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. 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. 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 concentration of key nutrients can, to some extent, determine the rate of microbial growth. Content Guidelines 2. 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. Vinegar and lemon juice have a similar effect. 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. Another problem in large-scale storage units such as grain silos occurs because the relative humidity of air is very sensitive to temperature. Exceptions occur in some foods, particularly highly structured ones where local microenvironments may be deficient in essential nutrients, or where nutrient limitation is used as a defence against microbial infection, for example the white of the hen’s egg. Microbial growth can occur over a temperature range from about — 8 °C up to 100oC at atmospheric pressure. FOOD SPOILAGE AND FOOD PRESERVATION Intrinsic Factors Affecting Microbial Growth • pH • Moisture Content • Water 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. 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. The measured Eh will also be influenced by the relative proportions of oxidized and reduced species present. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. Food inducers and Inhibitors– Food have some factors that either stimulate growth or adversely affect microorganism growth. 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 and extrinsic factors that affect growth of microorganism and help us understand how to preserve food 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. The storage of fresh fruit and vegetables requires very careful control of relative humidity. Microbial growth •The microbial growth is the increase in number of cells rather than in size of individual cells. Our mission is to provide an online platform to help students to share notes in Biology. 3. Redox potential exerts an important elective effect on the microflora of a food. If provided with the optimum conditions for growth, bacteria can multiply to millions over a small period of time via. In food microbiology mesophilic and psychrotrophic organisms are generally of greatest importance. 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. No single organism is capable of growth over the whole of this range; bacteria are normally limited to a temperature span of around 35 °C and moulds rather less, about 30 °C. Thus: where (aH) is the hydrogen ion activity and [H+] the hydrogen ion concentration. If it cannot cope with this it will increase in size and burst. 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. Probably the supreme example of this is the white or albumen of the hen’s egg which possesses a whole battery of inhibitory components. They are obligately halophilic because the integrity of their outer wall depends on a high concentration of sodium chloride in their environment. 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. Most foods are at least slightly acidic, since materials with an alkaline pH generally have a rather unpleasant taste (Table 3.2). It is a relatively simple matter to determine the water content of a food commodity by drying to constant weight under defined conditions. 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. Risk factors affecting microbial safety of foods in catering establishments. The measurement of water activity can thus be achieved by measuring the water content if the shape of the isotherm has been established. Carbon dioxide dissolves in water to produce carbonic acid which partially dissociates into bicarbonate anions and protons. Food Microbiology, Micro-Organisms in Food. Growth of microorganisms in food is dependent on various parameters. 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. TOS4. what the water activity is, unless the relationship between these two properties has been established. Relative humidity and water activity are interrelated, thus relative humidity is essentially a measure of the water activity of the gas phase. 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. If it is too high then condensation may occur and microbial spoilage may be initiated. 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. In this article we will discuss about the extrinsic as well as intrinsic factors affecting the growth of micro-organisms in food. Which organelle is known as “power house” of the cell? Home Economics: Food and Nutrition (CCEA). View food spoilage.pdf from PUPLIC HEA HSC 1104 at University of Guyana. 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. Before sharing your knowledge on this site, please read the following pages: 1. Able to grow on dry foods. These terms do not describe rigidly exclusive groups of micro-organisms but are useful in the context of studies of particular food commodities. Carbon dioxide is not uniform in its effect on micro-organisms. Temperature 3. pH 4. Some redox couples typically encountered in food materials and their E0’ values are shown in Table 3.4. Intrinsic and extrinsic factors play very important roles to maintain a microbiologically safe food system. Factors affecting microbial growth in food Intrinsic factors: by Alanoud Ahmad 1. Lag phase 5. This is usually ascribed to a combination of oxygen depletion and the production of reducing compounds such as hydrogen by the micro-organisms. Read on to learn more about which factors affect this, how you can keep food safer by monitoring it, and why the combined factors of temperature, moisture, and pH matter in keeping the public safe. 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. Factors Affecting. When S>>Ks, a micro-organism will grow at a rate approaching its maximum, but as S falls to values approaching Ks, so too will the growth rate. In fruits, however, a lower pH prevents bacterial growth and spoilage is dominated by yeasts and moulds. There are a number of instruments which measure relative humidity through its effect on the electrical properties, such as conductivity or resistivity, of materials. 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. This is known as the. 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. They lack the enzymes catalase and superoxide dismutase, which catalyse the breakdown of these species as outlined below. A parameter related to water activity is osmotic pressure which can be thought of as the force per unit area required to stop the net flow of water molecules from a region of high to one of low water activity. Spoilage of Food: 2 Factors | Food Microbiology, Food and Dairy Microbiology (Exam Questions) | Microbiology, Milk: Composition and Products | Microbiology. An easy way to remember the key conditions that affects the growth of microbes is the acronym FAT-TOM. As measured with the glass electrode, pH is equal to the negative logarithm of the hydrogen ion activity. 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. 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. 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 ‘. Increasing the access of air to a food material by chopping, grinding, or mincing will increase its Eh. 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. particularly protein foods such as chicken and dairy products are rich in nutrients and moisture and so promote bacterial growth. A large negative E0‘ indicates the reverse. The mechanism of CO2 inhibition is a combination of several processes whose precise individual contributions are yet to be determined. Similarly as the pH is decreased below the pKa the proportion of un-dissociated acid increases. Aero-tolerant anaerobes are incapable of aerobic respiration, but can nevertheless grow in the presence of air. They obtain their energy through aerobic respiration. 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. Food provides energy and nutrients for bacteria to grow. 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. 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. This partial dissociation of weak acids, such as acetic acid, plays an important part in their ability to inhibit microbial growth. Firstly, each organism exhibits a minimum, optimum and maximum temperature at which growth can occur. or even days 4. 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. This is why they grow on foods with high moisture content such as chicken. This is when a bacterium divides in two every 20 minutes. Share Your Word File Radiance and radiation Electromagnetic waves of different lengths display different effect on microorganisms • Infrared – no direct lethal effect (heat!) Since under redox potential, this will be confined to the microbiological effects of other gases commonly encountered in food processing. Heterogeneity and gradients of pH, oxygen, nutrients, etc., are key ecological factors in foods. How the vascular cambium is responsible for secondary growth? The most important requirement is that water should be present in the liquid state and thus available to support growth. -- Created using Powtoon -- Free sign up at http://www.powtoon.com/youtube/ -- Create animated videos and animated presentations for free. 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. 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). With the notable exception of oxygen, most of the couples present in foods, e.g. The water content, however, may not give a good indication of how available that water is, i.e. The tendency of an atom or molecule to accept or donate electrons is expressed as its standard redox potential, E0‘. An example of a water sorption isotherm is shown in Figure 3.10. 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. Factors Affecting Microbial Growth The key to a productive staff is creating and maintaining a safe and healthy work environment. Some micro-organisms are killed by prolonged exposure to CO2 but usually its effect is bacteriostatic. As redox conditions change there will be some resistance to change in a food’s redox potential, known as poising. 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 Plays a Key Role in Food Safety Worker Careers. This is achieved by the production of increasing concentrations of solutes which must not interfere with cytoplasmic function. 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. A cell suspension stained with methylene blue is examined under the microscope and viable cells with a reducing cytoplasm appear colourless. Redox dyes such as methylene blue or resazurin are sometimes used to indicate changes in Eh which are correlated with microbial levels. As a rule, fish spoil more rapidly than meat under chill conditions. ability to participate in chemical/biochemical reactions, and its. Learn vocabulary, terms, and more with flashcards, games, and other study tools. With a reduction of water activity in their environment the number of groups of micro-organisms capable of active growth decreases (Table 3.9). 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. Share Your PPT File. 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. glucose) for building blocks. Minimizing microbial growth on equipment, by cleaning and sanitizing, and in the product itself by adjusting storage temperature, pH, and other environmental factors. 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. 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. The value of these physical barriers can be clearly seen when they are breached in some way. 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. Ovotransferrin in egg white and lactoferrin in milk are proteins that scavenge iron from the medium. 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. Factors affecting microbial growth in food • Intrinsic Factors • Environmental Factors • Implicit Factors • Processing Factors Those fish that have a naturally low pH such as halibut (pH ≈ 5.6) have better keeping qualities than other fish. 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 cell multiply for few hr. Like us, micro-organisms can use foods as a source of nutrients and energy. . 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. 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