Microorganisms associated with food

Microorganisms associated with food

The most common microorganisms (MOs) associated with food are:

1.      Bacteria

2.      Yeast and

3.      Mold

The primary sources of MOs are

1.      Soil, water and air

2.      Plants and their products

3.      Animal and human beings

4.      Food handling equipments

The roles of MOs are:

1.      Spoilage

2.      Diseases

3.      Food fermentation

Bacteria: Bacteria are  unicellular organisms of aerobic or anaerobic nature an exhibit many morphological forms (mainly spherical, rod and spiral) having length in the range of 0.5 to 10.0 μm and width of about 0.5 to 2.5 μm

Three principles shapes have been well recognized namely

        i.            Spherical shape of cocci

     ii.            Rod shape of bacilli and

   iii.            Spiral form of spirilla

Important food spoilage bacteria:

Groups	Genus
Acetics	Acetobacter and Gluconobacter
Lactices	Lactobacillus,Leuconostoc, Pediococcus, Streptococcus
Butyrices	Clostridium
Propionics	Propionobacterium
Proteol Gytics	Bacillus, Pseudomonous, Clostridium, Proteus etc.

Some useful bacteria:

1.      Acetobacter sp. (A. Aceti, A. oreleansis, A. schutzenbachi): These bacteria also known as vinegar bacteria that oxidize ethyl alcohol to acetic acid. They are found on fruits, vegetables and alcoholic beverages.

2.      Lactobacillus sp. (L. plantarum): They ferment sugars with the products of lactic acid. So they are used at fermented plant and dairy products but they deteriorate some products i.e. wine, beer

other important species : Pediococcus cerevisiae, Leuconostoc mesenteroids, Streptococcus faecalis, Lactobacillus brevis

Yeast

Yeast are unicellular fungi of larger size compared tobacteria of oval, elongate, elliptical or spherical cell shape having length in the range of 10 to 20 μm and dia about 6 to 7 μm.

This organism produce pigments of many colors with red and black pigments producers being common. Optimum temperature of growth for most yeast is 25 to 30^oC and maximum range is 40 to 47^oC.

Most yeast used industrially is in the genus of Saccharomyces cerevisiae. Yeast are used in the manufacture of foods such as bread, beer, wine, vinegar and for surface ripening of cheese.

Yeast + Sugar          Alcohol +Carbon dioxide (gas)

Mold

Molds are multicelllular filamentous fungi growing as a network of hair like fibers called mycelia with a fuzzy or cottony appearance.

Optimum temperature of growing mold is 25 to 31^oC some grows in 35 to 37^oC. The mold thallus consists of a mass of branching, intertwined filaments called hyphae (singular hypha). The whole mass of these hyphae is known as mycellium. Molds are used in manufacturing of certain food such as: oriental food that includes soya sauce, jonti, miso etc.

The important molds are:

a)     Penicillium sp. (blue mold)

b)     Aspergillus sp. (black mold also known as laboratory fungi)

c)     Mucor sp. (gray mold)

d)     Byssochlamys fulva

Yeasts are grown well in sugar able condition. Bacteria cannot grow in minimum content of moisture but yeast and mold can grow.

Growth relation of MOs with a_w

Microorganism	Minimum aw
Bacteria	0.91
Yeasts	0.88
Molds	0.80
Halophilic bacteria	0.75
Xerophilic fungi	0.65
Osmophilic yeasts	0.60

Factors affecting the growth and survival of microorganisms are:

·        Nutrient

·        Time

·        Temperature

·        pH

·        Water activity (aw)

·        Oxygen

·        Chemical factors

·        Radiation

Food Wastage

Food wastage mainly occurs through:

1.      Contamination

2.      Consumption by pest

3.      Inefficient utilization

4.      Spoilage

Food spoilage can be defined as any change in the nature of a fresh or processed food material whereby changes in chemical, physical or organoleptic properties of the food take place, leading to its rejection as acceptable food by the consumers.

 Types of food spoilage and causative organisms:

Food	Type of spoilage	Causative microorganism
Fresh fruits and vegetables	Gray mold rot	Botrytis cinerea
	Rhizopus soft rot	Rhizopus nigricans
	Blue mold rot	Penicillium italicum
	Black mold rot	Aspergillus niger, Alternaria sp.
	Sliminess or souring	Saprophytic bacteria
Bread	Moldy	Rhizopus sp., Aspergillus sp.
	Ropy	Penicillium sp.
	Red bread	Bacillus sp. Serratia marcescens
Fresh meat	Putrefaction	Cholstridium sp., Pseudomonas sp., Proteus sp., Alcaligenes sp. Chromobacterium sp.
	Souring	Chromobacterium sp., Lactobacillus sp., Pseudomonas sp..
Fish	Discoloration	Pseudomonas sp.,
	Putrefaction	Choromobacterium sp., Halobacterium sp.
Poultry	Odor, slimy	Pseudomonas sp., Alcaligenes sp. Xanthomonous sp.
Eggs	Green rot	Pseudomonas flourescens
	Colorless rot	Pseudomonu sp., Alcaligenes sp., Chromobacterium sp., Cloiformis sp.
	Black rot	Proteus sp.
	Fungal rot	Penicillium sp., Mucor sp.
	Bacterial soft rot	Erwinia carotovera, Pseudomonas sp.