Cheese Technology

Module 4. Pretreatments of milk for cheesemaking

Lesson 8
CHILLING, STORAGE, CLARIFICATION AND BACTOFUGATION

8.1 Introduction

For centuries, milk for cheesemaking had been subjected to no pre-treatment before curdling, and many cheese varieties worldwide are still made from raw milk. However, predominantly for reasons of safety, but also for consistency of quality and manipulation of product characteristics, most cheesemaking today involves the treatment of milk by one or more processing steps prior to addition of coagulant and starter culture. Perhaps the simplest and earliest technological intervention, driven by safety concerns, was the pasteurization of milk. Pasteurisation inactivates some enzymes, reverses shifts in the mineral balance of milk and influences the microflora of non-starter lactic acid bacteria (NSLAB) in the final cheese. Pasteurization unacceptably impairs the cheese flavor as a result of its influence on NSLAB. On the other hand, more severe heat treatments than pasteurization result in significant denaturation of whey proteins and their resulting incorporation into cheese curd, with significant effects on cheese yield and composition. This chapter deals with the effect of the various treatments given to milk on cheese quality.

8.2 Chilling and Cold Storage

Raw milk is sometimes cooled to about 4°C and stored in refrigerated tanks or storage tanks prior to its conversion into cheese. This practice of storing milk at refrigerated temperature not only increases the possibility of growth of psychrotrophs but also alters the physico-chemical properties of milk components like casein and minerals. Due to these alterations, many properties change significantly like rennet coagulation time, firmness, moisture retention etc. Rennet coagulation time increases, firmness decreases and moisture retention increases due to cold storage of milk. The impaired technological properties as a result of chilling and cold storage of the raw milk can be improved by adopting measures such as:

1. Acidification of cheese milk with lactic acid to pH 6.5

2. Addition of calcium chloride @ 0.02%

3. Addition of more rennet within permissible limits

4. Use of higher renneting temperature

5. Use of higher cooking temperatures

Among all above stated measures, first two have been found to be most effective.

8.3 Clarification

Clarification is one of the centrifugal processes used in dairy industries. It is used to remove leukocytes, cellular debris and particles from earth or fodder gaining entry into milk. Centrifugation of milk by the clarifier removes much of the small sized particles in milk, having a specific gravity higher than 1.032, particularly dirt, cells and larger microorganisms if they are present in clumps. Clarification decreases the tendency of fat to form aggregates on standing, increases the rate of multiplication of starter organisms, may increase the fat losses in whey and may also decrease the moisture and yield of cheese. Due to the removal of anaerobic sporeformers, marked improvement in cheese quality has been recorded.

8.4 Bactofugation

Bactofugation is a physical process through which bacteria are removed from milk, the size and density of bacteria being the criteria for their removal. The other factors deciding the efficiency of bactofuge to remove bacteria are initial bacterial load, pre-treatment of milk, throughput of machine, volume of bactofugate to throughput, frequency of partial desludging and duration of the run. Bactofuge is a clarifier with one inlet for raw milk and one outlet for treated milk. There are 2-4 nozzles fitted into the bowl wall for the discharge of skim milk. Bacteria are subjected to a centrifugal force of the order of 7000- 9000 G. The process is also able to remove the heat resistant spores which otherwise are not removed by any other thermal process.

The bactofuge system has been used in the cheese industry where its high-cleaning capabilities have been used to remove spores from cheese milk that could cause late fermentation in semi-hard cheeses.

Effect of bactofugation on cheesemaking may be summarized as follows:

• Effective method for preventing late blowing defect in semi-hard or hard varieties of cheeses. It is mainly due to removal of anaerobic microorganisms

• Facilitates reduction or elimination in the use of nitrates (nitrates are added to prevent ‘late fermentation’ in cheese)

• Weakens the coagulum during cheesemaking which can be overcome by addition of calcium chloride