WATER ACTIVITY AND MICROBIOLOGICAL …

1 WATER ACTIVITY AND MICROBIOLOGICAL ASPECTS OF FOODSA KNOWLEDGE BASEPaul Gibbs1 and Vassilis Gekas21 Leatherhead Food Research Association., Randalls Road, Leatherhead, Surrey, UK2 University of Hania, Crete, GreeceIntroductionWater ACTIVITY , aW, is a physical property that has a direct implication formicrobiological safety of food. WATER ACTIVITY also influences the storage stabilitystability of foods as some deteriorative processes in foods are mediated by life of dry foods such as biscuits is generally longer then of moist foods suchas meat at the same temperature. In this connection freezing of foods is equivalentto drying the WATER is removed from the food matrix although it is still in the foodas of this strong association between the physical property, aw, and thechemical and microbological properties of food, it is appropriate to include theseaspects in the form of a Knowledge Base on this of WATER ActivityFrom the physicist s point of view, WATER ACTIVITY is defined in terms ofthermodynamic concepts such as the chemical potential and is related to the osmoticpressure of an aqueous solution.

2 When a substance such as salt (sodium cloride) isdissolved in WATER , the WATER ACTIVITY is reduced. This is why salting is an ancientway of preserving aW of a food or solution is the ratio of the WATER vapour pressure of the food orsolution (p) to that of pure WATER (po) at the same temperature: -aW = p/poThe aW is related to the boiling and freezing points, equilibrium relative humidity(ERH; see above equation), and osmotic pressure. The aW of a solution is a colligative property , dependent upon the number of particles (molecules orions) in solution. Increases in solute concentration decreases aW. Microorganismsrequire WATER for solution of cell contents and metabolic processes. The cellmembrane is semi-permeable (or more correctly selectively permeable), anddecreases in the aW of the suspending medium below a certain maximum value(dependent upon the specific organism) will withdraw WATER from the cell,concentrating the cellular contents until the internal and external aW values are inbalance.

3 This concentrating effect slows metabolic processes until at a limiting value,growth ceases. Many microorganisms under osmotic stress (low aW) can accumulateor synthesize compatible solutes to relieve the stress. These solutes generallyinterfere little with the metabolic functions of the cell, and may be accumulation ofK+ ions, accumulation or synthesis of proline, glutamine, betaine, certain sugars orsugar alcohols ( trehalose in yeasts), etc. However, this ACTIVITY also requiresenergy, diverting some of the metabolic activities from growth to accumulation ofsolutes, and resulting in lowering of growth definitionsHalophiles, halotolerant organisms, xerophiles,Micro-organisms generally grow best between aW values , while mostmicrobes cease growth at aW< However, halophiles ( salt-loving ) are unableto grow in salt-free media and often have an obligatory requirement for substantialconcentrations of salt (NaCl).

4 For example Halobacterium halobium will not grow insalt concentrations below ca 14% w/w, aW ca Halotolerant organisms, whilecapable of growth at low aW / high salt concentrations, grow best at high aW values, Staphylococcus aureus will grow at aW at ca 10% of the maximum rate ataW ca organisms grow best at low aW values adjusted with sugars, for examplethe mould Xeromyces bisporus grows best at aW ca , although is capable ofgrowth at aW (ca 10% of maximum rate), but ceases growth at aW ca ,when aW is adjusted with react not only to aW per se, but also to the solute adjusting the aW values for growth are often very different for different poisoning, food-borne infectionsFood poisoning is the result of ingesting a pre-formed toxin in food.

5 These toxinsmay result in vomiting ( Staph. aureus or Bacillus cereus enterotoxins), or othersystemic effects, botulinal neurotoxin paralysis of the nerve-muscle infections result from ingesting an organism capable of surviving theacidic environment of the stomach and growing in the intestinal tract, Salmonellaspp. Gastro-intestinal symptoms, diarrhoea, result from toxic metabolitesproduced in the spoilage of foods results from changes in the food composition, and/orappearance or structure as a result of the growth and metabolism of the evolution of obnoxious odours is the cause for rejection of foods, meats, although the appearance of mould colonies on semi-dry foods, , cheeses, is also common. A wide range of organisms can be responsible forspoilage, and therefore a wide range of changes in foods may be regarded as spoilage.

6 Certain controlled spoilage by micro-organisms is used to produce adifferent food from the starting ingredients, yoghurt or cheese from milk,fermented sausages from raw meats, sauerkraut from shredded Effects on Microbial Growth and/or DeathCell Membrane PhenomenaThe cell membrane is semi-permeable, or rather selectively permeable. Thus glycerolpenetrates the membrane readily, glucose penetrates poorly, sucrose very poorly,and NaCl is almost non-penetrating. When an organism is grown or exposed to lowaW conditions, the cells may accumulate from the environment or synthesize compatible solutes , glutamine, proline, betaine in bacteria, trehalose in internal solutes interfere little with the metabolism of the cell, althoughmetabolic energy must be diverted for synthesis, but increase resistance to lowexternal aW conditions, and also increases resistance to other injurious treatments, heat.

7 This effect differs with different external solutes, Staph. aureussynthesizes compatible solutes at high NaCl levels, but not in the presence of the partially dehydrated cell is exposed to a high temperature, then themicroorganism displays a greater thermal resistance than when grown at a higheraW. Proteins and other essential cellular components are more resistant to thermaldamage in the partially dehydrated state. WATER ACTIVITY plays an important role inthe heat resistance of microbes (see Table 3). Death curves are not always linearand interpolation of D-values (and z-values) into application of thermal processesmay not always be safe. Similarly, the ratio of effects of the different solutes on D-values (as in Table 3) differs for each organism.

8 Thus D-values in low aW solutionsor foods must always take into account the actual solute controlling the aW, and ifnecessary be determined in that with other physico-chemical parametersSince extra energy is required to combat the inimical effects of low aW, any otherconditions also requiring expenditure of extra energy, low pH, presence ofpreservatives, will result in an additive or synergistic effect in limiting microbialgrowth. Thus even moderate reductions in aW in combination with low levels ofpreservative or pH values, can be sufficient to inhibit growth. One good example isthat of inhibition of Clostridium botulinum. Under ideal conditions 10% NaCl isrequired to inhibit the proteolytic species; at the pH values typical of meats (ca. ) and in the presence of ca 100ppm nitrite, only NaCl is required toproduce botulinal-stable cured 1.

9 Aw values of sugar solutionsGlucose Syrups %w/wAwGlucose%w/wDE , Dextrose equivalent, the concentration (%w/w) of glucose in 2. Minimum* aw values for microbial growthFood poisoning organismsFood-borne infectious organismsNameMin aW for growth*NameMin aW for growthBacillus jejuni ca. botulinum:Salmonella *, the minimum aW for growth of bacteria is generally by addition of salt. MinimumaW for growth with other solutes may be different. For toxin production minimum aWvalues may be rather 3. Effect of solutes on D-values of Salmonella (mins)Solute%w/wS. typhimuriumS. Models describing microbial modellingTo assess the MICROBIOLOGICAL safety or shelf life of foods the traditional methodswere to inoculate foods with the organisms of concern, incubate under conditionsappropriate to the food and enumerate the organisms or assessspoilage/acceptability.

10 When the parameters / recipes of the foods changed theexercise had to be repeated with the new conditions, and no prediction could bemade regarding the behaviour of the organisms in the food. Predictive microbiologyis concerned with collecting growth/survival data for relevant organisms in simplemedia systems under a wide range of physico-chemical conditions, aW, pH,temperature, gas atmospheres, preservatives, etc. From this data, mathematicalequations (models) describing the growth/survival/death of the organisms over therange of conditions studied are constructed. These are then validated in real foodsystems for which the physico-chemical conditions are known. If good agreement isachieved between the predicted and observed values then the model may be used topredict, within the range of conditions studied, growth, survival or death of thoseorganisms under conditions for which data has not been collected.