Thermal Processing of Food Flashcards
There are two main temperature
categories employed in thermal processing:
Pasteurization and Sterilisation
The basic purpose for the thermal processing of foods
reduce or destroy microbial activity
The primary purpose of blanching
is to destroy enzyme activity in fruit and
vegetables.
Other functions of blanching
include:
- Reducing surface microbial
contamination - Softening vegetable tissues to
facilitate filling into containers - Removing air from intercellular
spaces prior to canning
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.
Steam Blanchers
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.
Individual Quick Blanching (IQB)
The blancher-cooker has three sections
preheating stage, a blanching stage, and
a cooling stage
is a relatively mild heat treatment in which food is heated to <100°C.
Pasteurization
organisms that can survive exposure to relatively high temperatures but do not necessarily grow at these temperatures e.g. Streptococcus and Lactobacillus.
Thermoduric
organisms that not only survive relatively high temperatures but require high temperatures for their growth.
Thermophilic
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.
High-Temperature-Short-Time (HTST)
a sterilisation treatment, can also
be performed using higher temperatures
and shorter times e.g. 1 s at 135°C
Ultra Heat Treatment (UHT)
Typical Equipment employed for this
method includes:
- 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
sized to ensure the correct treatment time is achieved
holding tube
for storage of the raw and pasteurised milk
Holding tanks
to assist in maintaining full flow, and to take returned milk if temperature not
achieved
balance tank
Maximum temperature between
the container and the liquid are
20°C for heating and 10°C for cooling.
the destruction of all bacteria including
their spores.
aim of sterilization
are processed so that they are
shelf stable. They should be ‘commercially
sterile’.
canned foods
Conditions Affecting the Growth of Microorganisms:
Water
Temperature
Oxygen requirements
Micro-organisms can be classified into
three general groups regarding their
oxygen requirements.
- 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
Thermophilic Spore Formers:
Flat Sours - B.sterothermophilus
Thermophilic Anaerobes –
C.thermosaccharolyticum
Sulphide types –
Desutfomotomaculum nigrificans
High heat resistance, product acid,
don’t produce gas, found in sugar, salt
and spices
Flat Sours - B.sterothermophilus
High heat resistance, product acid
and gas (CO2)
Thermophilic Anaerobes –
C.thermosaccharolyticum
High heat resistance, produce
H2S
Sulphide types –
Desutfomotomaculum nigrificans
Mesophilic Spore Formers
(The process should be designed to kill these microbes)
C.sporogenes, C.botulinium
Bacillus spp –
B.polymyxa, B.macerans etc
Produce gas (CO2 and sometimes H2,
moderate heat resistance
C.sporogenes, C.botulinium
Moderate to low heat resistance,
some may grow in acid foods
Bacillus spp –
B.polymyxa, B.macerans etc
Factors Affecting Spoilage of Canned Foods: (types)
Not processed
Under processed
Thermophilic Spoilage
Leaker Spoilage
The sterilization process in the canned
product can be subdivided into three
phases.
(phase 1 = heating phase). This temperature is maintained for a defined time (phase 2 = holding phase). In (phase 3 = cooling phase)
In order to reach temperatures above
100°C (“sterilization”), the thermal
treatment has to be performed under
pressure in pressure cookers, also called
Autoclaves or retorts
These are usually vertical autoclaves
with the lid on top. Through the opened
lid the goods to be sterilized are loaded
into the autoclave.
Simple small autoclaves
These are usually horizontal and loaded
through a front lid. Horizontal autoclaves
can be built as single or double vessel
system.
Larger autoclaves
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
Rotary Autoclaves
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.
Rotary Autoclaves
Types of Containers for Thermally Treated Products:
- Metal containers are cans
or “tins” - Glass jars
- Retortable pouches
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.
Metal containers are cans
or “tins”
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.
Glass jars
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.
Retortable pouches
is referred to as the decimal reduction
time or D value.
log reduction
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.
Clostridium Botulinum
Spoilage microorganisms are quickly killed
at temperatures of about
90 degree C
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.
Clostridium Butyricum
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.
The F_ value
refers to the power to which
a base must be raised to produce a given
number.
Logarithms
is a straight line when plotted using a logarithmic scale
death rate curve
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.
graphical method
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).
Trapezoidal Integration or General
Method
A number of factors affect the rate at
which a product heats inside a container:
- Type of container
- Size and shape of the container
- Retort temperature
- Agitation of the containers
- Type of product
- Headspace
obviously a large container will take longer
to heat than a small container
Size and shape of the container
a higher retort temperate will result in more rapid heating but also may lead to more over processing of product near the package surface.
Retort temperature
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.
Type of product
insufficient headspace can also affect the rate of heating, especially in an agitating retort.
headspace
are proteins which are denatured at high temperatures and lose their activity.
enzymes
can affect the growth of microbes in food
water
