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. The acidity of a product can have important implications for its microbial ecology, and the rate and character of its spoilage. 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. The storage of fresh fruit and vegetables requires very careful control of relative humidity. 6.1.1. If this were the complete explanation however, then the change in growth rate with temperature below the optimum might be expected to follow the Arrhenius Law which describes the relationship between the rate of a chemical reaction and the temperature. 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. 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. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in … Factors affecting microbial growth in food Processing factors • Processing factors: • Slicing • Washing • Packing • Irradiation • Pasteurization 43. 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. This is a useful, if rather arbitrary, convention, since the distribution of micro-organisms through the growth temperature range is continuous. Bacteria need the following conditions to grow: Our tips from experts and exam survivors will help you through. Probably the supreme example of this is the white or albumen of the hen’s egg which possesses a whole battery of inhibitory components. Similarly as the pH is decreased below the pKa the proportion of un-dissociated acid increases. 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. Content Guidelines 2. 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. Obligate anaerobes, such as Clostridia, are of great importance in food microbiology. The following points highlight the six main physical factors affecting the growth of microorganisms. Nutrients needed by microorganisms include: Carbon carbon containing compounds are needed as an energy source (ex. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. An oxidation-reduction (redox) reaction occurs as the result of a transfer of electrons between atoms or molecules. Foods that are dehydrated or freeze-dried can be stored for much longer as the moisture has been removed. 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. A rather different example of the importance of plant antimicrobials is provided by oleuropein, the bitter principle of green olives. Foods may contain multiple microenvironments. By carrying out measurements of weight change over a range of relative humidity it is possible to extrapolate to the relative humidity which would cause no weight loss and thus corresponds to the aw of the sample. 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. 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. 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. This knowledge will help you develop strategies to control them. 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. 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. It stands for food, acidity, time, temperature, oxygen, and moisture. Therefore, it’s important to have an understanding of the conditions that aid microbial growth. If it is too high then condensation may occur and microbial spoilage may be initiated. 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). Many lactic acid bacteria fall into this category; they can only generate energy by fermentation and lack both catalase and superoxide dismutase, but are able to grow in the presence of oxygen because they have a mechanism for destroying superoxide based on the accumulation of milli-molar concentrations of manganese. When the pH is equal to an acid’s pKa then half of the acid present will be un-dissociated. They lack the enzymes catalase and superoxide dismutase, which catalyse the breakdown of these species as outlined below. 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. Antimicrobial Barriers and Constituents: The best answers are voted up and rise to the top. The presence or absence of oxygen can naturally affect this, but for many anaerobes, oxygen exerts a specific toxic effect of its own. 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. 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. Animal products too, have a range of non-specific antimicrobial constituents. Like us, micro-organisms can use foods as a source of nutrients and energy. Name the types of nitrogenous bases present in the RNA. When microorganisms grow in food, they cause varying degrees of change in the food's characteristics as a result of metabolic activity. Spoilage of Food: 2 Factors | Food Microbiology, Food and Dairy Microbiology (Exam Questions) | Microbiology, Milk: Composition and Products | Microbiology. 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). 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. Disclaimer Copyright, Share Your Knowledge At this higher pH, the equilibrium shifts in favour of the dissociated molecule, so the acid ionizes producing protons which will tend to acidify the cytoplasm and break down the pH component of the proton motive force. 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. 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. These factors are the environmental factors that are implicated in food spoilage occurrences. They obtain their energy through aerobic respiration. It is in such regions that propagules which have remained viable, but unable to grow, may now germinate and grow. 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). 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). 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. 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. where n is the number of electrons, e, transferred. Most foods are at least slightly acidic, since materials with an alkaline pH generally have a rather unpleasant taste (Table 3.2). The inhibitory effect of carbon dioxide (CO 2) 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. 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. Methylene blue is also used to determine the proportion of viable cells in the yeast used in brewing. Carbon dioxide dissolves in water to produce carbonic acid which partially dissociates into bicarbonate anions and protons. 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. Milk also has the capacity to generate antimicrobials in the presence of hydrogen peroxide. 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). Some micro-organisms are killed by prolonged exposure to CO2 but usually its effect is bacteriostatic. Microaerophils are organisms that require a low concentration of oxygen (2% to 10%) for growth, but higher concentrations are inhibitory. Factors affecting microbial growth in food • Intrinsic Factors • Environmental Factors • Implicit Factors • Processing Factors 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. TOS4. 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. The factors influencing the growth of microorganisms are physical, chemical and biological in nature. Increasing the access of air to a food material by chopping, grinding, or mincing will increase its Eh. 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. Once micro­organisms have started to grow and become physiologically active they usually produce water as an end product of respiration. Factors Affecting. Sofos, in Encyclopedia of Food Safety, 2014. The water activity of a substrate is most conveniently defined as the ratio of the partial pressure of water in the atmosphere in equilibrium with the substrate, P, compared with the partial pressure of the atmosphere in equilibrium with pure water at the same temperature, P0. Most bacteria reproduce best at a neutral pH level of 7. Many natural constituents of plant tissues such as pigments, alkaloids and resins have antimicrobial properties, but limited practical use is made of these. It is a relatively simple matter to determine the water content of a food commodity by drying to constant weight under defined conditions. 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. Figure 3.9 shows the range of aw values associated with a number of different food commodities. Hence the intrinsic factor of redox potential is inextricably linked with the extrinsic factor of storage atmosphere. The high positive Eh values registered by fruit juices (see Table 3.5) are largely a reflection of their low pH. Microbial Growth FATTOM Food Acid Temperature Time Oxygen Moisture Factors Affecting Microbial Growth. 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. Oxygen comprises 21% of the earth’s atmosphere and is the most important gas in contact with food under normal circumstances. Why mitochondria is called as the power house of the cell? Heterogeneity and gradients of pH, oxygen, nutrients, etc., are key ecological factors in foods. Although each factor affecting growth is considered separately in the following discussion, these factors … Growth of microorganisms in food is dependent on various parameters. The tendency of an atom or molecule to accept or donate electrons is expressed as its standard redox potential, E0‘. Such a situation can occur in grain silos or in tanks in which concentrates and syrups are stored. Before sharing your knowledge on this site, please read the following pages: 1. In food microbiology mesophilic and psychrotrophic organisms are generally of greatest importance. Nitrogen needed for amino acids and nucleotides; some can synthesize … For example, plant products classed as vegetables generally have a moderately acid pH and soft-rot producing bacteria such as Erwinia carotovora and pseudomonads play a significant role in their spoilage. 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. Temperature 3. pH 4. This is achieved by the production of increasing concentrations of solutes which must not interfere with cytoplasmic function. Microbial growth •The microbial growth is the increase in number of cells rather than in size of individual cells. Food. 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. availability to facilitate growth of microorganisms. Hygienic food handlers` practices. 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. Moulds and oxidative Gram-negative bacteria are most sensitive and the Gram positive bacteria, particularly the lactobacilli, tend to be most resistant. 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. 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. 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). One factor often identified is the effect of CO2 on pH. 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. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Ovotransferrin in egg white and lactoferrin in milk are proteins that scavenge iron from the medium. or even days 4. 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. Factors affecting microbial growth in food Intrinsic factors: by Alanoud Ahmad 1. 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. 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. 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. Thus, the ability to synthesize amylolytic (starch degrading) enzymes will favour the growth of an organism on cereals and other farinaceous products. Water is a remarkable compound. The milk enzyme lacto-peroxidase will catalyse the oxidation of thiocyanate by hydrogen peroxide to produce inter alia hypo-thiocyanate. How the vascular cambium is responsible for secondary growth? The same water content seems to be associated with a higher aw, in the former case than in the latter. J.N. 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. 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. 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. Microbial Growth There are a number of factors that affect the survival and growth of microorganisms in food. Radiance and radiation Electromagnetic waves of different lengths display different effect on microorganisms • Infrared – no direct lethal effect (heat!) 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. Examples important in food microbiology are the lactobacilli and acetic acid bacteria with optima usually between pH 5.0 and 6.0. What are the characters Mendel selected for his experiments on pea plant? Intrinsic and extrinsic factors that affect growth of microorganism and help us understand how to preserve food 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. 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. 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. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes. 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. 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. This is well illustrated by the food poisoning outbreaks in aerobic foods caused by the strict anaerobe What are antibiotics? Factors affecting microbial growth in food (a) Intrinsic factors: These are inherent in the food. When that same side cools down again the relative humidity increases and, although water molecules migrate back again, the temporary increase in relative humidity may be sufficient to cause local condensation onto the grain with a localized increase in aw sufficient to allow germination of fungal spores and subsequent spoilage of the grain. Share Your PPT File. 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. To store, prepare and cook foods safely in order to reduce risk! Risk of bacteria multiplying and causing foodborne illness which catalyse the oxidation of thiocyanate by hydrogen to... Retards mould spoilage answers are voted up and rise to the microbiological effects of other gases commonly encountered in.... 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