Principle and Interpretation

Lee's HiVeg Agar is prepared by using HiVeg hydrolysate which is a vegetable peptone instead of Casein enzymic hydrolysate and hence the medium is free from BSE/TSE risks. Lee's HiVeg Agar is the modification of Lee's Agar which is formulated as per APHA (1) for differential enumeration of yoghurt starter bacteria, homofermentative Lactobacillus bulgaricus and heterofermentative Streptococcus thermophilus. Yoghurt is made by the controlled fermentation of milk held at 43°C using a starter culture of Streptococcus thermophilus and Lactobacillus bulgaricus. Streptococci grow first and produce a creamy, buttery aroma from diacetyl and similar metabolites. Redox potential is also lowered by Streptococci which enables Lactobacilli to grow thereby growth stimulatory products for Streptococci are synthesized by Lactobacilli. Hence the typical sharp acetaldehyde flavour of mature yoghurt is formed (2). HiVeg hydrolysate and yeast extract provide the essential nitrogenous nutrients to the yoghurt (lactic) starter bacteria. Lactose and sucrose are the fermentable carbohydrates. Calcium carbonate is added to medium along with the dipotassium phosphate to buffer the medium and avoid the drastic drop in pH due to lactic acid formation. Bromo cresol purple is the pH indicator which turns yellow in acidic condition and imparts yellow colour to the colony. It is recommended to dry the media plates for 18-24 hours prior to use.