Food Biochemistry and Food Processing (2 edition)

BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come

25 Biochemistry of Milk Processing 467

In this chapter, several of the main processes used in the

modern dairy industry will be discussed and the relationships

between the biochemical properties of milk and the processes

applied, explored.

THERMAL PROCESSING OF MILK

Introduction

The most common process applied to most food products is

probably heat treatment, principally used to kill microorgan-

isms (e.g., bacteria, yeasts, moulds and viruses) responsible for

foodborne disease, food poisoning or spoilage. Secondary objec-

tives of heat-treating foods include inactivation of enzymes in the

tissue or fluid, which would otherwise negatively influence prod-

uct quality, and effecting changes in the structure of the food,

for example through denaturation of proteins or gelatinisation

of starch.

Today, a wide range of thermal processes are commonly ap-

plied to milk, as summarised in Table 25.2.

The most widely used thermal process for milk is pasteurisa-

tion. In 1864–1866, Louis Pasteur, the famous French scientist,

discovered that the spoilage of wine and beer could be prevented

by heating the product to around 60◦C for several minutes; this

process is now referred to as pasteurisation. The historical de-

velopment of the thermal processing of milk was reviewed by

Westhoff (1978).

Although pasteurisation was introduced to improve the sta-

bility and quality of food, it soon became apparent that it of-

fered consumers protection against hazards associated with the

consumption of raw milk (particularly the risk of transmission

of tuberculosis from infected cows to humans); developments

in technology and widespread implementation occurred early

in the nineteenth century. The first commercial pasteurisers,

in which milk was heated at 74◦C–77◦C for an unspecified

time period, were made by Albert Fesca in Germany in 1882

(Westhoff 1978). The first commercially operated milk pas-

teuriser in the United States was installed in Bloomville, New

York, in 1893; the first law requiring the pasteurisation of liq-

uid (beverage) milk was passed by the authorities in Chicago

in 1908.

Many early pasteurisation processes used conditions not very

different from those proposed by Pasteur, generally heating milk

to 62–65◦C for at least 30 minutes, followed by rapid cooling

to less than 10◦C (now referred to as low-temperature long-

time, LTLT, pasteurisation). However, high-temperature short-

time (HTST) pasteurisation, in which milk is treated at 72–74◦C

for 15 seconds in a continuous-flow plate heat exchanger, was

introduced by APV in 1923 and gradually became the standard

industrial procedure for the heat treatment of liquid milk and

cream. Compared to the LTLT process, the HTST process has

the advantages of reduced heat damage and flavour changes,

reduced cost and increased efficiency and throughput (Kelly and

O’Shea 2002).

Later developments in the thermal processing of milk led to

the development of ultra-high temperature (UHT) processes, in

which milk is heated to a temperature in the range 135–140◦C

for 2–5 seconds (Lewis and Heppell 2000). UHT treatments can

be applied using a range of heat exchanger technologies (e.g.,

indirect and direct processes), and essentially result in a ster-

ile product that is typically shelf-stable for at least 6 months at

room temperature; eventual deterioration of product quality gen-

erally results from physicochemical rather than microbiological

or enzymatic processes.

Sterilised milk products may also be produced using in-

container retort systems; in fact, such processes were developed

before the work of Pasteur. In 1809, Nicholas Appert developed

an in-container sterilisation process that he applied to the preser-

vation of a range of food products, with varying degrees of suc-

cess. Today, in-container sterilisation, as first successfully and

consistently applied towards the end of the nineteenth century, is

generally used for concentrated (condensed) milks; typical con-

ditions involve heating at 115◦C for 10–15 minutes. A related,

although not sterile, class of product is preserved by adding a

high level of sugar to concentrated milk; the sugar preserves the

product through osmotic action (sweetened condensed milk was

patented by Gail Borden in 1856).

The primary function of thermal processing of milk is to kill

undesirable microorganisms; modern pasteurisation is a very

Table 25.2.Thermal Processes Commonly Applied to Liquid Milk Products

Process Conditions Heat Exchanger Reason

Thermisation 63 ◦C for 15 s Plate heat exchanger Killing psychrotrophic bacteria before

cold storage of milk

Pasteurisation 72–74◦C for 15–30 s Plate heat exchanger Killing of vegetative pathogenic bacteria

and shelf-life extension

UHT (indirect) 135–140◦C for 3–5 s Plate heat exchanger Production of safe long shelf-life milk

Scraped-surface heat exchanger

Tubular heat exchanger

UHT (direct) 135–140◦C for 3–5 s Steam infusion system Production of safe long shelf-life milk

Steam injection system

Sterilisation 118 ◦C for 12 min Batch or continuous Production of sterile

Retort Long-life milk

UHT, ultra-high temperature.