FOOD MICROBIOLOY: CURRENT STATUS

FOOD MICROBIOLOY: CURRENT STATUS

In the early 20th century, studies continued to understand the association and importance of microorganisms, especially pathogenic bacteria in food. Speci?c methods were developed for their isolation and identi?cation. The importance of sanitation in the handling of food to reduce contamination by microorganisms was recognized. Speci?c methods were studied to prevent growth as well as to destroy the spoilage and pathogenic bacteria. There was also some interest to isolate bene?cial bacteria associated with food fermentation, especially dairy fermentation, and study their characteristics. However, after the 1950s, food microbiology entered a new era. Availability of basic information on the physiological, biochemical, and biological characteristics of diverse types of food, microbial interactions in food environments and microbial physiology, biochemistry, genetics, and immunology has helped open new frontiers in food microbiology. Among these are:

A. Food Fermentation/Probiotics

• Development of strains with desirable metabolic activities by genetic transfer among strains
• Development of bacteriophage-resistant lactic acid bacteria
• Metabolic engineering of strains for overproduction of desirable metabolites
• Development of methods to use lactic acid bacteria to deliver immunity proteins
• Sequencing genomes of important lactic acid bacteria and bacteriophages for better understanding of their characteristics
• Food biopreservation with desirable bacteria and their antimicrobial metabolites
• Understanding of important characteristics of probiotic bacteria and development of desirable strains
• Effective methods to produce starter cultures for direct use in food processing

B. Food Spoilage

• Identi?cation and control of new spoilage bacteria associated with the current changes in food processing and preservation methods
• Spoilage due to bacterial enzymes of frozen and refrigerated foods with extended shelf life
• Development of molecular methods (nanotechnology) to identify metabolites of spoilage bacteria and predict potential shelf life of foods
• Importance of environmental stress on the resistance of spoilage bacteria to anti- microbial preservatives



C. Foodborne Diseases

• Methods to detect emerging foodborne pathogenic bacteria from contaminated foods
• Application of molecular biology techniques (nanotechnology) for rapid detection of pathogenic bacteria in food and environment
• Effective detection and control methods of foodborne pathogenic viruses
• Transmission potentials of prion diseases from food animals to humans Importance of environmental stress on the detection and destruction of pathogens
• Factors associated with the increase in antibiotic-resistant pathogens in food
• Adherence of foodborne pathogens on food and equipment surfaces
• Mechanisms of pathogenicity of foodborne pathogens
• Effective methods for epidemiology study of foodborne diseases.



D. Miscellaneous

• Application of hazard analysis of critical control points (HACCP) in food production, processing, and preservation
• Novel food-processing technologies
• Microbiology of unprocessed and low-heat-processed ready-to-eat foods
• Microbial control of foods from farm to table (total quality management)
• Food safety legislation

FOOD MICROBIOLOGY AND FOOD MICROBIOLOGISTS

From the above discussion, it is apparent what, as a discipline, food microbiology has to offer. Before the 1970s, food microbiology was regarded as an applied science mainly involved in the microbiological quality control of food. Since then, the technology used in food production, processing, distribution and retailing and food consumption patterns have changed dramatically. These changes have introduced new problems that can no longer be solved by merely using applied knowledge. Thus, modern-day food microbiology needs to include a great deal of basic science to understand and effectively solve the microbiological problems associated with food. The discipline includes not only microbiological aspects of food spoilage and food- borne diseases and their effective control and bioprocessing of foods but also basic information of microbial ecology, physiology, metabolism, and genetics. This Control of pathogenic parasites in food information is helping to develop methods for rapid and effective detection of spoilage and pathogenic bacteria, to develop desirable microbial strains by recombinant DNA technology, to produce fermented foods of better quality, to develop thermostable enzymes in enzyme processing of food and food additives, to develop methods to remove bacteria from food and equipment surfaces, and to combine several control methods for effective control of spoilage and pathogenic microorganisms in food.
An individual who has completed courses in food microbiology (both lecture and laboratory) should gain knowledge in the following areas:
• Determine microbiological quality of foods and food ingredients by using appropriate techniques
• Determine microbial types involved in spoilage and health hazards and identify the sources
• Design corrective procedures to control the spoilage and pathogenic microorganisms in food
• Learn rapid methods to isolate and identify pathogens and spoilage bacteria from food
and environment to cause foodborne diseases and food spoilage and to produce food and food ingredients. Many bacterial species and some molds and viruses, but not yeasts, are able to cause foodborne diseases. Most bacteria, molds, and yeasts, because of their ability to grow in foods (viruses cannot grow in foods), can potentially cause food spoilage. Several species of bacteria, molds, and yeasts are considered safe or food grade, or both, and are used to produce fermented foods and food ingredients. Among the four major groups, bacteria constitute the largest group. Because of their ubiquitous presence and rapid growth rate, even under conditions where yeasts and molds cannot grow, they are considered the most important in food spoilage and foodborne diseases.
Prion or proteinaceous infectious particles have recently been identi?ed to cause transmissible spongiform encephalopathies (TSEs) in humans and animals. How- ever, their ability to cause foodborne diseases is not clearly understood.
• Identify how new technologies adapted in food processing can have speci?c microbiological problems and design methods to overcome the problem.
Design effective sanitation procedures to control spoilage and pathogen problems in
• food-processing facilities. Effectively use desirable microorganisms to produce fermented foods
• Design methods to produce better starter cultures for use in fermented foods and probiotics.

• Know about food regulations (state, federal, and international).

E.
To be effective, in addition to the knowledge gained, one has to be able to communicate with different groups of people about the subject (food microbiology and its relation to food science). An individual with good common sense is always in a better position to sense a problem and correct it quickly.

Except for a few sterile foods, all foods harbor one or more types of microorganisms. Some of them have desirable roles in food, such as in the production of naturally fermented food, whereas others cause food spoilage and foodborne diseases. To study the role of microorganisms in food and to control them when necessary, it is important to isolate them in pure culture and study their morphological, physiolog- ical, biochemical, and genetic characteristics. Some of the simplest techniques in use today for these studies were developed over the last 300 years; a brief description is included here.

1. Carbohydrates in Foods
Major carbohydrates present in different foods, either naturally or added as ingre- dients, can be grouped on the basis of chemical nature as follows:
Monosaccharides
Hexoses: glucose, fructose, mannose, galactose Pentoses: xylose, arabinose, ribose, ribulose, xylulose
Disaccharides
Lactose (galactose + glucose) Sucrose (fructose + glucose) Maltose (glucose + glucose)
Oligosaccharides
Raf?nose (glucose + fructose + galactose)
Stachyose (glucose + fructose + galactose + galactose)

Polysaccharides
Starch (glucose units) Glycogen (glucose units) Cellulose (glucose units) Inulin (fructose units)
Hemicellulose (xylose, galactose, mannose units) Dextrans (a-1, 6 glucose polymer)
Pectins Gums and mucilages

Lactose is found only in milk and thus can be present in foods made from or with milk and milk products. Glycogen is present in animal tissues, especially in liver. Pentoses, most oligosaccharides, and polysaccharides are naturally present in foods of plant origin.
All microorganisms normally found in food metabolize glucose, but their ability to utilize other carbohydrates differs considerably. This is because of the inability of some microorganisms to transport the speci?c monosaccharides and disaccharides inside the cells and the inability to hydrolyze polysaccharides outside the cells. Molds are the most capable of using polysaccharides.
Food carbohydrates are metabolized by microorganisms principally to supply energy through several metabolic pathways. Some of the metabolic products can be used to synthesize cellular components of microorganisms (e.g., to produce amino acids by amination of some keto acids). Microorganisms also produce metabolic by- products associated with food spoilage (CO2 to cause gas defect) or food bioprocess- ing (lactic acid in fermented foods). Some are also metabolized to produce organic acids, such as lactic, acetic, propionic, and butyric acids, which have an antagonistic effect on the growth and survival of many bacteria. Some of these metabolic pathways are discussed in Chapter 7 and Chapter 11. Microorganisms can also polymerize some monosaccharides to produce complex carbohydrates such as dextrans, capsular materials, and cell wall (or outer membrane and middle membrane in Gram-negative bacteria). Some of these carbohydrates from pathogens may cause health hazards (forming complexes wit proteins), some may cause food spoilage (such as slime defect), and some can be used in food production (such as dextrans as stabilizers). Carbohydrate metabolism pro?les are extensively used in the laboratory for the biochemical identi?cation of unknown microorganisms isolated from foods.

2. Proteins in Foods
The major proteinaceous components in foods are simple proteins, conjugated pro- teins, peptides, and nonprotein nitrogenous (NPN) compounds (amino acids, urea, ammonia, creatinine, trimethylamine). Proteins and peptides are polymers of differ- ent amino acids without or with other organic (e.g., a carbohydrate) or inorganic (e.g., iron) components and contain ca. 15 to 18% nitrogen. Simple food proteins are polymers of amino acids, such as albumins (in egg), globulins (in milk), glutelins (gluten in cereal), prolamins (zein in grains), and albuminoids (collagen in muscle). They differ greatly in their solubility, which determines the ability of microorganisms to utilize a speci?c protein. Many microorganisms can hydrolyze albumin, which is soluble in water. In contrast, collagens, which are insoluble in water or weak salt and acid solutions, are hydrolyzed only by a few microorganisms. As compared with simple proteins, conjugated proteins of food on hydrolysis produce metals (metal- loproteins such as hemoglobin and myoglobin), carbohydrates (glycoproteins such as mucin), phosphate (phosphoproteins such as casein), and lipids (lipoproteins such as some in liver). Proteins are present in higher quantities in foods of animal origin than in foods of plant origin. But plant foods, such as nuts and legumes, are rich in proteins. Proteins as ingredients can also be added to foods.
Microorganisms differ greatly in their ability to metabolize food proteins. Most transport amino acids and small peptides in the cells; small peptides are then hydrolyzed to amino acids inside the cells, such as in some Lactococcus spp. Microorganisms also produce extracellular proteinases and peptidases to hydrolyze large proteins and peptides to small peptides and amino acids before they can be transported inside the cells. Soluble proteins are more susceptible to this hydrolytic action than are the insoluble proteins. Hydrolysis of food proteins can be either undesirable (texture loss in meat) or desirable (?avor in cheese). Microorganisms can also metabolize different NPN compounds found in foods.
Amino acids inside microbial cells are metabolized via different pathways to synthesize cellular components, energy, and various by-products. Many of these by- products can be undesirable (e.g., NH3 and H2S production causes spoilage of food, and toxins and biological amines cause health hazards) or desirable (e.g., some sulfur compounds give cheddar cheese ?avor). Production of speci?c metabolic products is used for the laboratory identi?cation of microbial isolates from food. An example of this is the ability of Escherichia coli to produce indole from tryptophan, which is used to differentiate this species from non-indole-producing related species (e.g., Enterobacter spp.)

3. Lipids in Foods
Lipids in foods include compounds that can be extracted by organic solvents, some of which are free fatty acids, glycerides, phospholipids, waxes, and sterols. Lipids are relatively higher in foods of animal origin than in foods of plant origin, although nuts, oil seeds, coconuts, and olives have high amounts of lipids. Fabricated or prepared foods can also vary greatly in lipid content. Cholesterols are present in foods of animal origin or foods containing ingredients from animal sources. Lipids are, in general, less preferred substrates for the microbial synthesis of energy and cellular materials. Many microorganisms can produce extracellular lipases, which can hydrolyze glycerides to fatty acids and glycerol. Fatty acids can be transported in cells and used for energy synthesis, whereas glycerol can be metabolized separately. Some microorganisms also produce extracellular lipid oxidases, which can oxidize unsaturated fatty acids to produce different aldehydes and ketones. In general, molds are more capable of producing these enzymes. However, certain bacterial groups such as Pseudomonas, Achromobacter, and Alcaligenes can produce these enzymes. Lysis of dead microbial cells in foods causes release of intracellular lipases and oxidases, which then can carry out these reactions. In many foods the action of these enzymes is associated with spoilage (such as rancidity), whereas in other foods the enzymes are credited for desirable ?avors (such as in mold-ripened cheeses). Some bene?cial intestinal micro- organisms, such as Lactobacillus acidophilus strains, can metabolize cholesterol and are believed to be capable of reducing serum cholesterol levels in humans.

4. Minerals and Vitamins in Foods
Microorganisms need several elements in small amounts, such as phosphorous, calcium, magnesium, iron, sulfur, manganese, and potassium. Most foods have these elements in suf?cient amounts. Many microorganisms can synthesize B vitamins, and foods also contain most B vitamins.
In general, most foods contain different carbohydrates, proteins, lipids, minerals, and vitamins in suf?cient amounts to supply necessary nutrients for the growth of molds, yeasts, and bacteria, especially Gram-negative bacteria normally present in foods. Some foods may have limited amounts of one or a few nutrients for rapid growth of some Gram-positive bacteria, especially some fastidious Lactobacillus species. When their growth is desired, some carbohydrates, essential amino acids, and B vitamins may be added to a food. It is not possible or practical to control microbial growth in a food by restricting nutrients.