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Written by: Gourley, J and Mullan, WMA

Summary

This study critically investigated and analysed the growth and numbers of thermophilic and thermoduric spore forming bacteria throughout the milk powder production process in a newly-commissioned commercial processing plant.  Various milk samples, including raw milk, pasteurised milk, concentrated milk and milk powder, were collected from a company situated in Northern Ireland, over the course of two full production runs; one completed in early December and the other in late January.  All samples were tested for thermophilic and thermoduric spores, and in addition to this, the bulk tanker milk was also tested for total viable count, coliforms, Escherichia coli, yeasts and moulds, to gain an indication of the quality of the raw milk.  Results concluded that during both pasteurisation and evaporation, thermoduric spore counts did increase slightly; however, it wasn’t until evaporation that the thermophilic spore count began to consistently increase; again only a small increase was noted.  The finished product, produced from trial one, contained much lower thermoduric and thermophilic spore counts, compared to that produced in trial two; and this is believed to have been brought about by the fact that higher quality raw milk, with mean thermoduric spore counts of 4.15x10³ CFU/ml and mean thermophilic spore counts of 2.5x10² CFU/ml was used in trial one.  Overall, this indicated that although a slight increase in the number of thermophilic and thermoduric spores did take place during processing, the majority of spore contamination took place at farm level.

How to cite this article

Gourley, Jill and Mullan, W.M.A. (2019). [On-line]. Available from: https://www.dairyscience.info/student-projects.html . Accessed: 29 September, 2023.  

 

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Written by: Michael Mullan

Every year I get requests from students for research topics for final year projects. The following is a short list of potential food science, food technology, food microbiology projects that reflect current issues in 2016.

More about undergraduate research students

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Written by: Gemma Devine, G. Doran and W.M.A. Mullan

Introduction

The growth of thermophilic streptococci in pasteurisers during long production runs has been known for some time (Driessen and Bouman, 1979). These organisms include Streptococcus thermophilus­ and organisms that are referred to as Str. thermophilus-like (Mullan, 2003); they have some of the phenotypic characteristics of Str. thermophilus but differ in that they grow in 4% salt solutions and utilise maltose and other carbohydrates. The Str. thermophilus-like bacteria were designated as Str waius by Flint et al. (1999) but were later reclassified as Str. macedonicus (Manachini et al., 2002).

Growth of Str. thermophilus and Str. thermophilus-like lactic acid bacteria to high concentration in milk in pasteurizers can cause significant problems in cheese making. These include excessive openness and overt gas production in cheese, off flavours, discoloration particularly in annatto-coloured cheeses and major disruption in process parameters in varieties in which the manufacture is tightly specified e.g. Mozzarella.

In this student project the authors investigated why less starter culture was required throughout the production run in one cheese making plant in Great Britain compared with a similar plant in Ireland.  

Aims

The project had two major objectives. To:

• determine the concentration of thermoduric LAB in raw milk and milk ex-pasteuriser at the two cheese plants
• identify the thermoduric LAB to species level.
  
  

Methods

Milk from two cheese production runs was sampled in Ireland. One production run was sampled in Great Britain (due to cost and time constraints).  Pasteurised milk samples were taken aseptically, using a syringe, from a sample port post-pasteurisation. Raw milk samples were taken from the pasteurizer-balance tank at both plants.  The samples were put into aseptic containers and held at 1-5°C prior to analysis normally within 24 hours of sampling.

Colonies were isolated, purified and allocated to genus as described by Billie et al. (1992) and Aprea et al. (2005) with the exception that the M17 agar used was acidified with hydrochloric acid to give a pH of 5.5 to make it slightly more selective for lactic acid bacteria and to inhibit swarming. API 50 CHL test kits and the API database from Biomerieux, Grafton Way, Basingstoke, Hampshire, England, RG22 6HY were used to confirm genus and allocate isolates to species.

Results

The growth of thermophilic bacteria in pasteurized milk from the cheese plants in Great Britain and Ireland.

No growth was recorded in milk samples plated on Rogosa agar at either plant but growth was recorded on M17 agar.  

More about growth of thermophilic bacteria in pasteurized milk from cheese plants