As all the food that is consumed is biological and not synthetic, they are always susceptible to get spoilt. The leading cause for much of food spoilage is microbes. Not all microbes have a negative effect on the food that they act upon. For example, microbes acting on milk and butter can produce yoghurt and cheese respectively. The resulting food items are not considered as spoilage.
The concern of this essay is with regard to those microbes whose action upon edible substances results in the deterioration of that substance and hence makes them toxic. The mechanism of action of these microbes is quite interesting. For example, some microbes release enzymes into the liquid surrounding them and absorb nutrition from the fluid digested externally. On top of that, this process of digestion leaves behind a residue of toxic waste, which are poisonous. That is why food items (both natural and processed) with high fluid content tend to be more susceptible for spoilage. So this is one internal or intrinsic condition for food spoilage.
Also, the vulnerability of a food item for a microbe attack is largely determined by its classification. Food such as carbohydrates, fats and proteins are made up of organic compounds, whose nutrition attract microbes. On the other hand vitamins and minerals have nothing to offer the microbes so they leave them alone. So, how organic a food is determines its vulnerability to microbes. This again is an intrinsic factor.
A fundamental extrinsic factor is the temperature. Only at a suitable temperature range can microbes survive (this is why refrigerators are effective in preserving food). It is measured that temperature between 0 and 60 degrees Celsius are the most conducive for microbe growth, with more microbial action at temperatures closer to 60 degrees and lesser microbial action at temperatures closer to 0 degrees. At the same time, too much heat is also detrimental for the survival of microbes. Any rise in surrounding temperature above 60 degrees can quickly destroy both the microbes and their enzyme excretions. Insights into the understanding of temperatures and microbe activity in food are applied in the design of refrigerators, pressure cookers and ovens. Different food items have different ranges of ideal temperatures.
We saw how the fluid content in food is a breeding ground for microbes. But external water, which usually manifests in the form of moisture or humidity, can also play a role in food spoilage. Storing food in an atmosphere where humidity is less than 70% will help prevent spoilage. A more advanced technique involves removing air altogether and thereby eliminating any possibility of spoilage. This is the concept behind vacuum-packed foods. In both these cases, humidity and air are external factors affecting the spoilage of food.
Another factor inducing the spoilage of food is pH level. Each organism has an ideal level of pH that suits its growth. A pH imbalanced food is particularly prone to fungal attacks as fungi have higher tolerance levels for acid than bacteria. For example, even a pH level of 4.5 will not thwart the growth of fungus. Fungus are known to survive even level lower than 4.5. But bacteria need the pH to be around 7. The food items that have a pH below 4.5 are classified as acid foods. Acid foods are more susceptible to spoilage due to yeast and moulds. The pH level in any natural food is an inherent property. So, this again is an intrinsic factor.
Oxygen is a particular element that has a lot of influence over microbial growth. For instance, a change in the partial pressure of oxygen can change the chances of spoilage. Having said so, different foods have different oxidizing potentials. So a combination of these two factors determines the chances of spoilage. In this case, it is an external factor in that the surrounding atmospheric oxygen that prompts microbial growth. At the same time, it is an intrinsic factor in that the oxidizing potential of the particular food item also determines the outcome.
The nutritional composition of the food also determines the kind of microbes that can grow. Each microbe differs in its ability to digest a particular nutrient. Nutrients that are easy to access, assimilate and digest will invite a larger number of microbes. For instance, foods that are rich I sugars have a greater chance of microbial infestation than ones that have a high proportion of polysaccharides. Sugar levels also determine the nature and spread of microbial growth. Other ingredients like lysozyme, which are found in eggs, prevent food spoilage. Thus, the nutrients and their proportions are an intrinsic factor.
The purpose of packaging food is to reduce the incidence of microbial infestation. This is achieved in two ways. First, the packaging prevents airborne and water borne microbes from entering the food. Secondly, by blocking out air and water, the essential conditions for microbial growth are eliminated. Having said this, each food item, whether naturally obtained or processed in factories, will have their own ideal packaging material. Choosing the right packaging material involves consideration of temperature, penetrable light, foil thickness, humidity, etc. So this is an extrinsic factor that influences the spoilage of food.
Many commonly used food items are susceptible to spoilage due to fungus. For example, Botrytis cinerea makes grapes and strawberry to rot quickly and blue mould rotting in tomatoes is a result of Penicilliumi infestation. Another fungus called Guignardia bidwellii also acts on grapes. The primary cause of spoilage in apples is Sclerotinia sclerotiorum and spoilage in oranges is induced by Penicillium digitatum. So, all varieties of foods can be spoilt and poisoned by micro-organisms.
In this context, a discussion of the “hurdle concept” is relevant. Hurdle concept is essentially placing hurdles before the factors discussed above, so that their detrimental impact on the food will be negated or reduced. In other words, creating conditions or adding ingredients in the processing and packaging of food so as to minimize food spoilage is called the “hurdle concept”. The application of the “hurdle concept” has advanced very much over the last few decades. Traditionally hurdles were determined based on common observations and knowledge of basic bio-chemistry. But with the advance in technology, hurdles are developed that act at a molecular level. Some hurdles even change the DNA of an organism so as to make it resistant to microbial intoxication.
Hurdles are applied to almost all food that we consume. Hurdles are used in the preservation of milk. There are lots of factors that could reduce the shelf life of milk. Bacteria can be a threat and so can high temperature. An internal factor that influences milk spoilage is the vulnerability of fat content in the milk. By applying one or more of the following techniques the shelf-life of milk can be prolonged. They include High-Temperature/Short-Time Pasteurization, Ultra-pasteurization, Enhanced Thermal Processing, Higher-Heat/Shorter-Time, etc.
The degree of success of any food preservation strategy can have profound implications. Diseases such as Salmonellosis, Enterohaemorrhagic E. coli and listeriosis, Cholera, Campylobacteriosis, etc., are all food borne diseases. Salmonellosis is caused by bacteria that are found in Salmonella. Such staple food items like meat, milk and eggs can carry these micro toxins. Campylobacteriosis is another common ailment. Some of the symptoms of this disease include severe stomach cramps, diarrhoea, vomiting and fever. These diseases consume numerous human lives each year. So the importance of food safety and the application of hurdles in preserving food items cannot be overemphasized.