Thermal Processing of Food

Question 1

There are two main temperature
categories employed in thermal processing:

Answer 1

Pasteurization and Sterilisation

Question 2

The basic purpose for the thermal processing of foods

Answer 2

reduce or destroy microbial activity

Question 3

The primary purpose of blanching

Answer 3

is to destroy enzyme activity in fruit and
vegetables.

Question 4

Other functions of blanching
include:

Answer 4

• Reducing surface microbial
  contamination
• Softening vegetable tissues to
  facilitate filling into containers
• Removing air from intercellular
  spaces prior to canning

Question 5

This is the preferred method for foods
with large cut surface areas as lower
leaching losses. Normally food material
carried on a mesh belt or rotatory cylinder
through a steam atmosphere, residence
time controlled by speed of the conveyor or rotation.

Answer 5

Steam Blanchers

Question 6

involves a first stage in which a single layer of the food is heated to suffcient temperature to inactivate enzymes and a second stage in which a deep bed of the product is held for suffcient time to allow the temperature at the centre of each piece to increase to that needed for inactivation.

Answer 6

Individual Quick Blanching (IQB)

Question 7

The blancher-cooker has three sections

Answer 7

preheating stage, a blanching stage, and
a cooling stage

Question 8

is a relatively mild heat treatment in which food is heated to <100°C.

Answer 8

Pasteurization

Question 9

organisms that can survive exposure to relatively high temperatures but do not necessarily grow at these temperatures e.g. Streptococcus and Lactobacillus.

Answer 9

Thermoduric

Question 10

organisms that not only survive relatively high temperatures but require high temperatures for their growth.

Answer 10

Thermophilic

Question 11

In this method the heating of every particle of milk to at least 72°C and holding for at least 15 seconds. Carried out as a continuous process.

Answer 11

High-Temperature-Short-Time (HTST)

Question 12

a sterilisation treatment, can also
be performed using higher temperatures
and shorter times e.g. 1 s at 135°C

Answer 12

Ultra Heat Treatment (UHT)

Question 13

Typical Equipment employed for this
method includes:

Answer 13

• Plate heat exchanger (PHE)
• Holding tube – sized to ensure the
  correct treatment time is achieved
• Holding tanks – for storage of the
  raw and pasteurised milk
• Balance tank – to assist in
  maintaining full flow, and to take
  returned milk if temperature not
  achieved
• Control and monitoring system

Question 14

sized to ensure the correct treatment time is achieved

Answer 14

holding tube

Question 15

for storage of the raw and pasteurised milk

Answer 15

Holding tanks

Question 16

to assist in maintaining full flow, and to take returned milk if temperature not
achieved

Answer 16

balance tank

Question 17

Maximum temperature between
the container and the liquid are

Answer 17

20°C for heating and 10°C for cooling.

Question 18

the destruction of all bacteria including
their spores.

Answer 18

aim of sterilization

Question 19

are processed so that they are
shelf stable. They should be ‘commercially
sterile’.

Answer 19

canned foods

Question 20

Conditions Affecting the Growth of Microorganisms:

Answer 20

Water
Temperature
Oxygen requirements

Question 21

Micro-organisms can be classified into
three general groups regarding their
oxygen requirements.

Answer 21

• Aerobes – can only grow in the
  presence of oxygen
• Anaerobes – Can only grow in the
  absence of oxygen
• Facultative Anaerobes – adaptable.
  Grows best aerobically but can grow
  anaerobically

Question 22

Thermophilic Spore Formers:

Answer 22

Flat Sours - B.sterothermophilus

Thermophilic Anaerobes –
C.thermosaccharolyticum

Sulphide types –
Desutfomotomaculum nigrificans

Question 23

High heat resistance, product acid,
don’t produce gas, found in sugar, salt
and spices

Answer 23

Flat Sours - B.sterothermophilus

Question 24

High heat resistance, product acid
and gas (CO2)

Answer 24

Thermophilic Anaerobes –
C.thermosaccharolyticum

Question 25

High heat resistance, produce H2S

Answer 25

Sulphide types – Desutfomotomaculum nigrificans

Question 26

Mesophilic Spore Formers (The process should be designed to kill these microbes)

Answer 26

C.sporogenes, C.botulinium Bacillus spp – B.polymyxa, B.macerans etc

Question 27

Produce gas (CO2 and sometimes H2, moderate heat resistance

Answer 27

C.sporogenes, C.botulinium

Question 28

Moderate to low heat resistance, some may grow in acid foods

Answer 28

Bacillus spp – B.polymyxa, B.macerans etc

Question 29

Factors Affecting Spoilage of Canned Foods: (types)

Answer 29

Not processed Under processed Thermophilic Spoilage Leaker Spoilage

Question 30

The sterilization process in the canned product can be subdivided into three phases.

Answer 30

(phase 1 = heating phase). This temperature is maintained for a defined time (phase 2 = holding phase). In (phase 3 = cooling phase)

Question 31

In order to reach temperatures above 100°C (“sterilization”), the thermal treatment has to be performed under pressure in pressure cookers, also called

Answer 31

Autoclaves or retorts

Question 32

These are usually vertical autoclaves with the lid on top. Through the opened lid the goods to be sterilized are loaded into the autoclave.

Answer 32

Simple small autoclaves

Question 33

These are usually horizontal and loaded through a front lid. Horizontal autoclaves can be built as single or double vessel system.

Answer 33

Larger autoclaves

Question 34

in which the basket containing the cans rotates during sterilization. This technique is useful for cans with liquid or semi-liquid content as it achieves a mixing effect of the liquid/semi-liquid goods resulting in accelerated heat penetration

Answer 34

Rotary Autoclaves

Question 35

During this phase, when the outside pressure is low but the pressure inside the containers is still high due to high temperatures there, the pressure difference may induce permanent deformation of the containers.

Answer 35

Rotary Autoclaves

Question 36

Types of Containers for Thermally Treated Products:

Answer 36

- Metal containers are cans or “tins” - Glass jars - Retortable pouches

Question 37

Produced from tinplate. They are usually cylindrical. However, other shapes such as rectangular or pear-shaped cans also exist. Tinplate consists of steel plate which is electrolytically coated with tin on both sides. The steel body is usually 0.22 to 0.28mm in thickness. The tin layer is very thin (from 0.38 to 3.08 µm). In addition, the interior of the cans is lined with a synthetic compound to prevent any chemical reaction of the tinplate with the enclosed food.

Answer 37

Metal containers are cans or “tins”

Question 38

are sometimes used for meat products but are not common due to their fragility. They consist of a glass body and a metal lid. The seaming panel of the metal lid has a lining of synthetic material. Glass lids on jars are fitted by means of a rubber ring.

Answer 38

Glass jars

Question 39

which are containers made either of laminates of synthetic materials only or laminates of aluminium foil with synthetic materials, are of growing importance in thermal food preservation.

Answer 39

Retortable pouches

Question 40

is referred to as the decimal reduction time or D value.

Answer 40

log reduction

Question 41

is the most dangerous, heat resistant spore forming pathogen (D121=0.1 to 0.2 min). It is anaerobic and so can survive and grow in a sealed can.

Answer 41

Clostridium Botulinum

Question 42

Spoilage microorganisms are quickly killed at temperatures of about

Answer 42

90 degree C

Question 43

It is a common soil borne micro-organism, and grows easily on surfaces in the food plant. It is not killed by processes commonly used for acid foods and can cause swelling/bursting of the cans in about 2 weeks.

Answer 43

Clostridium Butyricum

Question 44

is a measure of the “sterilising value” of a process. It can be thought of as the time required at a temperature of 121°C to reduce microbial numbers by the same amount as the actual process being considered.

Answer 44

The F_ value

Question 45

refers to the power to which a base must be raised to produce a given number.

Answer 45

Logarithms

Question 46

is a straight line when plotted using a logarithmic scale

Answer 46

death rate curve

Question 47

This involves drawing a graph of the product temperature vs time, then looking up the L-value of each temperature, and plotting L-value against time. The area under this graph is a measure of the L-value.

Answer 47

graphical method

Question 48

For this method, determine the L-value for each temperature measurement, add the L-value together then multiply by the time interval in minutes between temperature measurements (if temperatures are measured every minute there is no need to multiply).

Answer 48

Trapezoidal Integration or General Method

Question 49

A number of factors affect the rate at which a product heats inside a container:

Answer 49

* Type of container * Size and shape of the container * Retort temperature * Agitation of the containers * Type of product * Headspace

Question 50

obviously a large container will take longer to heat than a small container

Answer 50

Size and shape of the container

Question 51

a higher retort temperate will result in more rapid heating but also may lead to more over processing of product near the package surface.

Answer 51

Retort temperature

Question 52

obviously different products conduct heat more or less easily and have different heat capacities. Some products are more viscous than others which can have a particularly significant effect in agitating retorts. Therefore different products will heat at a different rate.

Answer 52

Type of product

Question 53

insufficient headspace can also affect the rate of heating, especially in an agitating retort.

Answer 53

headspace

Question 54

are proteins which are denatured at high temperatures and lose their activity.

Answer 54

enzymes

Question 55

can affect the growth of microbes in food

Answer 55

water