ELEMENTS OF FOOD MICROBIOLOGY AND SANITATION
Both the safety and quality of foods require an understanding of the basics of food microbiology.
Organisms important in food include:
 | bacteria |
| yeasts |
| molds |
| viruses |
Although not living organisms, viruses are generally included as biological agents of concern. These materials are combinations of proteins and nucleic acids that can take over cellular functions. In addition of causing disease, they also can infect microorganisms used in beneficial fermentation.
Microorganisms either can be beneficial or harmful.
Harmful effects of microorganisms include:
| spoilage of foods |
| food borne toxification |
| food borne infections |
| viral borne infections |
Beneficial effects results from the use of microoganisms to utilize carbohydrates and product fermented foods, which include
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Factors affecting microbial growth:
The effect of organisms on the safety and quality of foods is dependent on the initial numbers of organisms present, processing to eliminate the organisms, control of environment to prevent growth and sanitation.
The major factors that influence the growth of microorganisms in food are:
| pH |
| oxygen availability |
| moisture availability |
| nutrient availability |
| storage temperature |
| lag time |
| generation time |
The food industry depends on minimizing microbial populations in the food and/or control of the environment.
pH:
Generally microorganisms, and especially pathogens, can not grow at pH levels below 4.0. These are termed acid foods and depend upon the low pH to prevent or minimize growth. These include foods like fruit beverages and salad dressings.
Oxygen:
Organisms can be classified as: a) aerobic, b) microaerophylic and c) anaerobic.
Aerobic organisms require air (oxygen) to growth and will not grow in the absence of air. These include yeasts and molds and a number of bacteria. It is a common practice to hot fill foods and seal them – so that a slight vacuum is formed in the package. A number of foods will be labeled "refrigerate after opening" – and this is because yeasts and molds can growth one the product is open and contains air.
Anaerobic organisms only growth in the absence of acid. One such organisms is Clostridium botulinum, which produces a toxin that can be lethal if ingested. Where foods are packed under a vacuum, the protection against Cl. botulinum growth is to heat treat the food (sterilization) to a time and temperature where any organisms present are destroyed.
Some organisms, such as lactobacilli, either can grow (or grow best) under slightly reduced conditions with low levels of oxygen – but do not grow anaearobically.
Moisture availability:
Organisms need free moisture in order to grow. Drying foods removes the available moisture and prevents growth of bacteria, yeasts and molds.
Control of water activity is a common method of preventing the outgrowth of microorganisms. Water activity is a measure of free (unbound) water available for chemical and biological activity.
Aw = vapor pressure of food product at a specified temperature
Aw is a measure of the relative humidity of the food where:
Aw = 1.0 = 100% relative humidity
Aw = 0.0 = 0% relative humidity
Food generally have water activities that range between 0.1 for dried foods to 0.96 for fluid foods.
Different organisms require different levels of free water in order to growth, with bacteria requiring more free water than yeasts and molds. A thumb rule is that bacteria do not grow at an Aw < 0.85 and that yeasts and molds do not growth at an Aw <0.65. There are some exceptions.
Intermediate moisture foods depend upon the addition of large quantities of sugars or polyols to reduce the Aw to below 0.85 and on packaging to prevent the growth of yeasts and molds.
Nutrient Availability:
Most foods contain adequate nutrients to support the growth of microorganisms, especially foods that contain both a fermentable carbohydrate and a protein source.
Sugar is the most common carbon source. In starch based foods, the action of amylases will frequently increase the available sugar source.
Storage temperature:
Organisms can be classified on the basis of their ability to grow at different temperatures:
-psychrophiles – grow best at temperatures <10 ° C
-mesophiles – grow best at ambient temperatures 25-35 ° C
-thermophiles – grow best at temperature > 40 ° C
-psychrotrophic - tolerate low temperatures and can grow under refrigeration
-thermotrophic - tolerate high temperature and can grow at 55-60 ° C.
Most pathogenic bacteria are mesophilic, although a few (such as Listeria monocytogenes or Cl botulinum type E) can growth under refrigeration conditions.
Organisms that spoil refrigerated products are generally psychrotrophic, and frequently belong to the genus Psuedomonas.
Lag, generation time and numbers required to achieve an effect on the food:
The amount of time required for an organism to reach the log growth phase is termed the lag time and the time required to double the population of the organisms is termed the generation time. The amount of time required for an organism to reach a specific number is dependent upon the initial population, the lag time and the generation time. Under ideal growth conditions, microorganisms can double their number in about 30 minutes. As conditions move away from their optimum, the generation time is decreased – until eventually no growth occurs.
In processed foods, organisms surviving have generally been stressed and commonly may exhibit a lag time of 2-4 days. In fermented foods, where cultures are added, the organisms are added in an active growth phase and the lag time is 0 or very small.
A thumb-rule for the numbers of organisms required to produce toxins or to produce desired or undesired flavors is one million/g. (106/g). Thus for food born toxification (such as caused by Cl. botulinum, Staph. aureous and Bacillus cereus), there has to have been large number produced. Similarly, the numbers of organisms required for fermented foods (such as yogurt) is also very large.
However, in the case of organisms causing infectious disease (E. coli 0157, Listeria monocytogenes and Salmonella), the numbers of organisms that can cause the disease can be very small. FDA generally requires that no organisms can be recovered from 100 g of food after suitable incubation.
Under ideal growth conditions it can take a relatively short period of time to increase from 1/g to 1 million/g. (20 generations). At 4 generations/hour, the food can reach >1 million in just 5 hours. If the initial load is 1000/g, then the time required to reach 1 million would be 2.5 hours (10 generations).
Minimizing Microorganisms during processing and handling:
Keeping microbial loads at minimal levels is essential to provide safe food of high quality. This requires care in food handling and minimizing microorganisms in the product during processing. Key to this goal is preventing contamination of the food during contact with equipment (or food contact) surface. Cleaning and sanitizing are important steps in the operation of any food plant – and will become more important in the future as the industry deals new and emerging microorganisms – such as E. coli 0157 that first began to be associated with food borne infections in the 1980’s
Cleaning:
Cleaning of food plant equipment generally involves the use of relatively strong chemicals and can be considered a 4 step process: a) rinsing away excess soil, b) bringing the cleaning compound in contact with the soil, c) removing the soil from the equipment surface and final cleaning.
The factors that influencing cleaning efficiency are:
| nature of the soil and the cleaning compound used |
| concentration |
| time |
| force |
The removal of soil in a first order reaction in respect to the concentration of the cleaning compound and time of cleaning. This means that increasing concentration and time increase soil removal up to a point where further increases cause very little additional removal. Increasing force increases soil removal. Temperature increases soil removal, up to the point of protein denaturation – at which point protein denaturation decreases the effect of further increases in temperature. Generally a 10° C change in temperature causes a two fold change in soil removal [Q10 = 2]
The soil on a food contact surface may be one or more of the following:
| protein |
| fat |
| minerals |
| soluble solids (sugars, etc) |
Heated surfaces, especially those containing protein, are always more difficult to clean that unheated surfaces.
Alkaline cleaning compounds are used to remove fat and proteins, whereas acid is used to remove mineral soils. Minerals are frequently complexed with proteins and the addition of chlorine to the alkaline cleaner increases the ease of removal.
Cleaning compounds perform their cleaning via different functions, which include:
| emulsification (making fat into an emulsion that is water soluble and easily removed. |
| peptizing (chemically degrade proteins into smaller units and thus more easily removed |
| saponification (chemically convert oils and fats into soaps through interaction with sodium hydroxide |
| wetting (reducing the surface tension of the water) |
| water conditioning (removal of divalent cations that impede the action of the cleaning compound |
Cleaning may be achieved by hand or by circulation of the cleaning compound over or through the equipment. Circulation cleaning may be in place or may be the circulation of cleaning solution of cleaning compound of equipment components in a tank.
| CPI = cleaned in place |
| COP = cleaned out of place (circulation tank) |
CIP and COP are more efficient than hand cleaning because:
| can use stronger cleaning compounds (higher or lower pH)can use higher temperatures |
| provides for more force |
| eliminates potential contamination of equipment from contact with humans |
Sanination:
Cleaning reduces, but does not totally eliminate, microoganisms from the surface of the equipment. Generally chemical sanitizers are used to further reduce organisms on the equipment surfaces. However, sanitizers are only effective for clean surfaces – and will not penetrate residual soil to kill bacteria below the exterior surface.
Common sanitizers include:
| chlorine (100 - 200 ppm) |
| iodine (25 - 50 ppm) |
| acid/surfactant blends (100-200 ppm) |
| quaternary ammonium germicides. (200 ppm) |
The sanitizers are listed in order of decreasing effectiveness. The halogens are most effective over the widest range of different organisms.
Chlorine is especially affected by organic matter. Iodine sublimes at high temperatures. Quaternary ammonium compounds have a residual effect, but are not effective against viruses or gram - organisms.
Destruction of Microoganisms:
Microbial destruction is achieved either by heat or chemicals. The most common method of making foods safe is to use thermal processing to eliminate pathogens and then to use good sanitation practices to prevent them from re-entering the food after thermal processing.
Spores are more resistant than vegetative cells and thus require more heat to kill.
Two general heat processes are used:
| Pasteurization - heating to a specific temperature for a specific time to kill the most heat resistant vegetative pathogen |
| Sterilization - heat to a specific temperature for a specific time to kill the most heat resistant spore forming organism. |
Neither process kills all of the organisms in the food, and non-pathogenic, spoilage organisms can survive to some degree. Since pasteurization does not kill spores, pasteurized products are kept under refrigeration to control the growth of surviving spore formers that grow well at ambient temperature.
Two terms are used to evaluate the heat resistance of an organism:
D value = the time required at a given temperature to reduce the organism by 90%(one log cycle)
Z value = the temperature required to reduce the D value by 90% (one log cycle)
The industry uses the term Fo to describe the thermal process (time and temperature) to reduce the microbial population by a prescribed number of log cycles (generally 12). The FO required for non-acid foods is higher than than required for acid foods (See Chapter on Canned foods)
