MARKET MILK

Module 4. Procurement of milk

Lesson 8
LACTOPEROXIDASE-THIOCYANATE (LP) SYSTEM

8.1 Introduction

The perishable nature of milk makes it necessary to exercise extra care to preserve it. The tropical (hot and humid) climate of India further aggravates the problem. Milk meant for long distance transportation is always refrigerated to maintain its quality. However, refrigeration may not always be possible or available due to economic and/or technical reasons. Therefore the need to have a suitable alternative to refrigeration has been felt since long. An FAO/WHO Expert Panel on Milk Quality suggested addition of hydrogen peroxide (H₂O₂) as an alternative. This was not popular for several reasons, mostly related to the proper control of its usage.

• Milk of poor quality could be disguised and passed of as good quality milk

• Higher concentration of H₂O₂ is toxic

Research conducted in the 1960s on colostrum in Sweden led to the discovery of naturally occurring enzymes and their positive effect on preservation systems. One such enzyme was lactoperoxidase (LP).

• It is an oxido-reductase enzyme

• It occurs in milk, saliva, tears, cervical mucus

• It is a single polypeptide chain with a molecular weight of 77,000 – 100,000.

Cow milk has 1.4 units/ml of LP, whereas buffalo milk has 0.9 units/ml. The amount of LP required for preservation is 0.5-1 mg/l, much lower than its concentration in cow milk (30 mg/l). It is a relatively heat resistant enzyme which retains sufficient activity even after pasteurization. Thiocyanate is widely distributed in animal tissues and secretions. Its level is dependent on dietary habits and lifestyle (for example, smokers have a higher level of thiocyanate in their system). Thiocyanate is the product of detoxification of thioaminoacids and cyanides and is secreted in urine with a half life of elimination of 2.5 days under normal renal function. Bovine milk has 2-15 ppm thiocyanate, though higher values have also been reported.

Lactoperodixase can oxidise thiocyanate ions in the presence of H₂O₂, which is not normally present in milk. Even if present, it is in minute quantities, as a result of bacterial activity.

The LP-thiocyanate system can be explained with the following chemical reactions:

Hypothiocyanous acid dissociates into hypothiocyanite at the pH of milk

There are other end products besides these, such as sulphate, ammonia and carbon dioxide. The intermediary products of the LP-thiocyanate reaction are implicated in bacteriostatic and bactericidal activities.

The presence of large quantities of sulphydral groups (-SH) such as those occurring in boiled milk would reduce/eliminate the antibacterial effect because the OSCN- ions would then attach to these freely available groups. Raw milk has very few SH- groups and therefore, the LP-system is most effective in raw milk.

8.2 Effect of LP System against Bacteria

The bacteriostatic/ bacterioacidal effect of the LP system is species- and strain- specific. In a mixed flora dominated by mesophilic bacteria, the effect is largely bacteriostatic. The system exhibits bacteriocidal effect on some Gram negative organisms such as Pseudomonas sp. and E. coli. In case exogenous H₂O₂ is present, the effect may also be bacteriostatic. The LP system has an impact on the glycolytic enzymes and hence, the metabolic activity is adversely affected. Lysis occurs due to leaking of bacterial contents.

• Owing to the bacteriostatic effect, it is not possible to hide poor quality of milk. The high bacterial counts will be revealed during plating, MBRT etc.

• The antibacterial oxidation products are not heat stable. The stability is temperature dependent, leading to complete destruction by pasteurization.

• The oxidation products are not stable at neutral pH. Any surplus will decompose to thiocyanate.

• The preservation effect is directly proportional to the thiocyanate concentration, provided equimolar concentration of H₂O₂ is present. In practice small quantities of thiocyanate is added to make up the concentration

• The quantity of H₂O₂ to be added must be monitored. If excess quantity is added (for example, 300-800 ppm), lactoperoxidase is inactivated and preservation effect is then largely due to H₂O₂.

8.3 Method of Adoption

The LP system may be used when technical, economic and/or practical reasons do not permit chilling. It is not recommended for the use of individual farmers. It should be practiced only at collection centers with proper cleaning and sanitizing facilities. The manpower deputed to handle the system should be given adequate training. The dairy that receives the milk should monitor the system. Appropriate detection methods should be set up to avoid foul practices. The LP system does not eliminate the need for the normal clean milk production methods that are adopted at farm levels. It does not exclude the need for pasteurization before consumption

The preservation effect is temperature-dependent. The preservation effect under laboratory and field experimental conditions for stored milk which had been produced hygienically are listed in Table 8.1.

Table 8.1 Preservation effect of LP system at different temperatures

8.4 Practical Application

• At the village collection centre, each clean and washed milk can should be given a 3 min chlorine rinse (400 ppm) immediately before milk is poured into it.

• Sodium thiocyanate (NaSCN) should be added @ 14 mg/l milk and mixed well for a minute with a plunger.

• Sodium percarbonate (precursor of H₂O₂) should be added @ 30 mg/l milk and stirred well for 2-3 min
  
  
  • This sequence of addition should be followed because the enzyme action starts when H₂O₂ added and is completed within 5 min of addition of H₂O₂.
  
  
  • This rate of addition will provide 75 ppm of NaSCN and 50 ppm of H₂O₂ as percarbonate.
  
  
  • The LP activation must be done within 2-3 hours of milking
  
  
• After 10 hours of milking (or 7 hours of first treatment), a booster dose of 35 ppm H₂O₂ (~ 20 mg sodium percarbonate per litre milk) should be mixed into milk.

• To make the practice easy, the quantities of NaSCN and sodium percarbonate for a certain volume of milk (say 40 l) should be distributed to collection points in prepacked quantities, say for a few weeks at a time.
  

8.5 Monitoring the Practice

The usage of the system can be monitored through acidity, MBRT, Resazurin reduction test, total viable count and analysis of thiocyanate concentrations.

8.6 Effect on Milk Quality

• Field trials conducted in Kenya, Sudan, Mexico, Sri Lanka, Pakistan and India report that untreated milk spoiled within two hours whereas treated milk was preserved for different durations depending on the temperature of milk.

• There was no difference on the preservation effect of cow and buffalo milk.

• The quality of initial milk was, in all cases, proportional to the time taken for spoilage.

• A strong bacteriostatic effect was reported on mesophilic and thermophilic bacteria and also spores.

• Total and proteolytic psychrotrophs were also affected.

Selected Reading

FAO. 1999. Manual on the use of the LP-system in milk handling and preservation. Global Lactoperoxidase Programme, Animal Production Service, FAO Animal Production and Health Division. Food and Agriculture Organization of the United Nations. Rome.