3.4. Increasing the access of air to a food material by chopping, grinding, or mincing will increase its Eh. 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. More acidic foods can typically be stored longer without spoiling. Plotting microbial growth rate against pH produces an approximately symmetrical bell- shaped curve spanning 2-5 pH units, with a maximum rate exhibited over a range of 1-2 units. If provided with the optimum conditions for growth, bacteria can multiply to millions over a small period of time via. It is a relatively simple matter to determine the water content of a food commodity by drying to constant weight under defined conditions. This dynamic two way flow of water molecules is normally in a steady state and a living organism will only be stressed if there is a net flow out of the cytoplasm, leading to plasmolysis, or a net flow into the cell leading to rupture of the membrane, and the latter is normally prevented by the presence of a cell wall in the bacteria and fungi. In fruits, however, a lower pH prevents bacterial growth and spoilage is dominated by yeasts and moulds. Most fresh foods, such as fresh meat, vegetables, and fruits, have a. w values that are close to the optimum growth level of most. 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