Module 3. Microbiology of food commodities

Lesson 6

6.1 Introduction

Spoilage of food can be defined as any visible or invisible change which can makes food or product derived from food unacceptable for human consumption. Major causes of food spoilage are enlisted in Table 6.1.

Spoilage of food not only causes health hazard to the consumer but also cause large economic losses. Spoilage not only leads to loss of nutrients from food but also cause change in original flavor and texture. It is estimated that about 25% of total food produced is spoilt due to microbial activities only despite range of preservation methods available. Thus the spoilage of food is not only a health hazard but also carry lot of economic significance too.

In total, the food spoilage is considered a complex phenomena whereby a combination of microbial and biochemical activities take place. Due to such activities, various types of metabolites are formed which aid in spoilage. The detection of these metabolites help in detection of spoilage.

The ease with which foods are spoiled depend upon factors described. The foods are thus divided into different classes as:

6.1.1 Perishable foods

These foods are readily spoilt and require special preservation and storage conditions for use. The foods of this class are mostly used daily such as milk, fruits, vegetables, fish etc.

6.1.2 Semi-perishable foods

This class of foods if properly stored can be used for a long duration e.g. potatoes.

6.1.3 Non-perishable foods

These foods remain in good form for long duration unless handled improperly. It include sugar, flour etc.

Table 6.1 Major causes of spoilage


6.2 Factors Affecting Microbial Spoilage of Foods

While the spoilage by physical and chemical modes play important role, the microbial spoilage has most significant role. Combination of all these factors is ultimately responsible for overall decay of food. The spoilage of food can occur at different stages of production, processing. Various causes of spoilage at different stages are depicted in (fig. 6.1)

The spoilage of foods due to microbial activity initiates when undesirable microorganism colonize the food. Once colonization is established, the microbial community grow on the food constituents, thereby utilizing them for their metabolism. During the course of such microbial activity, the food become unsuitable for human consumption. Various parameters affect the proliferation of microbes. These include intrinsic to food such as water activity, acidity, oxidation-reduction potential, presence of antimicrobial compounds in food and food structure. Extrinsic parameters such as temperature, humidity and other storage condition aid in spoilage.


Fig. 6.1 Possible causes of spoilage and microbial contamination at different stages of food production

6.3 Microorganisms in Food

Microorganisms by virtue of their ubiquity and diversity in metabolism are most significant cause of food spoilage. Bacteria and fungi (including yeasts and moulds) are major cause of food spoilage.

Bacteria are round; rod or spiral shaped microorganisms and can grow under a wide variety of conditions. There are many types of bacteria that cause spoilage. Food spoiling bacteria are primarily divided two groups viz. spore-forming and nonspore-forming. Bacteria generally grow in low acid foods like vegetables and meat.

Yeasts growth causes fermentation which is the result of yeast metabolism. There are two types of yeasts true yeast and false yeast. True yeast metabolizes sugar producing alcohol and carbon dioxide gas. This is known as fermentation. False yeast grows as a dry film on a food surface, such as on pickle brine. False yeast occurs in foods that have a high sugar or high acid environment.

Molds grow in filaments forming a tough mass which is visible as `mold growth'. Molds form spores which, when dry, float through the air to find suitable conditions where they can start the growth cycle again.

Mold can cause illness, especially if the person is allergic to molds. Usually though, the main symptoms from eating moldy food will be nausea or vomiting from the bad taste and smell of the moldy food.

Both yeasts and molds can easily grow in high acid foods like fruit, tomatoes, jams, jellies and pickles. Both are easily destroyed by heat. Processing high acid foods at a temperature of 100°C (212°F) in boiling water in can for the appropriate length of time destroys yeasts and moulds.

a) Rod shaped Bacteria

b) Mould

c) Yeast

6.4 Changes in Foods Due to Microorganisms

As microorganisms grow in food, by virtue of their diversity in metabolism they utilize components of foods and convert them into variety of chemical compounds. An overview of these changes is given in Figure 6.2


Fig. 6.2 Changes in major food components during spolage

6.5 Change in Carbohydrates

Carbohydrates are used to obtain energy. While monosaccharide are preferred over complex carbohydrates, microorganisms have ability to convert polysaccharides to simpler forms before obtaining energy. The utilization of simple sugar such as glucose vary under aerobic and anaerobic conditions. In aerobic conditions it is converted into carbon dioxide and water through glycolysis and other related pathways. In absence of oxygen, the process yields a number of compounds in different organisms. This process is known as fermentation. These compounds include:

6.5.1 Alcoholic fermentation

It occurs due to yeasts and carbon dioxide and ethanol are the major end products.

6.5.2 Lactic fermentation

It is of two types viz. homolactic fermentation where primarily lactic acid is the end product and heterolactic fermentation where along with lactic acid, acetic acid, ethanol, glycerol, carbon dioxide are produced.

6.5.3 Coliform type fermentation

This type of fermentation occur in coliform bacteria. In this process acids such as lactic, acetic, formic. Ethanol, glycerol etc. are produced.

6.5.4 Propionic fermentation

It occurs in propionic bacteria and in it along with propionic acid, succinic acid and carbon dioxide are produced.

6.6 Change in Nitrogenous Compounds

Proteins are the major source of nitrogenous compounds in foods. Thus degradation of proteins, include hydrolysis by enzymatic reactions. The source of enzymes can be either microbes or foods own enzymes. Complex proteins are converted into polypeptides, simpler peptides and amino acids. The enzymes involved in conversion of proteins into polypeptide are termed as proteinase while those catalyzing conversion of polypeptides to amino acids are called peptidases. The decomposition of proteins can be aerobic or anaerobic. Usually the anaerobic decomposition of proteins results in obnoxious odors. This process is known as Putrefaction. Along with nitrogenous compounds, other compounds responsible for such smells also include sulfur compounds.

The microbial activity on amino acids cause either deamination (removal of amine group) or decarboxylation (removal of carboxyl group). Major organisms involved in conversion of nitrogenous compounds include Pseudomonas, E. coli, Clostridium, Desulfotomaculum etc.

6.7 Changes in Lipids

The hydrolysis of lipids is accomplished by lipase enzymes produced by different microorganisms. The major end products include glycerol and fatty acids, which are further used by microorganisms for their metabolism. The process of conversion is known as oxidation. The oxidation of fats is also done by enzymes of food itself. High fat containing foods are prone to such processes.

6.8 Public Health Aspect

Worldwide, foodborne pathogens cause numerous sufferings and deaths. In Africa, Asia, and Latin America, there are about 1,000 million cases of gastroenteritis per year in children under the age of 5, which leads to 5 million deaths. In Mexico and Thailand, half of the children aged 0–4 years suffer from the Campylobacter induced enteritis. In Europe, 50,000 cases/ million population suffer from acute gastroenteritis. In the Netherlands about 300,000 cases/million population occur yearly. In Northern Ireland and the Republic Ireland, about 3.2 million episodes of gastroenteritis are reported each year. In Australia, 5.4 million cases of foodborne gastroenteritis occur each year. In England 20% of population, i.e., 9.4 million people suffer from acute gastroenteritis each year and the primary contributing microorganisms are identified as Norovirus, Campylobacter species, rotavirus, and nontyphoidal Salmonella species. In the US, there are an estimated 6 million cases with 350,000 hospitalizations and 9,000 deaths associated with foodborne infections each year. Foodborne pathogen statistics show declines in incidences from 1996–1998 to 2005 for some pathogens but increased for others: the incidence of Shigella decreased by 43%, Yersinia species by 49%, Listeria monocytogenes by 32%, Campylobacter species by 30%, EHEC O157:H7 by 29%, and Salmonella Typhimurium by 42%; however, the incidence of Salmonella Enteritidis and S. HeidelbergS. javiana by 82%. Interestingly, the number of outbreaks and product recalls continued to increase thus placing a huge economic burden on producers and processors. Mycobacterium paratuberculosis is the causative organism of Johne’s disease in dairy cows. However it has also been linked to Crohn’s disease in human beings. The organism may survive conventional pasteurization treatment of milk increased, each by 25%, and

Mycotoxins are fungal metabolites that get formed in foods due to the growth of some strains of Pencillium, Aspergillus and some other molds. They are secondary metabolites and are of low molecular weight. They are highly thermo stable and withstand the conventional processing temperatures. They are highly toxic to a large number of animals and also to human beings. The food borne disease resulting from the ingestion of toxin in mold contaminated food is called mycotoxicosis. They affect kidney, liver, cause skin irritation, birth defect and death. They have also been found to be carcinogenic in nature.

Table 6.2 Mycotoxins produced by Fungi in foods


Aflatoxins are produced by some strains of Aspergillus flavus and A. parasiticus and they occur in different chemical forms like B1, B2, G1 and G2. Aflatoxin M1 and M2 are the hydroxylated derivatives of B1 and B2. Milch animals fed with aflatoxin B1 and B2, excret aflatoxin M1 and M2, in milk, urine and feces. Aflatoxin B1 is the most toxic among all aflatoxins. However M1 and M2, are relatively less toxic as compared to the parent compounds B1 and B2. Aflatoxins are found in nuts, spices and figs and they are produced under hot and humid conditions during storage.

Viruses are obligate intracellular parasites. They are host specific. Several viruses have been implicated in food borne outbreaks. There are four acute gastro enteritis namely calici virus, rota virus, astro virus and adeno virus. The infectious hepatitis A virus enters a person through the contaminated food or water and it causes gastro enteritis. Viruses responsible for poultry diseases have also been implicated in human aliments.
Last modified: Friday, 2 November 2012, 10:18 AM