Module 3. Microorganisms associated with milk

Lesson 13

ROLE OF PSYCHROTROPHS IN MILK

13.1 Introduction

As per the estimates world’s one-fourth of the food produced is spoiled by the microbial activities alone. In the developed nations, spoilage is mainly caused by the psychotrophic, moulds and yeasts, while in developing world it is mostly due to the power failure and severe lacking of proper storage facilities, especially for perishable food commodities. Psychrotrophs can grow in the range of 0 to 7°C or below, irrespective of their optimal growth temperature. Psychotropic spoilage of milk and milk products is a quite complex event of both microbial and biochemical activities. However, most of the spoilage in dairy products is caused by nonpathogenic psychrotrophic strains such as Pseudomonas.

13.1.1 Major psychrotrophs involved in the spoilage of milk and milk products

A) Gram-Negative: Major psychrotrophs of raw milk are Gram negative rods, among these, Pseudomonas spp., individually contributes to the extent of 50% of total genera involved. The other genera involved in spoilage include Achromobacter, Aeromonas, Alcaligenes, Chromobacterium, and Flavobacterium. Many of these produce heat resistant proteases and lipases that have deleterious effect on the quality of the final product. Coliforms contribute to the extent of 5 to 33 % of total psychotrophic microflora of the raw milk.

B) Gram-Positive: These are much less in numbers than the Gram-negative species of bacteria in raw milk. The most common genera in this category included are Arthrobacter, Bacillus, Clostridium, Corynebacterium, Lactobacillus, Listeria, Microbacterium, Micrococcus, Sarcina, Staphylococcus and Streptococcus.

Members of the genus Bacillus and Clostridium are spore formers and can survive even after pasteurization treatment. The leading spoilage contributors of hygienic milk and milk products stored at longer period is expected to be Gram positive spore formers, for example Bacillus cereus.

C) Pathogenic psychrotrophs: Listeria monocytogenes, Yesinia enterocolitica and Bacillus cereus are the most potential psychrotrophs of importance to the dairy industry.

13.2 Influences on Milk and Dairy Products

There are four different conditions of psychrotrophs that are mainly responsible for the spoilage of milk and milk products:

· Growth of psychrotrophs prior to pasteurization process

· Activity of thermo-resistant enzymes

· Growth of thermo-resistant psychrotrophs

· Post pasteurization contamination that must be the important factor out of above three

Due to lack of suitable transport systems and gaps in communication the milk received at dairy processing plants from farms is 5 to 7 days old. Such a delay adds psychrotrophs to the milk and subsequent processing of the milk to removes microbes from the unprocessed milk also aggravates the growth of psychrotrophs in the product. Most of the psychrotrophs are sensitive to pasteurization, for this reason, the situation is not so serious but on the other hand, their extracellular enzymes are heat resistant and are not inactivated by pasteurization.

13.3  Influence of Psychrotrophs on Milk and Milk Products

13.3.1  Raw milk

Psychrotrophs may produce different off-flavours in the milk products such as:

· Rancid flavor: Due to the production of lipase, formation of free fatty acids; rancid flavor (C₄-C₈ chain length) may be increased by the action of phospholipase enzyme.

· Unclean soapy flavor: Due to the production of lipases, formation of free fatty acids; rancid flavor (C₁₀-C₁₂ chain length)

· Metallic flavors: Due to the production of lipases; released unsaturated fatty acids are oxidized to aldehydes and ketones

13.3.2 UHT milk

· Rancidity (1 to 7 month storage): due to the production of heat resistant lipases

· Gelation: due to the production of heat resistant proteases

13.3.3  Pasteurized milk

· Formation of precipitate, when milk is added to hot beverages (bitty cream): due to the production of phospholipase and proteases from bacillus, fat destabilization

· Gelation: thermo-resistant proteases; requires 8.7×10^{8 CFU}/ml

· Fruity smell: due to the synthesis is of esters

· Short shelf-life: stimulated by the presence of ethanol

· Sweet curdling: by protein hydrolysis

· Fouling in heat exchangers: the proteases decrease heat stability of the proteins

13.3.4  Pasteurized cream

· Rancidity during storage: The creams with high lipase comprise a group of Gram-positive, low-GC, acid-tolerant, generally non-sporulating, rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria are usually associated with milk and milk products, and produce lactic acid as a major metabolic end-product of sugar fermentation. Many of the strains produce proteinaceous bacteriocins; highly effective against pathogenic microorganisms. These are generally used for the industrial applications due to their GRAS (generally regarded as safe) status prior to pasteurization or from post pasteurization contamination by Pseudomonas (mostly fluorescent in nature).

13.3.5  Ice cream

· Off-flavors: The cream of inferior quality (due to the enzymatic action)

13.3.6  Butter

· Rancidity: The production of lipases in milk and cream by Pseudomonas, or by yeast in butter; C₄-C₆ fatty acids

· Soapy, bitter: Similar to that as for rancidity, but C₁₀-C₁₂ free fatty acids are responsible for the soapy flavor in butter

13.3.7  Powders

· Rancidity: The confectionary, fat containing food of long storage in which milk or whey powders are added as the ingredients; some lipolysis takes place in whole milk powder upon extended storage (i.e. >1 year)

13.3.8  Cheese

· Taints, off-flavors in Cottage cheese: Due to the post pasteurization contamination by bacteria

· Rancidity in cheeses during 1 to 12 months of maturation: The production of thermo-resistant lipases that remain in curd and continue their activity during the storage

· Lower yields: due to proteolysis

· Shorter making times, Cheddar: the starters are stimulated by proteolysis

· Longer making times: the starter are inhibited by the free fatty acids released following lipolysis

13.3.9  Control of psychrotrophs

This group of microbes may be controlled via different methods as given in Table 13.1, while applying alone or in combination.

Table 13.1 Different factors that affect the growth of microorganisms

Method used	Mode of action of different factors
Low temperature	Slowing down the growth of psychrotophs
High temperature	Denatures vital cell-components; hence, temperatures lower than pasteurization can be applied
H2O2	Activation of lactoperoxidase system8 to 12 ppm of H2O2 is sufficient
Glucose oxidase	Enzyme generate H2O2 that activates lactoperoxidase
Lactic acid bacteria	Generate H2O2 that activates lactoperoxidase
Mixed culture of lactic and propionic acid bacteria	A combined action of propionic and acetic acid
Chlorine	It is added to acid-wash water used in cottage cheese manufacturing; which can kill contaminant psychrotrophs
CO2 injection in milk	Influences the membrane permeability and decarboxylating enzymes
Sorbate	Inhibits enzymes; that are active against catalase positive bacteria
Lactoferrin	Combines iron in milk, consequently depriving bacteria that requires it for growth
Lysozyme	It attacks cell-wall components, induces cell-lysis
Spore germination activators	Hippuric acid