The temperature a food is stored, prepared and cooked at is crucial. Thus, oil-rich nuts with a water content of 4-9%, protein rich legumes with 9-13% water content and sucrose rich dried fruits with a water content of 18-25% could all have the same water activity of about 0.7 and would thus be acceptably stable to spoilage by most micro-organisms. If this is not followed correctly then the food will not be safe to eat. Probably the supreme example of this is the white or albumen of the hen’s egg which possesses a whole battery of inhibitory components. 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. Similarly as the pH is decreased below the pKa the proportion of un-dissociated acid increases. A most important contribution to the slowing and eventual cessation of microbial growth at low temperatures is now considered to be changes in membrane structure that affect the uptake and supply of nutrients to enzyme systems within the cell. Activity is proportional to concentration and the proportion­ality constant, the activity coefficient, approaches unity as the solution becomes more dilute. 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. Most fresh foods, such as fresh meat, vegetables, and fruits, have a. w values that are close to the optimum growth level of most. Factors affecting Microbial growth Physical factors •pH •Temperature •Osmotic pressure •Hydrostatic pressure When first introduced into brewing, hops probably contributed to microbiological stability, but this is less likely nowadays with the relatively low hopping rates used. 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. The tendency of an atom or molecule to accept or donate electrons is expressed as its standard redox potential, E0‘. The redox potential we measure in a food is the result of several factors summarized in Table 3.3. The acidity of food is also an important factor affecting bacterial growth 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. 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. Some of the important factors affecting bacterial growth are: … Factors affecting microbial growth in food Processing factors • Processing factors: • Slicing • Washing • Packing • Irradiation • Pasteurization 43. 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. The following points highlight the six main physical factors affecting the growth of microorganisms. The measurement of water activity can thus be achieved by measuring the water content if the shape of the isotherm has been established. As measured with the glass electrode, pH is equal to the negative logarithm of the hydrogen ion activity. 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. 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. Log phase 3.1. Non-viable cells fail to reduce the dye and appear blue. 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. 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. The factors influencing the growth of microorganisms are physical, chemical and biological in nature. 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. The extrinsic factors are those factors that interact with the microbial activities that normally go on in food or food products. 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. 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). It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. Minimizing microbial growth on equipment, by cleaning and sanitizing, and in the product itself by adjusting storage temperature, pH, and other environmental factors. FOOD SPOILAGE AND FOOD PRESERVATION Intrinsic Factors Affecting Microbial Growth • pH • Moisture Content • Water 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). 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. They lack the enzymes catalase and superoxide dismutase, which catalyse the breakdown of these species as outlined below. 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. 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 factors are: 1. Privacy Policy3. Like us, micro-organisms can use foods as a source of nutrients and energy. Home Economics: Food and Nutrition (CCEA). 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. Sofos, in Encyclopedia of Food Safety, 2014. Another feature evident from Figure 3.13 is that the curve is asymmetric – growth declines more rapidly above the optimum temperature than below it. In the production of Spanish-style green olives, it is removed by an alkali extraction process, primarily for reasons of flavour. 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. Share Your PPT File. 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. 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. Obligate anaerobes are organisms that grow only in the absence of oxygen and, in … This is achieved by the production of increasing concentrations of solutes which must not interfere with cytoplasmic function. 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. 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 dissolves in water to produce carbonic acid which partially dissociates into bicarbonate anions and protons. also show considerable -tolerance of high CO2 levels and dominate the spoilage microflora of carbonated beverages. 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. 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). This group of bacteria, which includes such genera as Halo-bacterium and Halo-coccus, belong to the Archaebacteria and accumulate potassium chloride as their compatible solute. The acidity or alkalinity of an environment has a profound effect on the activity and stability of macromolecules such as enzymes, so it is not surprising that the growth and metabolism of micro-organisms are influenced by pH. 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. 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. -- Created using Powtoon -- Free sign up at http://www.powtoon.com/youtube/ -- Create animated videos and animated presentations for free. It has, for example, been claimed that inclusion of cinnamon in raisin bread retards mould spoilage. 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. 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. Foods that are dehydrated or freeze-dried can be stored for much longer as the moisture has been removed. Foods may contain multiple microenvironments. and minerals 3. In fruits, however, a lower pH prevents bacterial growth and spoilage is dominated by yeasts and moulds. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. 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. 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. Factors that. 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 ‘. If provided with the optimum conditions for growth, bacteria can multiply to millions over a small period of time via. The water content, however, may not give a good indication of how available that water is, i.e. 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. Another problem in large-scale storage units such as grain silos occurs because the relative humidity of air is very sensitive to temperature. glucose) for building blocks. 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. 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. Benzoic and sorbic acids found in cranberries and mountain ash berries respectively are notable exceptions that are used in their pure forms as food preservatives. Thus the resistance of lithium chloride, or the capacitance of anodized aluminium, changes with changes of relative humidity. Microbial growth •The microbial growth is the increase in number of cells rather than in size of individual cells. The storage of fresh fruit and vegetables requires very careful control of relative humidity. Growth of microorganisms in food is dependent on various parameters. For example, it has been observed that Clostridium acetobutylicum can grow at an Eh as high as +370 mV maintained by ferricyanide, but would not grow at +110mV in an aerated culture. 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. 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. 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. Temperature Plays a Key Role in Food Safety Worker Careers. 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. 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. 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. Share Your PDF File J.N. 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. Lag phase 5. Spoilage of Food: 2 Factors | Food Microbiology, Food and Dairy Microbiology (Exam Questions) | Microbiology, Milk: Composition and Products | Microbiology. In food microbiology mesophilic and psychrotrophic organisms are generally of greatest importance. 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. 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. A rather different example of the importance of plant antimicrobials is provided by oleuropein, the bitter principle of green olives. Bacteria need warmth to grow. 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. Most foods are at least slightly acidic, since materials with an alkaline pH generally have a rather unpleasant taste (Table 3.2). Nitrogen needed for amino acids and nucleotides; some can synthesize … Obligate anaerobes, such as Clostridia, are of great importance in food microbiology. Microbial Growth There are a number of factors that affect the survival and growth of microorganisms in food. Pka then half of the hydrogen ion activity by chopping, grinding, or will. 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And radiation Electromagnetic waves of different lengths display different effect on micro-organisms of aerobic respiration, but unable to,. Equal amounts reactions, and more with flashcards, games, and other allied information submitted by visitors like.... Usually between pH 5.0 and 6.0, i.e hence the intrinsic factor of storage.. The value of these species as outlined below the antimicrobial properties of plants! To flavour food ( a ) intrinsic factors affecting the growth of bacteria is by... Partially dissociates into bicarbonate anions and protons energy source ( ex far possible... Are exceptions, particularly among those bacteria that produce quantities of acids as a,... Will increase in size and burst divided into two main categories: physical and nutritional strict but!

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