Both heat and organic acids (such as acetic acid) can be used to ensure the safety of acidified vegetable products.

Processing of Food: Heat and acid killing of microbes

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Both heat and organic acids (such as acetic acid) can be used to ensure the safety of acidified vegetable products.  Based upon our current knowledge, acidified vegetable products that have a pH value above 3.3 must receive a heat treatment to ensure safety.  For products acidified with acetic acid that have a final pH at or below 3.3, the acetic acid present can ensure the death of pathogens, given a sufficient holding time.

Bacterial Cells 

Bacterial cells vary widely in their ability to survive heat treatments.  Some very sensitive species die rapidly at a temperature of 120°F (49°C), while some thermophilic bacteria grow well at this temperature.  There are even some bacteria which grow in deep ocean volcanic vents that can only live and grow at very high pressures and boiling (212oF or 100oC or greater) temperatures.  The vegetative cells of most bacteria, including food pathogens, spoilage bacteria, and the lactic acid bacteria used in vegetable fermentations, are readily destroyed by heating to 160°F (71°C), especially when the pH is low.

Acid and low pH are also toxic to most bacteria. This is why vegetable fermentations are useful for food preservation.  The lactic and acetic acids produced by lactic acid bacteria during fermentation, as well as other metabolic end-products, preserve vegetables by preventing the growth of other microorganisms, including human pathogens.  In acidified foods, acid (typically acetic acid or vinegar) is added for the same reason.

Bacterial Endospores 

The spores of bacteria, including the spores of C. botulinum, are extremely heat-resistant compared to vegetative cells.  It would be necessary to use a retort with high pressure steam to destroy these spores.  The spores of C. botulinum are widespread in the environment and present in many foods we eat, but pose no danger since only the vegetative or growing cells of C. botulinum can produce botulism toxin and cause disease. 

In most foods, the oxygen present in air and competing microorganisms will keep C. botulinum from growing.  The germination of spores of anaerobic microorganisms in sealed jars or containers of acidified foods is prevented by keeping the pH at or below 4.6.  It is important to remember that heat treatments designed for acidified foods will not kill most spores, so keeping the pH of acidified foods at or below 4.6 is required to prevent botulism.  Fortunately, E. coli, Listeria, and Salmonella do not form spores. The sensitivity to heat of vegetative cells makes thermal processing a very useful process for ridding acidified foods of these pathogens and other microorganisms which could cause disease or spoilage.

Yeasts and Molds 

Most yeasts and molds are heat-sensitive and destroyed by heat treatments at temperatures of 140-160°F (60-71°C).  Some molds make heat-resistant spores, however, and can survive heat treatments in pickled vegetable products.  These molds, however, require oxygen to grow.  When jars or containers of thermally processed acidified vegetables are improperly sealed or cracked, oxygen may get in.  Under these conditions, spores that survived heat treatment may germinate and grow on the surface of the liquid inside the container where air is present.  These molds can consume the acid present in these products, causing the pH to rise above 4.6, which in turn can lead to the growth of C. botulinum and potentially the production of deadly botulinum toxin.  Therefore, it is very important to make sure that containers of acidified foods are properly sealed.

There are relatively few spore-forming microorganisms which can grow without oxygen and at pH values below 4.6.  One such organism is a mold named Byssochlamys fulva.  This organism has been responsible for the spoilage of thermally processed canned fruits.  It is quite heat-resistant, requiring about 1 minute at boiling temperature to kill cells of the organism, and may survive the heat treatment. 

Fortunately, this mold has not been reported to be a problem in acidified vegetable foods.  When spoilage of thermally processed acidified food products does occur, it is usually because some jars or containers were not heated for the required time at the correct temperature.

Factors influencing the survival of microbes

Not only do different species of microorganisms vary widely in resistance to heat and acid, but sensitivity within a single species is also variable.  Many factors, including the time and temperature of exposure and the amount of acid present, can affect the heat and acid resistance of microorganisms.  A particularly important factor is the conditions the microorganisms have been exposed to prior to exposure to acid or heating. 

For example, some bacteria can become more heat-resistant than normal by exposure to low levels of heat that do not kill but only stress the bacteria.  When thermal processes are developed for acidified foods, all these factors must be taken into account to assure that the final products produced are safe and will not spoil.

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1. Number of Cells Present 

Individual cells of microorganisms in a population are not all killed at the same time.  With most bacteria, only a percentage of the cells present are killed in a given time increment.  It is very important that manufacturers of acidified foods understand this and keep the microbial population of foods prior to final processing to a minimum.  Equipment should be cleaned on a regular schedule and the food moved through the manufacturing operation rapidly.  This is one important reason why good sanitation is so important in food processing plants.

2. Relationship of Time and Temperature 

 The time required to kill microorganisms with heat changes dramatically with temperature.

3. Physiological Condition of the Microbe 

 The conditions under which microorganisms have been living can greatly influence the ability of an organism to be killed by acid conditions or heating.  Important factors include:  (a) the culture medium or food in which the organism has grown, (b) the temperature of growth, and (c) the phase of growth or age of the culture.  In addition, when bacteria are subjected to one kind of stress, such as chemical sanitizer treatments, surviving bacteria can become more resistant than usual to heating or other environmental insults.

4. pH and Acidity

Most bacteria are rapidly killed at pH and acid concentrations typical of acidified foods, but the type and concentration of acid present can play a significant role.  Not all acids are equally effective in killing bacteria. 

The killing effects of a given type of acid can be differentiated from pH effects. Thermal processes for acidified foods are routinely less severe than processes for high pH foods (low-acid foods).  This is because acid and pH enhance the killing effect of heat.  It is important to remember that the killing effects of heat and acid combined on vegetative cells of bacteria are more effective than either alone. 

Bacterial endospores are also killed more rapidly as pH is lowered, but the effects are not significant because most spores are very resistant to acid and heat.  As mentioned above, spores usually survive heat treatments applied to acidified foods.

5. Sugar and Salt 

 As with acid and pH, sugar and salt concentration can affect the ability of bacteria to survive in acidified foods. However, at concentrations typically used in acidified vegetable products, evidence suggests that sugar and salt have relatively little effect on the survival of bacteria.  Some yeasts, however, which grow best in high concentrations of sugar become more heat-resistant as sugar concentration is increased.

6. Other Food Ingredients 

 Food constituents, such as oils and fats, may protect microorganisms from heat to varying degrees because moist heat is much more effective than dry heat in killing microbes.  Cells killed almost instantly in boiling water may survive for hours on dry surfaces in an oven at the same or higher temperatures.  Cells suspended in oil respond very much like cells on dry surfaces.  This is an important consideration in developing thermal processes for acidified foods marinated in oil.  These foods may require longer heating times or higher temperatures than other acidified foods.  It is also important to acidify oil-marinated foods prior to adding oil because oil may impede the penetration of acid into the food product.

Products Not Requiring Heat Treatments 

Acidified foods with pH values above 3.3 must be heated to ensure the destruction of pathogenic bacteria.  For products where acetic acid is used as the primary acidulent that have a pH value at or below 3.3, safety can be assured without a heat process.  However, to safely produce these products without a heat treatment they must be held at 77oF (25oC) or higher for a minimum of 48 hours prior to distribution.  The 48-hour holding time is required to ensure that the acetic acid present has sufficient time to kill the vegetative cells of bacterial pathogens that may be in the product.

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