Response to adverse environments 731 Temperature

Cold temperatures are the foundation of most storage technologies. Chemical reactions proceed at a slower rate at lower temperatures. Changes in reaction rates with temperature are often characterized by their respiratory quotient (Q10). The rate of simple chemical reactions is halved by a 10 °C reduction in temperature and is therefore said to have a Q10 of 2. The rate of many enzymatic reactions shows a more pronounced decline with temperature and these have a Q10 greater than 2. For example, the rate of respiration of harvested broccoli is reduced by a quarter (a Q10 of 4) as the temperature is reduced from 20 °C to 10 °C, and by another third (a Q10 of 3) as it is further reduced to 0 °C. Broccoli that would remain marketable for 3 days at 20 °C, would therefore have 12 and 36 days of shelf-life at 10 and 0 °C, respectively. The length of shelf-life is often inversely related to the rate of respiration (Figure 7.2). However, not all fruits and vegetables respond so favorably to reduced storage temperatures.

Fresh fruits and vegetables can be categorized by their sensitivity to cold, chilling temperatures (Table 7.2). Plants indigenous to the tropics and sub-tropics often suffer from a physiological disorder called 'chilling injury' if stored at non-freezing temperatures below 10 °C (50 °F). The level of sensitivity varies greatly among plants and tissues, with some being damaged by less than a day at 0 °C (e.g. avocados, bananas), while others (e.g. cantaloupe, peppers) tolerate many days at 0 °C before exhibiting any chilling-injury symptoms. Symptoms characteristic of chilling injury include altered and abnormal ripening, elevated respiration and ethylene production, increased water loss which often produces surface pitting, tissue and vascular browning, and increased senescence and susceptibility to disease. Tolerance to chilling injury is affected by growing conditions and temperature conditioning before chilling, while symptom development during or after chilling is affected by factors such as humidity, sanitation, wounding, packaging, atmospheric composition and rapidity of use. If the level of

CP O

CP O

10 15 20 25 30

Temperature (°C)

Fig. 7.2 Relationship between the rate of respiration and the shelf-life of a non-chilling-sensitive commodity. Relationships are given for a commodity with a respiratory quotient (Q10) of 2 (dashed lines) and one with a Q10 of 3 (solid lines). Shelf-life decreases as the rate of respiration (and other associated metabolic reactions) increases with increasing temperature.

Table 7.2 Some common fruits and vegetables grouped by their chilling sensitivity. Very chilling-sensitive crops should not be stored below 7-10 °C, moderately chilling-sensitive crops should not be stored below 3 °C, while chilling-tolerant crops can be stored at 0 °C (Gross et al., 2005)

Very chilling sensitive Moderately chilling sensitive Chilling tolerant

Avocado (e.g. Fuerte)

Banana

Basil

Bean (Snap, Lima) Citrus (most) Cucumber

Eggplant (aubergine)

Ginger

Jicama

Mango

Melons

Okra

Papaya

Pepper

Pineapple

Pumpkin

Summer squash

Sweet potato

Tomato (mature-green)

Apple (e.g. Mcintosh)

Asparagus

Cantaloupe

Cowpeas

Cranberries

Lychee

Olive

Orange

Peach

Pepper

Pomegranate

Potato

Tomato (ripe) Watermelon

Apple (other cultivars)

Apricot

Artichoke

Beetroot

Berries

Broccoli

Cabbage

Carrot

Celery

Cherry

Collards

Corn, sweet

Grapes

Leeks

Lettuces

Onions

Pears

Strawberries chilling is moderate, symptoms may only develop after removal to a warmer, non-chilling temperature. Many chilling-sensitive fruits and vegetables can therefore be stored for a short time at chilling temperatures (the duration depends on the sensitivity of the tissue) if they are rapidly used after removal from storage.

Elevated temperatures reduce the quality of harvested fruits and vegetables. Moderately high temperatures (20-35 °C) increase respiration and the associated reactions that reduce quality, while higher temperatures (>35 °C) denature crucial enzymes and thereby prevent many reactions necessary for producing or maintaining high-quality fruit. Elevated rates of respiration reduce quality because many of the same compounds that contribute to product quality (e.g. sugars and organic acids) are substrates for respiratory metabolism. In contrast, higher temperatures inhibit essential reactions (softening and the production of characteristic pigments and flavor compounds) that contribute to product quality. If the high temperatures have not caused cellular death (i.e. sunburn or sunscald on tomatoes), the tissue may recover normal metabolic processes during storage at proper storage temperatures.

A compromise must therefore be made between the detrimental effects of high and low temperatures (Figure 7.3). Chilling injury can occur if the sensitive product is held at too low a temperature, while the rate of deterioration will increase if the temperature is too high. The optimal storage temperature may vary as a fruit progresses through various stages of ripeness. For example, a mature-green tomato is more sensitive to chilling than is a ripe tomato. Unripe fruit may appear to be more sensitive to extreme ffl CL

80 60 40 20

Level of chilling injury

Shelf-life of non-chilling-sensitive commodity

Level of chilling injury

Shelf-life of non-chilling-sensitive commodity

Shelf-life of chilling-sensitive commodity

80 60 40 20

Threshold for chilling .1

Shelf-life of chilling-sensitive commodity

Threshold for chilling .1

Fig. 7.3 Comparison between the shelf-life of a chilling-sensitive (dashed lines) and non-chilling-sensitive (solid lines) commodity. The level of chilling injury (dotted line) increases as the temperature declines from a threshold at 12.5 °C to 2.5 °C, and then decreases as the temperature declines further to 0 °C.

temperatures because the physiological changes they must undergo during ripening are more sensitive than are the physiological processes involved in maintaining the ripened tissue. However, the increases in ion leakage and respiration induced by chilling are similar in unripe and ripe tissue. It appears that a similar level of physiological damage is produced by chilling in unripe and ripe fruit tissue, but it is not as apparent in the ripe tissue since it has already ripened and any damage that would have occurred to the ripening process is now irrelevant.

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