Microbial Analysis Of Food

Different between viable count and direct microbial count (DMC)

Dry reduction test

Two dye are commonly employed in dye reduction test to estimate viable number of organism

1. Methylene blue reduction time (MBRT) TEST : MBRT test is commonly used to determinate a number of viable organisms raw milk . In this method , methylene blue is mixed with raw milk and incubated . Microorganisms present in milk reduce methylene blue to from colorless Leuchomethylene blue . So that milk becomes blue to colorless. The time of decolourization of milk is indicative of a number of viable of the organism .If the no.of organism is high it is decolourize in a short time and vice versa . In this method, microbial quality of milk is assessed by reduction time . It is a rapid test tube to which 1ml of methylene blue solution is added and after homogeneously mixing it is placed in an incubator at 37°c and observation is made at a regular interval of half hours .Athe best grade of milk decolourized the dye late( about 5 and 1/2 hours ) or does not decolorize the dye.
2. Resazurin reduction test : It is an example of rapid dye reduce on the test used to the determined number of the viable organism in food such as raw milk. In this test resazurin, the dye is mixed with raw milk microorganism present in milk reduces resazurin so , that is its color is a change from blue to pink or colorless. If the number of an organism is higher . Therefore microbial load of milk can be predicted by reduction time obtained within10m.In laboratory 10m of milk is taken in a test tube . Where 1ml of resazurin solution is added and mixed . It is placed in an incubation at 37c and color appearance is observed after 10 mints .

Advantage of dry reduction test

• It is simply easier , rapid and inexpensive test.
• Only viable cells actively reduced the dye no.of viable organisms can be predicted .

Disadvantage

• Not all microorganisms the dye equally .
• They are not applicable for as reducing enzymes unless specials are employed.

Microbial analysis of food

Steps in the microbial analysis of food are as follows.

Sampling

Sampling is an import step in the microbial analysis of food can be obtained from processing line from storage house or market.

Sample :The sample should be representative of the whole of food.The sample should be collected randomly from the lot . The principle is to avoid biased and to draw a sufficient number of sample units.

sample device :Serial poon , pipetter , swab sample serial scissors , the knife can opener is used for opening food package liquid and small particles are sampled with sampling tube spoon dropper.large food materials ( eg-cheese) are cut by a sterile knife,for the surface , sample , knife sticky or sticky cellophane tape is used sticky cellophane tape is presented on the surface of solid agar medium to transfer microorganisms.

A number of samples :It food is mined for Salmonella usually 10 sample are collected and 5 sample are used for another organism.

Techniques sample collection :

When the product is in big container steps in sample collection are:

• The outer surface of the container is cleaned by washing by 70% ethanol.
• The container is opened with the help of a serial cutting instrument .
• If the food is in buck the sample is collected from the various place within a container.
• In the case of a liquid sample, it is first agitated to form a homogeneous mixture and the sample is taken .

When a food is in a small container : when a sample is in the small container . They should be taken directly to the lab for analysis.

Preparation of sample :

• For solid food :Solid is mixed with the sterile diluent in a mechanical blender to obtained homogenous suspension . For eg. when 1ogm food is added to 100ml diluent 1/10 dilution is obtained
• For liquid :Liquid food is serially diluted to obtained 1/10 , 1/100,1/1000 dilution

Transportation of sample

Multiplication and death of microorganisms in the sample should be prevented during transport of sample .

• The sample should be delivered to lab immediately.
• The perishable and unfrozen sample should be cooled to (0-5°c) and transported in the insulated container.
• Frozen product must remain frozen conditionUntil analysis.
• Sample report should be prepared and sent the sample with sampling report.

Sampling report : The report should provide the following information.

• Name and address of the person collecting the sample .
• Date ,time , place and purpose of sample collection.
• Nature of food.
• Name of manufacturing date , expiry date etc.
• The method of sampling size and no.of sampling temperature of produce at the time of sampling.
• Suggested test and also be given.

Sample plan.

Choice of sample plan depends on upon the organism present in food . The nature of food and whether the food is raw or processed type of consumer i.e infant old person or ill person . The sampling plan is basically a statement of criterium of acceptance based upon the examination of required number of sample unit by specific method sampling plane one a two types.

Two class plan :It consist of three specifications i.e 'n' ,' c' ,and'is plan is simplest and is used to accept or reject food product.

For example : 2 class of certain food from coliform is n=5 , c=2 , m=10² =100 . It means sample must be tested . In order to food to be acceptable , not more than two sample most contains greater than 100 poly from per gram . The food will be rejected if 3 or more sample units exceed 100 poly from gram . n=5 , c=0 and m= o for ( Salmonella). It means food will be rejected if any one sample out of 5 contain organism where,

n= no .of sample unit to be tested

m= maximum no.of organism 1gram. a value above this is non-acceptable.

c= The maximum no.of allowable sample that may exceed the microbiology criteria' . When this no.is exceeded the is rejected.

Three class plan :It consist of four specifications i.e 'n' , 'c' , 'm' and 'M' . It is used to spareprate good quality from marginally accepted quality.

M = The quality is used to separate marginally accepted quality from unacceptable quality . Number or value above'M' in any sample are unacceptable . Example n=5 , c=2 , m=100 and M = 150 eg. For coliform . It means 5 sample must be tested . It not more than 2 sample contains more than 100 organisms , it is acceptable . If anyone sample contains equal or greater than 'M' sample is nonacceptable

Sample procedure for dairy product milk and milk product : No. of samples .

• Fluid milk and ice cream- 5 field sample
• Dried milk and ice cream- 10 field sample
• Cheese - 5 filed sample
• Butter - 5 filed sample

References

Frazier, WC and Westhoff DC. Food Microbiology. 3rd. New Delhi: Tata McGraw-Hill Publishing Company Limited, 1986.

Jay, MJ. Modern Food Microbiology. Ed. India. 3rd. Delhi: CBS Publications and Distributions, 1987.