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.