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. 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. 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. Figure 3.9 shows the range of aw values associated with a number of different food commodities. 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). Psychrotrophs or facultative psychrophiles will grow down to the same low temperatures as strict psychrophiles but have higher optimum and maximum growth temperatures. 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. Thus: where (aH) is the hydrogen ion activity and [H+] the hydrogen ion concentration. Microbial Growth FATTOM Food Acid Temperature Time Oxygen Moisture Factors Affecting Microbial Growth. 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. 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. Moulds and oxidative Gram-negative bacteria are most sensitive and the Gram positive bacteria, particularly the lactobacilli, tend to be most resistant. Microbial growth in foods is very complex and diversified, which is governed by biochemical, environmental, and genetic factors along with their nutritional class. 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. Factors Affecting. No of cells produced is equal to the no. J.N. 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. These terms do not describe rigidly exclusive groups of micro-organisms but are useful in the context of studies of particular food commodities. FOOD SPOILAGE AND FOOD PRESERVATION Intrinsic Factors Affecting Microbial Growth • pH • Moisture Content • Water The pH can interact with factors such as a w, salt, temperature, redox potential, and preservatives to inhibit growth of pathogens and other organisms. 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 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. Intrinsic and extrinsic factors play very important roles to maintain a microbiologically safe food system. 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. Its presence and its influence on redox potential are important determinants of the microbial associations that develop and their rate of growth. 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). 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. It has, for example, been claimed that inclusion of cinnamon in raisin bread retards mould spoilage. 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. The factors can be generally classified as intrinsic and extrinsic 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. 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. Considerable attention has been directed to the antimicrobial properties of those plants used as herbs and spices to flavour food (Table 3.6). Increasing the access of air to a food material by chopping, grinding, or mincing will increase its Eh. (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. : It should be noted that water activity is a colligative property, that is to say it depends on the number of molecules or ions present in solution, rather than their size. Stationary phase 1.1. 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. 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. 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. In food microbiology mesophilic and psychrotrophic organisms are generally of greatest importance. Thus the resistance of lithium chloride, or the capacitance of anodized aluminium, changes with changes of relative humidity. Outbreaks of botulism associated with crushed garlic in oil and home canned peppers demonstrate that even in relatively high concentrations plant antimicrobials are not a complete guarantee of safety. Factors affecting microbial growth in food Processing factors • Processing factors: • Slicing • Washing • Packing • Irradiation • Pasteurization 43. 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. 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. 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. 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. Read about our approach to external linking. A cell suspension stained with methylene blue is examined under the microscope and viable cells with a reducing cytoplasm appear colourless. As a rule, fish spoil more rapidly than meat under chill conditions. Redox dyes such as methylene blue or resazurin are sometimes used to indicate changes in Eh which are correlated with microbial levels. and minerals 3. 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. 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. The growth rate decrease due to shortage of growth factors such as vit. Antimicrobial Barriers and Constituents: The best answers are voted up and rise to the top. Factors affecting microbial growth in food Intrinsic factors: by Alanoud Ahmad 1. Therefore, it’s important to have an understanding of the conditions that aid microbial growth. Bacteria need moisture in order to grow. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. Micro-organisms can be classified into several physiological groups based on their cardinal temperatures. 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 ‘. 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 temperature a food is stored, prepared and cooked at is crucial. Bacteria need the following conditions to grow: Our tips from experts and exam survivors will help you through. 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. Water activity can be measured by measuring the equilibrium relative humidity of the atmosphere in contact with the sample. Analysis of their volatile flavour and odour fractions, known as essential oils, has frequently identified compounds such as allicin in garlic, eugenol from allspice (pimento), cloves and cinnamon, thymol from thyme and oregano, and cinnamic aldehyde from cinnamon and cassia which have significant antimicrobial activity (Figure 3.4). 3. 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. what the water activity is, unless the relationship between these two properties has been established. 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. 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