factors affecting microbial growth in food

Name the types of nitrogenous bases present in the RNA. 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). Microbial Growth There are a number of factors that affect the survival and growth of microorganisms in food. Nitrogen needed for amino acids and nucleotides; some can synthesize … The presence or absence of oxygen can naturally affect this, but for many anaerobes, oxygen exerts a specific toxic effect of its own. 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. The factors influencing the growth of microorganisms are physical, chemical and biological in nature. The acidity of food is also an important factor affecting bacterial growth 1. Microbial Growth FATTOM Food Acid Temperature Time Oxygen Moisture Factors Affecting Microbial Growth. 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. Thus, the ability to synthesize amylolytic (starch degrading) enzymes will favour the growth of an organism on cereals and other farinaceous products. Temperature Plays a Key Role in Food Safety Worker Careers. Food Microbiology, Micro-Organisms in Food. If it is too low then many vegetables will lose water and become flaccid. 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. Influence. 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. 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. 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. 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. 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. 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. Microbial growth in foods is very complex and diversified, which is governed by biochemical, environmental, and genetic factors along with their nutritional class. They are obligately halophilic because the integrity of their outer wall depends on a high concentration of sodium chloride in their environment. 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. 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. Those fish that have a naturally low pH such as halibut (pH ≈ 5.6) have better keeping qualities than other fish. 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). The latter might be particularly important in discussions about the availability of water in a complex matrix such as cake. Share Your PPT File. 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. 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. With the exception of those soft drinks that contain phosphoric acid, most foods owe their acidity to the presence of weak organic acids. They are thus known as compatible solutes and include such compounds as the polyols glycerol, arabitol and mannitol in the fungi and amino acids or amino acid derivatives in the bacteria. water + solute), i.e. 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. When first introduced into brewing, hops probably contributed to microbiological stability, but this is less likely nowadays with the relatively low hopping rates used. also show considerable -tolerance of high CO2 levels and dominate the spoilage microflora of carbonated beverages. Redox dyes such as methylene blue or resazurin are sometimes used to indicate changes in Eh which are correlated with microbial levels. It may take a very long time for the bulk of the commodity to increase in water activity, but condensation may occur on surfaces giving rise to localized regions of high water activity. In food microbiology mesophilic and psychrotrophic organisms are generally of greatest importance. 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. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. 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 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. 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. This type of phenomenon can often account for localized caking of grain which had apparently been stored at a ‘safe’ water content. 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. The milk enzyme lacto-peroxidase will catalyse the oxidation of thiocyanate by hydrogen peroxide to produce inter alia hypo-thiocyanate. 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. 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. Sofos, in Encyclopedia of Food Safety, 2014. An easy way to remember the key conditions that affects the growth of microbes is the acronym FAT-TOM. 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. Consequently it is important to the farmer and food processor that harvesting and transport maintain these barriers intact as far as possible. (With Methods)| Industrial Microbiology, How is Cheese Made Step by Step: Principles, Production and Process, Enzyme Production and Purification: Extraction & Separation Methods | Industrial Microbiology, Fermentation of Olives: Process, Control, Problems, Abnormalities and Developments. 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. Stationary phase 1.1. Carbon dioxide dissolves in water to produce carbonic acid which partially dissociates into bicarbonate anions and protons. The mechanism of CO2 inhibition is a combination of several processes whose precise individual contributions are yet to be determined. The factors can be generally classified as intrinsic and extrinsic factors. Temperature 3. pH 4. 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. This is usually ascribed to a combination of oxygen depletion and the production of reducing compounds such as hydrogen by the micro-organisms. Although each factor affecting growth is considered separately in the following discussion, these factors … . The most important requirement is that water should be present in the liquid state and thus available to support growth. 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. 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. Hence the intrinsic factor of redox potential is inextricably linked with the extrinsic factor of storage atmosphere. 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. Milk also has the capacity to generate antimicrobials in the presence of hydrogen peroxide. Factors Affecting Microbial Growth The key to a productive staff is creating and maintaining a safe and healthy work environment. As redox conditions change there will be some resistance to change in a food’s redox potential, known as poising. 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 pH of the food also significantly impacts the lethality of heat treatment of the food. Spoilage of Food: 2 Factors | Food Microbiology, Food and Dairy Microbiology (Exam Questions) | Microbiology, Milk: Composition and Products | Microbiology. 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. 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. Disclaimer Copyright, Share Your Knowledge -- Created using Powtoon -- Free sign up at http://www.powtoon.com/youtube/ -- Create animated videos and animated presentations for free. 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. A large positive E0‘ indicates that the oxidized species of the couple is a strong oxidizing agent and the reduced form only weakly reducing. The water content, however, may not give a good indication of how available that water is, i.e. Moulds and oxidative Gram-negative bacteria are most sensitive and the Gram positive bacteria, particularly the lactobacilli, tend to be most resistant. They obtain their energy through aerobic respiration. This is why they grow on foods with high moisture content such as chicken. For aqueous solutions, pH 7 corresponds to neutrality (since [H+][OH–] = 10-14 for water), pH values below 7 are acidic and those above 7 indicate an alkaline environment. If this is not followed correctly then the food will not be safe to eat. Some micro-organisms are killed by prolonged exposure to CO2 but usually its effect is bacteriostatic. Thus, if there is a preponderance of the oxidant over its corresponding reductant, then this will tend to increase the redox potential and the oxidizing nature of the medium. 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. 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. Destruction or weakening of this layer causes the cell to rupture (lyse) under osmotic pressure. 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. 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. 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. Antimicrobial Barriers and Constituents: The best answers are voted up and rise to the top. Relative humidity and water activity are interrelated, thus relative humidity is essentially a measure of the water activity of the gas phase. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in … 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. Radiance and radiation Electromagnetic waves of different lengths display different effect on microorganisms • Infrared – no direct lethal effect (heat!) 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. 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. 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. 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. The addition of fruits containing sucrose and other sugars to yoghurt increases the range of carbohydrates ‘available and allows the development of a more diverse spoilage microflora of yeasts. 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. Carbon dioxide is not uniform in its effect on micro-organisms. 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. As a consequence, herbs and spices may contribute to the microbiological stability of foods in which they are used. This website includes study notes, research papers, essays, articles and other allied information submitted by visitors like YOU. Examples important in food microbiology are the lactobacilli and acetic acid bacteria with optima usually between pH 5.0 and 6.0. 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). Factors affecting microbial growth in food (a) Intrinsic factors: These are inherent in the food. No of cells produced is equal to the no. 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