The isolation of lactic acid bacteria from raw and pasteurized milk and from fermented dairy products is discussed.
Isolation of lactic acid bacteria from raw milk
It can be challenging to isolate lactic acid bacteria (LAB) from raw milk that has been refrigerated without pre-incubation since the flora tends to be dominated by Gram-negative bacteria and the LAB are present in low numbers.
There is no single agar medium that is suitable for the selective isolation of strains of all genera of LAB present in raw milk.
George Doran graduated with a 2:1 honours degree in Food Technology from the College of Agriculture and Food Technology in Northern Ireland in 2015. Mr Doran's final year research project was entitled "An Investigation of Biofouling in Two Mozzarella Cheese Manufacturing Plants".
George completed his food technology internship at Cottage Catering, Dromore, N. Ireland and gained experience in Quality Assurance, New Product Development and Production.
George has achieved several academic distinctions and has extensive work experience gained through part-time work in the security and retail sectors.
Included amongst George's achievements are:
- Member of the winning team for the Chesapeake Product Development Challenge in December 2013
- Represented IFST Ecothrophelia in London at Food Matters Live in November 2014
Refrigerated storage of raw milk is used to limit the growth of microorganisms in milk prior to processing. It has been known for some time that the quality and yield of cheese produced from bulk cooled milk may be adversely affected by this procedure (Weatherup et al., 1988; Weatherup and Mullan, 1993). The reduced yield and poor quality may be due to physico-chemical changes in the state of several milk components e.g. dissociation of micellar casein, mainly Κ-casein into a soluble phase, occurs during the first 48 h of storage at 4° and 7° C. This results in losses of fat and curd fines, weaker curd, more moist curd and a slightly lower yield. Partial reversal of dissociation occurs after further storage. The reduced yield and quality can also be due to the activity of proteases and lipases produced by psychrotrophic bacteria.
Despite the work that has been done over many years milk is still being stored for extended periods (1-3+ days on some farms) and cheesemakers are again (2019) reporting problems with the yield and quality of cheese produced using this milk.
Following several queries related to milk quality and cheese manufacture I am providing a report written by Wilf Weatherup and me some years ago that may be helpful.
A simple calculator has been provided using the total viable count of milk prior to pasteurisation and a regression equation to predict the grade value of Cheddar cheese.
It is important that students understand accuracy, precision and error before embarking on research projects and reflect this understanding in reports and dissertations.
There is a free tutorial by Cecil McIntosh on Sophia that explains these concepts and also provide self assessed questions to test understanding.