Starter Cultures and Fermented Milk Products

Module 7. Preservation and forms of starters

Lesson 11
NEED FOR PRESERVATION, CRITERIA FOR SELECTION OF METHOD, PRESERVATION TECHNIQUES

11.1 Introduction

The starter cultures have to be preserved so as to use them in when starting the preparation of fermented milk products.

11.2 Purpose

1. To maintain an available stock organisms especially in case of starter failure

2. Successive sub culturing can induce mutant strains, which may alter the overall

behavior and general characteristics of the starter

11.3 Basic Principles Adopted to Maintain the Activity of the Culture

1. Reducing or controlling the metabolic activity of the organisms by ordinary refrigeration. This is a short time storage of cultures where they can be kept viable for a week or fortnight

2. Separate the organisms from their waste followed by concentration and re-suspension in a sterile medium and preserving by freezing and drying for extended periods of storage. This is used during the production of concentrated active bulk starter culture either in a continuous fermented or batch process for direct inoculation of milk in the vat.

11.4 Criteria for Selection of Method

The type of starter culture used depends on the desired product.Culture supply companies can provide processors with a variety of cultures tailored for their operation that can be purchased frozen or dehydrated,typically as a mixture of several strains. It is very important to follow the supplier’s advice on the handling, storage,rotation, use rate, and incubation temperature for their cultures. Generally,dairy cultures are classified as Mesophilic cultures with optimum growth at 21-35°C or Thermophilic cultures with optimum growth at 42-45°C. Varying the incubation temperature of certain cultures can influence the flavor profile and other attributes of the final product.

11.5 Methods for Preservation of Starters

Starters are preserved in liquid, frozen and dried forms. Liquid sub-cultures is for routine use, while frozen and freeze dried forms are for long term preservation.

1. Sub-culture by transferring in skim milk
2. Use of chalk-litmus milk
3. Use of buffered media
4. Slant culture
5. Stab culture
6. Storage under oil/ glycerol stock
7. Deep freezing at -20°C
8. Ultra-deep freezing at -40°C
9. Storage under liquid nitrogen at -196°C
10. Spray drying
11. Vacuum drying
12. Freeze drying/ lyophilization