This chapter more focused on the reduced temperature processes. Therefore, processes like chilling, freezing, freeze drying and freeze concentration as well as modified or controlled atmosphere storage and packaging are the main lessons covered

1. CHAPTER FOUR
PROCESSING BY THE REMOVAL OF HEAT
CHILLING
CA/MA-STORAGE & PACKAGING
FREEZING
FREEZ DRYING and FREEZ CONCENTRATION

2. Introduction
• In the unit operations described in this section, a reduction
in the temperature of foods slows the biochemical and
microbiological changes that would otherwise take place
during storage.
• Preservation by lowering the temperature of foods has
important benefits in maintaining their sensory
characteristics and nutritional value to produce high
quality products.
• Many of the developments in minimal processing methods
as well as storage of fresh foods rely on chilling as a main
preservation component.

3. Introduction…
• Rapid expansion of ready-to-eat chilled foods, which may
also be packed in modified atmospheres has been an
important development over the last ten years.
• In general, the lower the storage temperature, the longer
foods can be stored, and
• Freezing continues to be an important method of processing
to produce foods that have a long shelf life.
• Freeze drying and freeze concentration remain important
processes for some high-value products, but the high
operating costs of these technologies remain important
deterrents to their more widespread adoption.

4. Introduction…
• Micro-organisms and enzymes are inhibited at low
temperatures, but unlike heat processing, they are not
destroyed. Any increase in temperature can therefore permit
the growth of pathogenic bacteria or increase the rate of
spoilage of foods.
• Careful control is needed to maintain a low storage
temperature and prepare foods quickly under strict hygienic
conditions to prevent spoilage or food poisoning.
• The need to maintain chill- or frozen temperatures
throughout the distribution chain is a major cost to
producers and retailers, and this area has seen significant
developments to improve efficiency, reduce costs and
reduce the risk of spoilage and food poisoning.

5. 4.1 Chilling
• Chilling is the unit operation in which the temperature of a food
is reduced to between 1ºC and 8ºC. It is used to reduce the rate
of biochemical and microbiological changes, and hence to
extend the shelf life of fresh and processed foods.
• It causes minimal changes to sensory characteristics and
nutritional properties of foods and, as a result, chilled foods are
perceived by consumers as being convenient, easy to prepare,
high quality and ‘healthy’, ‘natural’ and ‘fresh’.
• Chilling is often used in combination with other unit operations
(e.g. fermentation or pasteurization) to extend the shelf life of
mildly processed foods.
• There is a greater preservative effect when chilling is
combined with control of the composition of the storage
atmosphere than that found using either unit operation alone.

6. 4.1 Chilling…
• However, not all foods can be chilled and tropical,
subtropical and some temperate fruits, for example, suffer
from chilling injury at 3–10ºC above their freezing point.
• Chilled foods are grouped into three categories according
to their storage temperature range as follows
a) -1ºC to +1ºC (fresh fish, meats, sausages and ground meats,
smoked meats and breaded fish).
b) 0ºC to +5ºC (pasteurized canned meat, milk, cream,
yoghurt, prepared salads, sandwiches, baked goods, fresh
pasta, fresh soups and sauces, pizzas, pastries and unbaked
dough).
c) 0ºC to +8ºC (fully cooked meats and fish pies, cooked or
uncooked cured meats, butter, margarine, hard cheese,
cooked rice, fruit juices and soft fruits).

7. 4.1 Chilling…
• Chilling equipment is classified by the method used to
remove heat, into:
– mechanical refrigerators
– cryogenic systems.
• Batch or continuous operation is possible with both types of
equipment, but all should lower the temperature of the product
as quickly as possible through the critical warm zone (50–10ºC)
where maximum growth of micro-organisms occurs.
Effect of chilling on foods
• The process of chilling foods to their correct storage temperature
causes little or no reduction in the eating quality or nutritional
properties of food.
• The most significant effect of chilling on the sensory characteristics
of processed foods is hardening due to solidification of fats and oils.

8. Effect of chilling on foods …
• Chemical, biochemical and physical changes during
refrigerated storage may lead to loss of quality, and in many
instances it is these changes rather than micro-biological
growth that limit the shelf life of chilled foods.
• These changes include; enzymic browning, lipolysis,
colour and flavour deterioration in some products and
retrogradation of starch to cause staling of baked products.
• Lipid oxidation is one of the main causes of quality loss in
cook–chilled products, and cooked meats in particular
rapidly develop an oxidized flavour termed ‘warmed-over
flavour’ (WOF).

9. 4.2. Controlled- or modified-atmosphere storage
and packaging: (CAS, CAP, MAS, MAP)
• A reduction in the concentration of O2 and/or an increase in
CO2 concentration of the storage atmosphere surrounding a food,
reduces the rate of respiration of fresh fruits and vegetables and
also inhibits microbial and insect growth.
• Modified-atmosphere storage (MAS) and packaging (MAP) are
the use of gases to replace air around non-respiring foods
without further controls after storage (in case of MAS) or
packing (in case of MAP).
• In controlled-atmosphere storage (CAS) and packaging (CAP),
the composition of gas around respiring foods is monitored and
constantly controlled. but with advances in ‘active’ packaging
systems the distinction between MAP and CAP is no longer
clear.

10. 4.2. Controlled- or modified-atmosphere storage
and packaging: (CAS, CAP, MAS, MAP)…
• An increase in the proportion of CO2 and/or a reduction in
the proportion of O2 within specified limits maintains the
original product quality and extends the product shelf life.
• This is achieved by:
inhibiting bacterial and mould growth
protecting against insect infestation
reducing moisture loss
reducing oxidative changes
controlling biochemical and enzymic activity to
slow down senescence and ripening.

11. MAS and CAS
• In MAS, the store is made airtight, and respiratory
activity of fresh foods is allowed to change the
atmosphere as O2 is used up and CO2 is produced.
• In CAS, the concentrations of O2, CO2 and
sometimes ethylene (ethene) are monitored and
regulated.
• When CO2 and O2 levels change due to respiration
in MAS, or when adjustment to atmospheric
composition is needed in CAS, solid or liquid CO2
can be used to increase gas concentration;

12. MAS and CAS…
• Controlled ventilation is used to admit oxygen or
‘scrubbers’ may be used to remove CO2 and thus
maintain a constant gas composition in the
atmosphere.
• MAS and CAS are useful for crops that ripen after
harvest, or deteriorate quickly; even at optimum
storage temperatures CA stores have a higher
relative humidity (90–95%) than normal cold stores
and therefore retain the crispness of fresh foods and
reduce weight losses.

13. MAS and CAS…
• Limitations of MAS and CAS are as follows: the low
levels of O2, or high levels of CO2, which are needed to
inhibit bacteria or fungi, are harmful to many foods.
• CAS conditions may lead to an increase in the concentration
of ethylene in the atmosphere and accelerate ripening and
the formation of physiological defects.
• An incorrect gas composition may change the biochemical
activity of tissues, leading to development of off-odours,
off-flavours, a reduction in characteristic flavours, or
anaerobic respiration.
• Tolerance to low O2 and high CO2 concentrations, varies
according to type of crop, conditions under which a crop is
grown and maturity at harvest.

14. Modified-Atmosphere Packaging (MAP)
• MAP (gas flushing) is the introduction of an atmosphere, other
than air, into a food package without further modification or
control. CAP; continuous monitoring and control of gas
composition in bulk containers.
• MAP is used to extend a product shelf life to give processors
additional time to sell the food without sacrificing quality or
freshness.
• Successful MAP requires raw materials with a low
microbiological count and strict temperature control throughout
the process.
• The three main gases used in MAP are nitrogen, oxygen and
CO2, although others, including carbon monoxide, nitrous oxide,
argon, helium and chlorine have also been investigated,

15. 4.3. Freezing
• Freezing is the unit operation in which the temperature of a food
is reduced below its freezing point and a proportion of the water
undergoes a change in state to form ice crystals.
• The immobilization of water to ice and the resulting
concentration of dissolved solutes in unfrozen water lower the
water activity (aw) of the food.
• Preservation is achieved by a combination of low temperatures,
reduced water activity and, in some foods, pre-treatment by
blanching.
• There are only small changes to nutritional or sensory qualities
of foods when correct freezing and storage procedures are
followed.
• Frozen foods and chilled foods have an image of high quality
and ‘freshness’ and, particularly in meat, fruit and vegetable

16. 4.3. Freezing…
• During freezing, sensible heat is first removed to lower the
temperature of a food to the freezing point. In fresh foods, heat
produced by respiration is also removed.
• Most foods contain a large proportion of water, which has a high
specific heat (4200 J /kg/ K) and a high latent heat of
crystallization (335 kJ /kg).
• A substantial amount of energy is therefore needed to remove
latent heat, to form ice crystals and hence to freeze foods.
• The latent heat of other components of the food (for example
fats) must also be removed before they can solidify but in most
foods other components are present in smaller amounts and
removal of a relatively small amount of heat is needed for
crystallization to take place.

17. 4.3.Freezing…
• Freezers are broadly categorised into:
 Mechanical refrigerators, which evaporate and compress a
refrigerant in a continuous cycle and use cooled air, cooled
liquid or cooled surfaces to remove heat from foods
 Cryogenic freezers, which use solid or liquid carbon dioxide,
liquid nitrogen (or until recently, liquid Freon) directly in
contact with the food.
• An alternative classification, based on the rate of movement of
the ice front is:
 slow freezers and sharp freezers (0.2 cm/ h) including still-air
freezers and cold stores
 quick freezers (0.5–3 cm /h) including air-blast and plate freezers
 rapid freezers (5–10 cm /h) including fluidised-bed freezers
 ultrarapid freezers (10–100 cm /h), that is cryogenic freezers.

18. Effect of freezing
• The main effect of freezing on food quality is damage
caused to cells by ice crystal growth.
• It causes negligible changes to pigments, flavors or
nutritionally important components, although these may be
lost in preparation procedures or deteriorate later during
frozen storage.
• Food emulsions can be destabilized by freezing, and
proteins are sometimes precipitated from solution, which
prevents the widespread use of frozen milk.
• The main changes to frozen foods during storage are as
follows: Degradation of pigments., Loss of vitamins.
Residual enzyme activity., Oxidation of lipids.

19. 4.4. Freeze drying and freeze concentration
• freeze drying and freeze concentration is a preservative
method in which preservation is achieved by reduction in
water activity without heating the food, and as a result
nutritional qualities and sensory characteristics are better
retained.
• However, both operations are slower than conventional
dehydration, evaporation or membrane concentration.
• Energy costs for refrigeration are high and, in freeze
drying, the production of a high vacuum is an additional
expense. This, together with a relatively high capital
investment, results in high production costs for freeze-dried
and freeze concentrated foods.

20. 4.4. Freeze drying and freeze concentration…
• Freeze drying is the more important operation commercially and
is used to dry expensive foods which have delicate aromas or
textures.
• For example coffee, mushrooms, herbs and spices, fruit juices,
meat, seafoods, vegetables and complete meals for military
rations or expeditions for which consumers are willing to pay
higher prices for superior quality.
• In addition, microbial cultures for use in food processing are
freeze dried for long-term storage prior to inoculum generation.
• Freeze concentration is not widely used in food processing but
has found some applications such as pre-concentrating coffee
extract prior to freeze drying, increasing the alcohol content of
wine and preparation of fruit juices, vinegar and pickle liquors.

21. 4.4.1. Freeze drying (lyophilisation)
The main differences between freeze drying and conventional hot air
drying are shown in Table below.

22. 4.4.1. Freeze drying (lyophilisation)…
• The first stage of freeze drying is to freeze the food in
conventional freezing equipment.
• Small pieces of food are frozen rapidly to produce small ice
crystals and to reduce damage to the cell structure of the
food.
• In liquid foods, slow freezing is used to form an ice crystal
lattice, which provides channels for the movement of water
vapour.
• The next stage is to remove water during subsequent drying
and hence dry the food.

23. 4.4.1. Freeze drying (lyophilisation)…
• Freeze-dried foods have a very high retention of sensory
characteristics and nutritional qualities and a shelf life of
longer than 12 months.
• When correctly packaged. aroma retention of 80–100% is
possible. The texture of freeze-dried foods is well
maintained; there is little shrinkage and no case hardening .
• Changes in thiamin and ascorbic acid content during freeze
drying are moderate and there are negligible losses of other
vitamins. However, losses of nutrients due to preparation
procedures,

24. 4.4.2. Freeze concentration
• Freeze concentration of liquid foods involves the
fractional crystallization of water to ice and subsequent
removal of the ice.
• This is achieved by freezing, followed by
mechanical separation techniques or washing
columns.
• Freeze concentration comes closest to the ideal of
“selectively removing water from a food without
alteration of other components.”
• In particular, the low temperatures used in the process
cause a high retention of volatile aroma compounds.

25. 4.4.2. Freeze concentration
• However, the process has
– high refrigeration costs,
– high capital costs for equipment required to handle the
frozen solids,
– high operating costs and
– low production rates, compared with concentration by
boiling.
• The degree of concentration achieved is higher than
in membrane processes, but lower than
concentration by boiling.
• As a result of these limitations, freeze concentration
is only used for high-value juices or extracts.

26. Questions on Chapter 4
1.Explain about processing by the removal heat
(importance with the respect to processing the
application of heat)?
2.What is chilling and state the categories of chilled
foods?
3.Explain about the effect of chilling on food quality?
4.Identify between (MAP & MAS) and (CAP & CAS)?
5.How an increase in the proportion of CO2 and/or a
reduction in the proportion of O2 within specified limits
maintains the original product quality and extends the
product shelf life. (i.e How this is achieved?)

27. Questions on Chapter 4…
6. What are the limitation of MAS and CAS?
7. What is MAP, its use and gases used in MAP.
8. What is freezing?, how preservation achieved by
freezing?, its effect on food quality?
9. What is freeze drying and freeze concentration?
Compare freeze drying with other conventional
drying?
10. Indentify between freeze drying and freeze
concentration?

28. Questions on Chapter 5
1.What is food preservation and what is the starting
point of food preservation?
2.What are the main reason for food preservation and
the important point to be considered in food
preservation?
3.Why it is important to consider “for whom food is
preserved”?
4.Explain the courses of food deterioration in detail?
5.Based on the mode of actions, the major food
preservation techniques can be categorized as?

29. Questions on Chapter 5
6.What is the use chemical as preservative?
7.What is the minimum water activity and give
examples for some microorganisms category?
8.Explain food preservation by controlling
atmosphere?
9.State advantage and disadvantages of using heat
treatment as preservation method?
10.Explain in detail food preservation by food safety
concept/systems? (GMP, SOP, HACCP, TQM, ISO
9000 and 22000 and hurdle technology).
THE END