Standard Plate Count-SPC

Standard Plate Count-SPC

	Standard Plate Count-SPC - This determines the total number of bacteria in a specified amount of milk, generally a milliliter (ml). This is used for grading of milk. Under aseptic conditions, one ml of milk is added to 99 ml of distilled water or buffer. One ml and one-tenth ml samples are then transferred to sterile Petri dishes.. These give 1: 100 and 1: 1000 dilutions of the milk respectively. Other dilutions may also be prepared successively.
A growth medium like plate count agar or tryptone glucose yeast extract agar is then added, and the milk samples are mixed with the medium. The dishes are incubated at 37°C for 24-48 hours. The plates are then placed on a counting device like a Quebec colony counter and the number of bacterial colonies is recorded. The colony count falling between 30 and 300 is selected and multiplied by the reciprocal of the dilution factor to obtain the bacterial count per ml of milk. If 248 colonies appeared on the 1 : 100 plate and 16 colonies on ,the 1:1000 plate, 248 would be selected and multiplied by 100, giving 24,800 total bacteria per ml of milk sample.
The 30 to 300 range is used because under 30 the chance of sampling error may be significant, and over 300, due to overcrowding on the plate, there may be formed a single colony by more than one bacteria.