FDSC 4075 Chapter : Part 2
PRESERVATION BY TEMPERATURE REDUCTION
PRESERVATION OF FOODS BY STORAGE AT CHILLING TEMPERATURES
I. INTRODUCTION
At temperatures above 0 and below 15EC food is considered to be refrigerated or chilled.
This is a short term preservation method which retards:
1. Growth of microorganisms
2. Postharvest and postslaughter metabolic activities
3. Deterioritive chemical reactions
4. Moisture loss
II. CONSIDERATIONS RELATING TO STORAGE OF FOOD AT CHILLING TEMP.
1. Plant tissues
Most important characteristics of intact storage of fruits, vegetables and seeds is aerobic respiration or metabolism of carbohydrates for
energy with production of water, CO2, and heat. Need O2
Maximum storage life depends on balance respiration rate and rate of quality loss
Usually inverse relationship such that foods with high respiration have short shelf life do to quality losses so should use lowest
temperature to retard deterioration without inhibiting (totally) respiration.
Table 1shows the respiration rates of some plant products-as move rightward in table those materials with lower respiration rates are
less “perishable”
TABLE 1
Rates of Respiration of Fruits and Vegetables
(BTU’s of heat/ ton product / 24 hr @ 40EF)
Rapid
(>10000)
Moderately
Rapid
(5000-10000)
Moderately
Slow
(2000-5000)
Slow
(<2000)
Asparagus Beans (lima) Apples (yellow) Apples
Beans (green) Brussel sprouts Beets (topped) Cabbage
Broccoli Lettuce (leaf) Carrots (topped) Cranberries
Corn (sweet) Raspberries Cauliflower Grapefruit
Okra Spinach Celery Grapes
Peas (green) Strawberries Cherries (sour) Lemons
Lettuce (head) Onions (mature)
Potatoes (sweet) Oranges
Turnips Peaches
Plums
Potatoes (mature)
Tomatoes
In general, intact plant products are fairly resistant to microorganism invasion
because of the outer coverings (skin) and the active living nature of the tissue
Some true fruits are “climacteric” but vegetables are not. This response usually
occurs near optimum maturity at which time there is a significant rise in
respiration rate. It separates “pre-ripe” from “overmature” fruit. After
climacteric, the respiration rate of these fruits declines rapidly as can be seen in
Fig. 1
Non-climacteric fruits and vegetables display a gradual decrease in respiration
with storage time.
Besides reducing respiration lower temperatures also delay senescence and decay
unless susceptible to “chilling injury”
Chilling injury is a physiological disorder of fruits and vegetables (usually
subtropical or tropical) that occurs when storage temperatures are above freezing
but below the critical temperature which is the lowest temperature that product
can be held without damage.
2. Animal Tissues
Unlike plants which normally continue aerobic respiration after harvest, animals
tissues rapidly enter anaerobic respiration or glycolysis after slaughter. This
occurs because oxygen is no longer carried to the tissues by blood.
Glycogen and glucose are converted to lactic acid and tissue pH drops rapidly from 7 to 5.1-6.5. All respiration eventually ceases after
1-36 hrs depending animal and tissue type.
As pH drops and ATP content dissipates muscles enter a state of “rigor” or firm non-pliable condition caused by the locking of actin
and myosin proteins. Eventually the bonds formed between these two proteins begin to break and the muscle again becomes pliable or
tender
Since animal tissue is usually skinned and cut into smaller pieces the mechanical barrier to microorganisms is removed and meat
becomes susceptible to invasion. The normal lowering of pH does offer some inhibition to spoilage.
3. Non-tissue
Most non tissue foods follow the pattern of plant and animal tissues. If the food has a skin, rind or shell then it will resist attack. If
like milk, it does not, then it will spoil rapidly unless chilled
B. Causes of Quality Loss in Food Stored at Refrigerated Temperatures
1. Microbiological Activity - major cause of spoilage of fresh food. As mentioned above, susceptibility depends on barriers and
product conditions (e.g. pH). Unchecked growth leads to decline in quality
2. Physiological or Chemical Activity - senescence in plant materials, autolysis and glycolysis in fish and animal tissue lead to
degradation in quality. Chemical reactions of oxidation, denaturation and vitamin degradation lead to loss of quality. If too
cold, get chilling injury in some plants and suspension of ripening.
3. Physical Damage - Mechanical bruising and dehydration cause quality losses. Dehydration also causes a net loss in weight
which is often the basis for payment.
C. Temperature
1. Benefits of Chilling Temperatures
Pathogenic microorganisms grow most rapidly at 10-37EC, slowly at 3.3-10EC and not at all below 3.3EC.
Most spoilage microorganisms are mesophilic and their growth is retarded at low temperatures
Psychrotropic microorganisms grow at 0-15EC but growth is usually slow.
As with microbial growth, enzymatic, respiratory and chemical reactions are also slowed at refrigerator temperatures. Table 2 shows
effect of temp. on respiration.
TABLE 2
Effect of Temperature on Respiration Rate
BTU per ton per 24 hours
Commodity 0EC (32EF) 4.5EC (40Ef) 15.5EC (60EF)
Apples 300-1500 600-2700 2300-7900
Green Peas 8200-8400 13200-16000 39300-44500
Strawberries 2700-3800 3600-6800 15600-20300
Fig. 2 and Table 3 show the relationship between storage temperature and quality losses in plant products
TABLE 3
Loss of Vitamin C in Fresh Vegetables During Storage
Commodity
Storage Temperature EC Vit. C (mg/100g)
in fresh harvested
% loss in Vit. C
after 24 hr
% loss in Vit. C
after 48 hr
French beans 4
12
20
25.6 22
26
36
34
40
52
Peas 4
12
20
36 7
18
23
10
29
36
Spinach (spring) 4
12
20
39.8 20
27
34
32
43
54
Spinach (fall) 4
12
20
72.3 27
41
56
33
51
79
Spinach (winter) 4
12
20
120.5 5
5
14
8
8
20
In many cases, small (2-5 degree) differences in temperatures are enough to significantly alter quality loss
