FOOD MICROBIOLOGY

 

• Introduction

DEFINITION

Food Microbiology is the study of microbes (micro-organism)that create, inhibit or/and contaminate food.

Food Microbiology thus deals with

  • Food Spoilage
  • Disease causing pathogens
  • Beneficial roles of microbes e.g. fermentation, pro-biotics
  • Microorganism groups important in food microbiology

Viruses

Definition

Virus, infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria. The name is from a Latin word meaning “slimy liquid” or “poison.”

Viruses, especially norovirus and hepatitis A, are an important cause of food borne diseases. Viral contamination of food occurs through contact with infected persons, e.g. food handlers.

Food contaminated with viruses can cause dangerous infections in humans ranging from mild diarrhea to severe hepatitis. Foods that are handled manually and are not processed before consumption are at particular risk of viral contamination.

Bacteria

Definition

Bacteria are unicellular micro organisms which do not have cell wall and nucleus.

Bacteria have both beneficial and harmful role in food.

Beneficial role of Bacteria in Food

Bacteria are commonly used in the production of variety of dairy products. Bacteria are used to make a wide range of food products. The most important bacteria in food manufacturing are Lactobacillus species, also referred to as lactic bacteria. Bacillus subtilis is used to produce amylase and also used to produce hyaluronic acid, which is useful in the joint-care sector in healthcare.

 

Harmful Role of Bacteria in Food

 

Bacteria Inhabitation Mode of Spread Illness Caused

Acinetobacter

baumannii

Soil

Water

Fresh Refrigerated Products

Uncooked Contaminated Food

Pneumonia

Skin&Wound Infections

Meningitis

Bacillus Cereus

Soil

Food

Milk

Meat

Uncooked

Cooked

Refrigerated food

Milk

Diarrheal Illness
Clostridium botulinum

soil and sediments

 

Improperly Canned food products like meat,poultry Botulism
Escherichia coli Different Media and Many Food

Fecal Pollution

Food

Food Borne Gastroeneritis

Foodborne-illness can be placed into five groups: enteropathogenic (EPEC), enterotoxigenic (ETEC), enteroinvasive (EIEC), enterohemorrhagic (EHEC) and facultatively enteropathogenic (FEEC).

Salmonella (S. typhimurium, S. typhi, S.enteritidis)

Food

Finished Food Products

Milk,Meat,Poultry

Leading cause of Food borne illness in humans.

Enteric Fever

Typhoid

Paratyphoid

Staphylococcus aureus Food Food

Enterotoxins

Gastro intestinal disorders

Shigella Faeces Contaminated food Shigellosis

 

Fungi (Yeast & Molds)

Fungus, plural fungi, any of about 144,000 known species of organisms of the kingdom Fungi, which includes the yeasts, rusts, smuts, mildews, molds, and mushrooms. Fungi are among the most widely distributed organisms on Earth and are of great environmental and medical importance. Many fungi are free-living in soil or water; others form parasitic or symbiotic relationships with plants or animals.

Yeast are single-celled microorganisms that are classified, along with molds and mushrooms, as members of the Kingdom Fungi.

A mold or mould is a fungus that grows in the form of multicellular filaments called hyphae.

Algae

 

Algae are a diverse group of aquatic organisms that have the ability to conduct photosynthesis. Certain algae are familiar to most people; for instance, seaweeds (such as kelp or phytoplankton), pond scum or the algal blooms in lakes.

Their gelling, thickening and stabilizing properties have led to the development of such products as agar, alginate and carrageenan. Moreover, algae are used in the food industry as food supplements and an addition to functional food. Algae are also added to meat products, such as pasty, steaks, frankfurters and sausages, as well as to fish, fish products, and oils, to improve their quality. Cereal-based products, such as pasta, flour and bread, are another group of products enriched with algae. Due to their properties algae may also be used for construction of fermented functional food. Fermented products containing algae are, most of all, dairy products, such as cheese, cream, milk deserts, yoghurt, cottage cheese, and processed cheese. Combination of fermented products offering a high content of lactic acid bacteria with algae possessing biologically active metabolites of natural origin allows not only to compose products with a high content of nutrients, but also to create a brand new segment of fermented food.

 

Parasites

A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites.

Numerous parasites can be transmitted by food including many protozoa and helminth.The most common foodborne parasites are protozoa such as Cryptosporidium spp., Giardia intestinalis, Cyclospora cayetanensis, and Toxoplasma gondii; roundworms such as Trichinella spp.

 

• Factors affecting the growth of microbes

The most important factors that affect microbial growth in foods can be categorized as under (i) factors related to the food itself, the “intrinsic factors,”

  • Nutrient content,
  • Water activity,
  • Ph value,
  • Redox potential, and
  • the presence of antimicrobial substances and mechanical barriers to microbial invasion;

(ii) factors related to the environment in which the food is stored, the “extrinsic factors,”

  • including the temperature of storage,
  • the composition of gases and
  • relative humidity in the atmosphere surrounding the food;

(iii) factors related to the microorganisms themselves, the “implicit factors,”

  • interactions between the microorganisms contaminating the food and
  • between these microorganisms and the food, e.g., their abilities to utilize different nutrient sources, tolerate stresses, and produce promoters or inhibitors of growth of other microorganisms, etc.;

(iv) processing factors, which include treatments such as heating, cooling, and drying that affect the composition of the food and also affect the types and numbers of microorganisms that remain in the food after treatment; and

(v) interaction between the above‐described factors can also affect the growth of microorganisms in foods in a complicated way; the combined effects may be additive or synergistic.

 

• Beneficial role of Microbes

  • Adds variety
  • Flavour
  • Colour
  • Fermentation
  • Supplement
  • Important component of Food processing industry
  • Beer,Wine making

 

 

FOOD CONTAMINATION AND SPOILAGE

 

Classification Of Food

 

Classification-1

  1. Milk and milk products

1.1 Milk

1.2 Cream

1.3 Kefir

1.4 Yoghurt

1.5 Whey

1.6 Other fermented milk products

1.7 Cheese

1.8 Cheese substitutes

1.9 Ice cream

1.10Milk and milk products for dietetic use

  1. Egg and egg products
  2. Meat and meat products
  3. Fish, molluscs, reptiles, crustaceans and their products
  4. Oils, fats and their products
  5. Grains and grain products
  6. Pulses, seeds, kernels, nuts and their products
  7. Vegetables and vegetable products
  8. Fruit and fruit products
  9. Sugar, chocolate and related products
  10. Beverages (non-milk)
  11. Miscellaneous, soups, sauces, snacks and products
  12. Products for special nutritional use

Classification 2

1 Bread and rolls

2 Breakfast cereals

3 Flour

4 Pasta

5 Bakery products

6 Rice and other cereal products

7 Sugar

8 Sugar products excluding chocolate

9 Chocolate

10 Vegetable oils

11 Margarine and lipids of mixed origin

12 Butter and animal fats

13 Nuts

14 Pulses

15 Vegetables excluding potatoes

16 Starchy roots or potatoes

17 Fruits

18 Fruit juices

19 Non alcoholic beverages

20 Coffee, tea, cocoa powder

21 Beer

22 Wine

23 Other alcoholic beverages

24 Red meat and meat products

25 Poultry and poultry products

26 Offals

27 Fish and seafood

28 Eggs

29 Milk

30 Cheese

31 Other milk products

32 Miscellaneous foods

33 Products for special nutritional use

 

Classification of food Based on Perishability

 

Some foods have longer shelf life than others. Perishability refers to the quickness with which a food gets spoilt. Foods can be classified into three groups depending on how long they can be kept without any treatment.

Perishable foods can be kept at room temperature for only few hours or 1 or 2 days before spoiling. For example- milk and milk products, meat, fish, poultry, fruits, leafy vegetables and cooked food. These foods keep well under refrigeration at household as well as commercial level. In general, the most perishable foods contain a high level of protein or have moisture and carbohydrates in them. Special methods are used to preserve such foods. The rate of spoilage varies with the temperature, moisture and or dryness of the environment. Storage of perishable foods should be done by keeping following points in mind.

Flesh foods like meat, chicken and fish need to be kept frozen at -60°C in a deep freeze for long term storage. These foods should not be left at room temperature for more than an hour or two. Organ meats tend to spoil faster than muscle meat. Ground meats spoil faster because of high surface area exposed to contamination.

Eggs are best kept in a cool place or in a basket in an airy room refrigerator. Never wash eggs before storing. Store eggs with their pointed end downwards.

Milk in boiled form can be kept at room temperature for 6 to 12 hours during winters. Inside a refrigerator milk can last 3 to 4 days or even more in closed container.

The keeping quality of a vegetable depends upon its nature. Leafy vegetables wilt and deteriorate within minutes of buying unless they are kept wrapped in a damp cloth or inside a plastic bag in the refrigerator wherein they last for more than a day or two.

All other vegetables keep well in a cool place with relatively high humidity in a basket covered with a damp cloth.

Vegetables must be kept in plastic bags to prevent drying by evaporation, if stored in a refrigerator.

Do not wash fruits before storing as they spoil faster.

Remember not to, refrigerate bananas, pineapples, papayas and avocadoes, as these fruits undergo undesirable changes in texture and flavour at refrigerator temperature. Most other fruits keep well, when refrigerated.

 

Semi -perishable foods can be stored for a couple of weeks or even a month or two without any detectable signs of spoilage. Temperature and humidity of the environment again affects the shelf stability of such foods. Proper handling and storage can result in fairly long storage without spoilage. Examples are all cereal and pulse products like wheat flour, semolina, vermicelli, broken wheat, Bengal gram flour, and some fruits and vegetables like citrus fruits, aonla, apples, pumpkin, roots and tubers, yams, potatoes, onions, garlic etc. Following points should while storing semi-perishable foods.

Processed cereal products develop an off-flavour or are infested by insects very easily if not taken care. Therefore, they should be sieved and cleaned of all such contamination, exposed to the sun for a few hours, allowed to cool and then stored in tightly covered bottles or other containers.

Especially onions and potatoes should be stored in a cool, dry and airy place to prevent them from developing moulds or growing shoots. They are best hung up from the ceiling in a wire or plastic-mesh basket, or kept in mesh containers which permit air circulation.

Nuts become rancid and get infested with insects very easily, therefore, they should be bought in large quantities only when storage space is available.

Fruits like apples, oranges and semi-ripe mangoes do last for a few weeks and should be put in a basket lined and covered with paper to prevent them from drying up. They need a cool environment to last long.

 

Non- perishable foods will keep for months or years without spoiling unless handled and stored carelessly. Examples of such foods are all preserved food products (canned, dried, pickled etc.), whole cereal, pulse and millet grains, oil seeds, nuts, fats and oils, honey, sugar, jaggery, salt, some spices and essence. Following points should be followed while storing non-perishable foods:

Food should be carefully cleaned i.e. free from gravel, husk and other foreign matter etc. and dried thoroughly in the sun/ drier before storage.

Storage of foods should be done in clean containers with tight-fitting lids. Containers can be made of tin, aluminum, plastic or glass. Clay pots or gunny bags may also used in case of large quantities.

A dry, cool and dark area should be chosen for storage of non-perishable foods.

The perishability of food dictates to a considerable extent the preservation techniques that are used to keep that food in good quality. In case of non-perishable foods, preservation techniques are dedicated to keeping out insects, rodents and other pests and keeping the foods dry to prevent it from becoming moldy. Perishable and semi-perishable foods depend a great deal on the technologies of refrigeration, drying, freezing, canning and the use of chemical preservatives to give shelf stability. These treatments can make such food commodities keep for many months or years if they are performed properly

Contamination And Cross Contamination

 

Microbial Contamination of food can be defined as accidental and/or intended addition of contaminants like bacteria, virus, fungus etc. and/or their toxins & by-products in food causing spoilage of food and diseases in human beings.

Cross Contamination

Cross-Contamination in food can be defined as the physical movement or transfer of microbial contaminants from one person, object, or place to another. There are three main types of cross contamination: food-to-food, equipment-to-food, and people-to-food.

 

Contamination Spoilages Of Various Food With The Storing Method

 

Spoilages of various foods

  • Many food borne microbes are present in healthy animals (usually in their intestines) raised for food. Meat and poultry carcasses can become contaminated during slaughter by contact with small amounts of intestinal contents.
  • Fresh fruits and vegetables can be contaminated if they are washed or irrigated with water that is contaminated with animal manure or human sewages.
  • Some types of Salmonella can infect a hen’s ovary so that the internal contents of a normal looking egg can be contaminated with Salmonella even before the shell in formed.
  • Oysters and other filter feeding shellfish can concentrate Vibrio bacteria that are naturally present in sea water, or other microbes that are present in human sewage dumped into the sea.
  • Later in food processing, other food borne microbes can be introduced from infected humans who handle the food, or by cross contamination from some other raw agricultural products.
  • For example, Shigella bacteria, hepatitis A virus and Norwalk virus can be introduced by the unwashed hands of food handlers who are themselves infected.
  • In the kitchen, microbes can be transferred from one food to another food by using the same knife, cutting board or other utensil to prepare both without washing the surface or utensil in between.
  • A food that is fully cooked can become re-contaminated if it touches other raw foods or drippings from raw foods that might contain pathogens microbes responsible for spoilage.
  • The way in which food is handled after it is contaminated can also make a difference in whether or not an outbreak occurs.
  • Lightly contaminated food left out overnight can be highly infectious by the next day. Given warm moist conditions and an ample supply of nutrients, a bacterium that reproduces by dividing itself every half hour can produce 17 million progeny in 12 hours.
  • If the food is refrigerated promptly,the bacteria multiply at a slower rate.However, Listeria monocytogenes and Yersinia enterocolitica can actually grow at refrigerator temperatures.

 

Food Storage

 

Food preservation aims at preventing the microbial spoilage of food products and the growth of the food borne pathogens. Thus, the two principal goals of food preservation methods are:

(i) increasing the shelf life of the food and

(ii) ensuring the safety for human consumption. There are a variety of food preservation methods.

 

Principles of Food Preservation include the following:

  1. i) Prevention or delay of microbial decomposition.
  • By keeping microbes out (asepsis).
  • By removal of microbes (e.g. filtration).
  • By reducing the rate of microbial growth (e.g. by low temperature,

      drying, anaerobic conditions and chemical inhibitors).

  • By killing microbes (e.g. by heat or radiation).
  1. ii) Prevention or delay of self-decomposition of food
  • By inactivation of food enzymes (e.g. blanching).
  • By prevention of chemical reactions (e.g. by using antioxidants).

 

 

Methods To Prevent Food Spoilage and Contamination

 

Operation                                                                      Intended effect

Cleaning, washing                                                       Reduces microbial load

Cold storage (below 8OC)                                          Prevents the growth of most pathogenic bacteria; slows the growth of spoilage microbes

Freezing (below – 10OC)                                           Prevents growth of all microbes

Pasteurizing (60-80OC)                                              Kills most non-sporing bacteria, yeast and molds

Blanching (95-110OC)                                              Kills surface vegetative bacteria, yeast and molds

Canning (above 100OC)                                             ‘Commercially sterilizes’ food; kills all pathogenic bacteria

Drying                                                                        Stops growth of all microbes when aw<0.60

Salting                                                                         Stops growth of most microbes

Syruping (sugars)                                                        Halts growth when a w < 0.70

Acidifying                                                                   Halts growth of most bacteria (effects depend on acid type)