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 negatively affect 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, whether or not a food item will be affected by microbes 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 responsible for food spoilage 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 affect the condition of food. 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 that affect the condition 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 easily affected due to yeast and moulds. The pH level in any natural food is an inherent property. So, this again is an intrinsic factor responsible for food spoilage.
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 in sugars have a greater chance of getting affected by microbes 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 that affect food freshness.
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 responsible for 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 responsible for reducing 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.
There are other factors responsible for food spoilage in an indirect way. A good example is naturally occurring toxins such as Aflatoxin and Ochratoxin A (both are Mycotoxins). BSE (bovine spongiform encephalopathy or “mad cow disease”) is caused by an unconventional agent. Then there are compounds that gather in the atmosphere – Persistent Organic Pollutants (POPs). A good example from this category would be Dioxins and polychlorinated biphenyls. Finally, some metals such as lead, cadmium and mercury can have severe health implications including brain damage, kidney dysfunction, etc.
In the final analysis, control of microbiological spoilage of food requires a clear understanding of the underlying biomechanical and chemical processes that transform palatable food into a poisonous one. Knowledge of different manifestations of the same microbe, the chances of any particular strain of microbe with a given food item, etc., all help in taking effective preventative steps. Checking the merchandise for “expiry” or “best-before” dates, maintaining proper hygiene in the kitchen and storage places, choosing the right temperature and moisture levels for storing food ingredients, etc., are some of the fundamental measures to prevent food spoilage and its consequences of intoxication/poisoning.
The necessity of such measures becomes all the more compulsory when seen in light of the following statistics. Every year 325,000 people are hospitalized due to diseases caused by food. Nearly 76 million people suffer from gastrointestinal problems. More importantly 5,000 people lose their lives due to this. These numbers pertain to the American population alone, which is economically and technologically the most advanced country. The distress and turmoil caused by food-related diseases in the lesser developed parts of the world are much higher.
References:
AS Battey, S Duffy, DW Schaffner, Modeling Yeast Spoilage in Cold-Filled Ready-To-Drink Beverages with Saccharomyces cerevisiae, …, Applied and Environmental Microbiology, 2002.
I. Ashie, JP Smith, BK Simpson, Spoilage and shelf-life extension of fresh fish and shellfish., Critical Review- Food Science and Nutrition, 1996.
D. Mossel, J Westerdijk – Antonie van Leeuwenhoek, The physiology of microbial spoilage in foods, published by Springer publications in 1949.
T Deák, LR Beuchat, Handbook of Food Spoilage Yeasts, published in 1996.
E Lück, M Jager, Antimicrobial Food Additives: Characteristics, Uses, Effects, 1997.
A Maxcy, Differentiation of Food Spoilage Bacteria on the basis of their Ability to Utilize Different Proteins…, JOURNAL OF FOOD SCIENCE, 1979.
L Leistner , Basic aspects of food preservation by hurdle technology,
International Journal of Food Microbiology, 2000.
T. McMeekin, K Presser, D Ratkowsky, T Ross, M …, Quantifying the hurdle concept by modelling the bacterial growth/no growth interface, International Journal of Food Microbiology, 2000.
V. Hughey, EA Johnson, Antimicrobial activity of lysozyme against bacteria involved in food spoilage and food-borne disease …, Applied and Environmental Microbiology, 1987.
Microbial and biochemical spoilage of foods: an overview, International journal of food microbiology, 1996.