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. If it is too low then many vegetables will lose water and become flaccid. 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). Before sharing your knowledge on this site, please read the following pages: 1. 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. The redox potential we measure in a food is the result of several factors summarized in Table 3.3. What are antibiotics? Relative humidity and water activity are interrelated, thus relative humidity is essentially a measure of the water activity of the gas phase. Oxygen comprises 21% of the earth’s atmosphere and is the most important gas in contact with food under normal circumstances. 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. Temperature Plays a Key Role in Food Safety Worker Careers. Since under redox potential, this will be confined to the microbiological effects of other gases commonly encountered in food processing. 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. Therefore, itâs important to have an understanding of the conditions that aid microbial growth. TOS4. Destruction or weakening of this layer causes the cell to rupture (lyse) under osmotic pressure. Share Your PDF File
Yeast: Origin, Reproduction, Life Cycle and Growth Requirements | Industrial Microbiology, How is Bread Made Step by Step? The storage of fresh fruit and vegetables requires very careful control of relative humidity. Water is a remarkable compound. 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. When the pH is equal to an acid’s pKa then half of the acid present will be un-dissociated. water + solute), i.e. FOOD SPOILAGE AND FOOD PRESERVATION Intrinsic Factors Affecting Microbial Growth â¢ pH â¢ Moisture Content â¢ Water A graph showing the variation of growth rate with temperature illustrates several important features of this relationship (Figure 3.13). This relationship for a single couple is expressed by the Nernst equation: where Eh and E0â 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. 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. The extrinsic factors are those factors that interact with the microbial activities that normally go on in food or food products. Obligate or strict aerobes are those organisms that are respiratory, generating most of their energy from oxidative phosphorylation using oxygen as the terminal electron acceptor in the process. The optimum temperature range for bacterial growth is between 5-63℃. The measured Eh will also be influenced by the relative proportions of oxidized and reduced species present. Factors Affecting Microbial Growth The key to a productive staff is creating and maintaining a safe and healthy work environment. 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. 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 Â°C. 6.1.1. Explain its significance. If it is too high then condensation may occur and microbial spoilage may be initiated. Some of the important factors affecting bacterial growth are: â¦ Able to grow on dry foods. A cell suspension stained with methylene blue is examined under the microscope and viable cells with a reducing cytoplasm appear colourless. Temperature 3. pH 4. 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. The latter might be particularly important in discussions about the availability of water in a complex matrix such as cake. As a rule mesophiles grow more quickly at their optima than psychrotrophs and so spoilage of perishable products stored in the mesophilic growth range is more rapid than spoilage under chill conditions. 3 MPa) in a Gram-positive bacterium or as little as 5 atm (ca. 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. (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. Solutes and Water Acidity 2. 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. Most foods are at least slightly acidic, since materials with an alkaline pH generally have a rather unpleasant taste (Table 3.2). 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. In fruits, however, a lower pH prevents bacterial growth and spoilage is dominated by yeasts and moulds. Microbial growth â¢The microbial growth is the increase in number of cells rather than in size of individual cells. Vinegar and lemon juice have a similar effect. 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. Influence. also show considerable -tolerance of high CO2 levels and dominate the spoilage microflora of carbonated beverages. 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. If this is not followed correctly then the food will not be safe to eat. Factors Affecting. The cell multiply for few hr. Foods may contain multiple microenvironments. 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. These factors are the environmental factors that are implicated in food spoilage occurrences. 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. As with all generalizations there are exceptions, particularly among those bacteria that produce quantities of acids as a result of their energy-yielding metabolism. It stands for food, acidity, time, temperature, oxygen, and moisture. 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. This is why they grow on foods with high moisture content such as chicken. Food inducers and Inhibitorsâ Food have some factors that either stimulate growth or adversely affect microorganism growth. Carbonic acid is a weak dibasic acid (pKa 6.37 and 10.25); in an un-buffered solution it can produce an appreciable drop in pH, distilled water in equilibrium with the CO2 in the normal atmosphere will have a pH of about 5, but the effect will be less pronounced in buffered food media so that equilibration of milk with 1 atmosphere pCO2 decreased the pH from 6.6 to 6.0. 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. 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. The tendency of an atom or molecule to accept or donate electrons is expressed as its standard redox potential, E0‘. Hops and their extracts are ubiquitous ingredients in beer. Firstly, each organism exhibits a minimum, optimum and maximum temperature at which growth can occur. Microaerophils are organisms that require a low concentration of oxygen (2% to 10%) for growth, but higher concentrations are inhibitory. This force is an electrochemical potential produced by the active translocation of protons from the cell interior to the external environment. 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. An oxidation-reduction (redox) reaction occurs as the result of a transfer of electrons between atoms or molecules. Factors that. 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. Risk factors affecting microbial safety of foods in catering establishments. 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. 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