Fruit storage and refrigeration

Today our pace of life, food needs and supply make fruit ripening a necessity to satisfy the customer. To achieve this, the use of cooling and gas mixing processes is of vital importance.

When entering a shopping center where they sell fruits and vegetables we automatically look for those with the best ripening point or that are close to this condition. However, we don't always consider the whole process and technology involved in being able to get products right.

Ethylene is a non-toxic, highly flammable, colorless gas with characteristic smell and sweet taste, it is also a natural hormone of plants, which actively complies with the growth, development, maturation and aging of the same and in turn, is very important for the ripening of some fruits such as bananas, tomatoes, papayas, melons, pineapples and citrus fruits. Despite this, it can also be very harmful since it immediately accelerates the aging process, thus decreasing the quality of the product and therefore its shelf life. It is important to first know the nature of the fruit to consider its ethylene content, its metabolic pathway, how it is synthesized and where it acts.

Maturation process: The ripening of fruits is linked to complex processes of transformation of their components. When collected, they are separated from their natural source of nutrients, but their tissues continue to breathe and remain active. Sugars and other components undergo important modifications, forming carbon dioxide (_CO2) and water. These processes are of great importance because they influence changes produced during storage, transport and marketing. The phenomena especially prominent during maturation are breathing, sweetening, softening, changes in aroma, coloration and nutritional value.

Respiration: The intensity of breathing of a fruit depends on its degree of development. It is measured as the amount of_CO2 (mg) shed from each kilogram of fruit per hour.

There are fruits in which after reaching the minimum ripening again increases the respiratory intensity until reaching a maximum value, called climacteric peak; these fruits are called "climacteric fruits".

During breathing Ethylene is produced, if this gaseous compound, produced by a ripe fruit, accumulates near unripe fruits, it will quickly trigger its ripening and therefore the deterioration of all the others.

Taste: At the beginning of ripening, the carbohydrate content and the typical sweetness of ripe fruits increases. The acids decrease and the sour taste disappears, to give rise to a mild one.

Softening: The protopectin traps water forming a kind of mesh, which provides the unripe fruit with its particular texture. With maturation, this substance decreases into soluble pectin, which is dissolved in the water it contains, producing softening.

Changes in aroma: The formation of such aromas depends largely on external factors, such as temperature and variations throughout the day.

Color changes: Usually the transition goes from green to another color when the chlorophyll decomposes revealing previously masked dyes. It also increases the production of red and yellow dyes typical of ripe fruits. In some cases the color variation also indicates chemical changes such as in the mango by increasing the content of carotenes, while dyes such as anthocyanins, activate the light.

Nutritional value: In general, fruits lose vitamin C when ripe on the tree and during storage; in this case, the loss depends largely on the temperature, being much lower while it is closer to 0°C.Other elements such as provitamin A are sensitive to contact with oxygen in the air, so peeling, chopping and smoothing fruits, must be done just before consumption.

On some occasions, the maturation process is promoted by applying Ethylene externally before the natural internal concentration reaches 0.1 to 1.0PPM. This does not mean that it is an artificial process, it simply accelerates the normal process.

How do I know what kind of fruit I'm going to handle for optimal results?

It is always necessary to know the products with which you are going to work since during their handling, preservation and refrigeration, some specific characteristics of these require special care related to temperature, relative humidity, etc. In the case of fruits, it is necessary that the providers of refrigeration and conditioning services of environments know the varieties of fruits and for the purposes of this text there are two large groups: climacteric and non-climacteric.

Climacteric fruits, such as tomatoes, are initially green and change to characteristic tones of their variety as chlorophyll decreases as they ripen. Respiration decreases oxygen (_O2) and increases carbon dioxide (_CO2) and ethylene, starch, soluble solids and ascorbic acid.

Another fruit of this type is the banana, in which by decreasing chlorophyll carotenes and xanthophylls increase. The amount of dry matter, starch and hemicellulose decreases and results in higher sugar content. As ripening increases, the banana transpires like other climacteric fruits, so the atmosphere where it is is important, since O₂ decreases and_CO2 increases.

If this type of fruit is left on trees it ripens more slowly than when collected (by a substance existing in the branches). A study in avocados showed that in the tree they take months to ripen, while when collecting them this process takes between 3 and 4 weeks, in addition to ripening increases their respiratory intensity and their storage period depends on it.

On the other hand, non-climacteric fruits, such as vegetables in general, strawberries and citrus fruits, do not produce self-catalysts such as Ethylene, which they take from the environment. As long as this element is in the environment the maturation will continue, otherwise the respiratory intensity decreases and the maturation remains static. For example, if the green pepper in the plant is applied Ethylene it ripens turning red, but if it is outside the plant it will remain green, modifying only in case of previously being somewhat reddish. This is why some products ripen on the tree or plant and others do not; for example, if we cut melons before ripening they will never ripen regardless of the amount of Ethylene applied.

Fruits and vegetables breathe while in the plant when cut, taking O₂ and giving off_CO2; in addition to perspiring (losing moisture). The difference between both states is that together with the plant the flow of sap and other nutrients that compensate for losses by respiration is maintained, while when separated from the plant, the product must be kept with its reserves, in suitable conditions they lose properties. It is here that the conditions achieved by traders and suppliers take on greater importance so that the vegetables reach the consumer in optimal conditions.

How does temperature affect enzymatic activity?

Generally, at 30°C the decrease in enzymatic activity begins, at 35°C it decreases even more and at 40°C it stops. If we maintain 30°C for a long time, there will be no normal maturation, this being irregular.

The lower limit for enzymatic inactivity is between 0°C and 2°C, but at this temperature the water of the product freezes giving expansion of this that affects the tissue cells. When thawing the tissue does not reabsorb the water because the cells are damaged and with it the texture is modified. So, high or very low temperature is not convenient, the ideal is to stay a little above the freezing temperature of the fruit, to be able to store longer and prolong the shelf life.

When you have non-climacteric fruits, the cold delays the deterioration and in the climacteric ones the beginning of ripening is delayed, and if they are kept at a low temperature for a long time you have to apply Ethylene (for longer) so that they ripen. The optimal range for organoleptic maturation is between 10°C and 30°C, with the optimum being 20°C. There is benefit when the temperature is lowered, but factors such as maturation, storage life, cold damage, etc. must be taken into account.

Climacteric fruits such as bananas, avocados and mangoes, should be harvested immature when exported to distant markets and shipped when they are still hard and green, in order to reduce damage and losses during travel and handling.

Storage conditions

Relative humidity

High relative humidity: involves the development of microorganisms and rot.

This variable is important in terms of freshness; allows the fruit to look better or promotes mold formation or other undesirable characteristics.

When you do not have adequate or balanced control of the relative humidity it can lean to two parts: low, which involves dehydration of the product, wilting and weight loss, among others; or high, whose tendency is to allow and / or promote the development of microorganisms and rot.

Therefore, it is important to play with high or low relative humidity; the most recommended are between 85% and 95%, but you should always take into account whether the product supports it or not. There are exceptions that must have a relative humidity greater than 95% such as nuts, bulbs, lettuce, celery, etc.

Air renewal

Air is the medium that helps remove the heat contained in the canning environment. For the temperature to be uniform there must be a constant flow of air, without in case of high air speed, this can affect the quality of the fruits.

In cases where this flow has greater speed than necessary, it affects the product with water loss and therefore weight. Then, the citizen in control of refreshing air flows must necessarily be leveled to avoid burning or dehydrating the product. Flows of 17.66 CFM (30 m 3/h⁾ are recommended.

In addition to the above, ideally the air used should be free of agents that may be harmful to the quality of the fruits, point where the filtering and control of the air must be taken care of.

Ripening, storage and refrigeration of bananas

The banana is a climacteric product, it continues to ripen once it has been harvested, but the fruit must be harvested when it is physiologically ripe (green). Its shape, size and color, depend on the variety grown.

For preservation, storage cellars must be cooled prior to the entry of the product, the temperature of which should drop to 13°C as soon as possible.

Green-ripe fruit can be preserved for one to four weeks; depending on the driving conditions and state of maturity of the same. Once ripe, the banana has satisfactory shelf life beyond two to four days, depending on the ambient temperature. It must also remain stored at 14°C, with relative humidity of 90% to 95% for green-ripe bananas and 85% if ripe. Temperatures of the order of 11°C to 12°C will cause cooling damage. The green-ripe fruit is slightly more sensitive to cold than the ripe one. A few hours of exposure to 10°C can result in the color of the shell; while 12 hours at 7°C are enough to affect the quality of consumption of this fruit.

The maturation process can be induced and accelerated by the external application of Ethylene in special maturation chambers. The process takes about 24 hours with proper temperature and relative humidity control. In this case, it is recommended to keep the temperature between 14°C and 18°C.

During the ripening process the pulp temperature should never be above 19°C, as there is damage known as "cooked", an effect that results in fruits with softened pulps.

The ambient relative humidity, should be maintained in the order of 95% to 98% below these, the fruits have more sensitivity to staining with simple rubbing, impairing the quality and presentation of the product.

Ripening, storage and refrigeration of tomatoes

The appropriate temperature, humidity, air circulation, ventilation and ethylene are required for proper maturation.

The average ripening temperature is 64°F to 70°F (18°C to 21°C). The humidity for maturation and storage is 85% to 95% RH (90% ideal). The air circulation should be sufficient to provide a homogeneous temperature throughout the ripening room and for ventilation it is necessary to use a flow through the room of 10 to 20 minutes every 12 hours.

It is recommended to ripen tomatoes as soon as possible, avoid "holding on" and delay ripening. Tomatoes will respond better and ripen in a homogeneous way when using external Ethylene after harvest. On average, fruit ripening is between 18.0°C and 21.0°C. They can be stored for more than two weeks at 12.5°C until they are completely red.

One of the causes of the loss of taste of tomatoes is the cold. If the temperature of the tomato pulp is outside the appropriate range, visible internal damage to the appearance of the tomato and decline in its taste may occur. So, it is crucial to make sure that the pulp temperature is always above 12.5°C during all steps of ripening. When shipping mixed loads at a lower temperature, some form of protection, such as a plastic or insulated cover, should be used to maintain the temperature of the fruit.

Maturation, storage and refrigeration of papaya

To deliver this fruit in its optimum quality, it is recommended that freshly packaged and pre-refrigerated it be stored between 9°C and 10°C, at this temperature, the papaya has a very slow ripening process. Transportation must be at this temperature either using a trailer, container or refrigerated vessel cell. When the papaya has reached its destination, the temperature can be increased to accelerate its ripening. If the temperature is less than 18°C, color change and maturation are relatively slow.

The storage and handling of the fruit should not go down to more than 10° C to avoid burns and lack of consistency once ripe to 100% coloration. Avoid abrupt and long temperature changes as this results in hard fruit.

A closed storage room is ideal for controlling the temperature and consequently the maturation process. The ideal relative humidity for preserving papayas is between 80% and 95% to avoid dehydration of the fruit, since in case of drying it looks wrinkled and old. When it is between the beginning and the first half of its maturation, it should be stored at 26°C for 40 to 60 hours.

To reach a maturity level of 75% in general, a uniform and controlled temperature is essential to ensure gradual and controlled ripening, while the fruit is in storage and ensure maximum product quality. For example, if it is stored at 10°C it can have up to three weeks of shelf life, but if the temperature rises to 24°C, it can be kept for up to 5 days.

Remember that papaya is like a melon, it is ready to eat when it is half yellow. Do not allow the fruit to overripe because this decreases its life span and taste.

Another point to consider is that papaya produces ethylene and its sensitivity is high, so care should be taken with mixed storage (with other fruits).

The conservation of the papaya once split, lies in storing it in a container with a lid (glass or plastic) in the refrigerator between 5°C and 6°C. The flavor and constitution of this fruit will last approximately 6 days, then its flavor will decrease and the texture will change until it corrugates.

Maturation, storage and refrigeration of melon

Cantaloupe melons are harvested by maturity, not by size. Its commercial maturity is identified when the fruit is gently cut, it detaches from the plant. These melons ripen after harvest, but their sugar content does not increase. The color of its skin is typically gray to opaque green when the fruit has no commercial maturity, uniform dark green at commercial maturity and light yellow at full consumption maturity, at which time another characteristic is the presence of a well-formed and enhanced network on the surface of the fruit.

At temperatures between 2.2°C and 5.0°C the storage life is up to 21 days, although its sensory quality can be reduced. Generally, they can spend 12 to 15 days as normal post-harvest life within the optimal temperature range. Sometimes, during short-term storage or in transport, lower temperatures are applied that outside this range can give rise to cold damage.

High humidities of 90% to 95% are essential, maximizing post-harvest quality and preventing desiccation. Water loss can be significant through damaged or battered areas of the fruit. Prolonged periods in humidity above the optimal range or condensation can stimulate mold growth on the surface.

This type of melon is moderately sensitive to environmental ethylene and overripe can be a problem during its short-term distribution and storage.

Honeydew melons are also not harvested by size. Their maturity is difficult to judge since they do not present a clear abscission process (detachment of the fruit from the plant). The degrees of maturity are grouped according to the change in the "background" color (general color of the skin or peel, not its greenish or yellowish tints) of the fruit, which goes from greenish to cream with yellow dyes.

Temperatures between 7°C and 10°C are preferred, with humidity from 85% to 90%. The storage life is between 12 and 15 days at 7°C and can be extended up to 21 days. If melons are ripe for consumption or matured with Ethylene at 100 PPM for 24 hours, the commercial recommendations for short-term shipping or storage range from 2.2°C to 5°C, as longer periods result in damage to the product.

The application of Ethylene from 100 PPM to 150 PPM for 18 to 24 hours at 20°C will be used to induce the maturation of consumption of physiologically mature Honeydew melons. Physiologically immature fruits do not soften or develop a characteristic sensory quality even when applying Ethylene treatment.

Ripening, storage and refrigeration of pineapple

For harvest, pineapples must have reached their consumption maturity with the maximum sugar content and aroma typical of the species. It is important that who determines the time of harvest, knows the maturity criteria to be applied since, the pineapple is non-climacteric and therefore if it is harvested immature, its presentation and flavor do not improve after the harvest.

To cool pineapples, forced air systems are recommended that allow the temperature to be lowered quickly, eliminating the heat it brings from the field. Rapid cooling prevents weight loss and wilting. Special care should be taken with temperature control which should not be less than 10°C; since like other tropical fruits, this one is very susceptible to cold damage. It can withstand temperatures below 10°C for short periods depending on their degree of maturity, because the less mature they are, the more susceptible they are. The damage starts at 6°C and manifests itself with brown spots on the surface and crown of the fruit. Part of the pulp also becomes brown, acquiring unpleasant aroma and flavor, it is worth mentioning that at 7°C the pineapple will last several days.

In many parts of the world, pineapple is eaten in different ways, one of the most common is chopped, but what happens when they are left over?

Once the pineapple is chopped, to preserve it, it must be peeled, cut, put in plastic bags and frozen directly. Approximately 10 hours before consumption, it is allowed to thaw, and thus, the pineapple retains its nutritional properties (including its fiber and vitamin C content), and its delicate sweet taste, in addition to allowing you to make the most of this fruit.

Maturation, storage and refrigeration of citrus fruits

Citrus fruits are non-climacteric fruits; if they are cut immature their taste and sweetness will not improve, this is because they will not continue to ripen after harvest, so they should not be collected green. It is very important to harvest them when physiologically they are already ripe, that is, when they have already reached their maximum development and good relationship between the concentration of sugars and acidity. Usually, a change in shell color can be a good indicator of maturity. The fruit is ripe, when the color of the peel goes from dark green to light, yellowish or orange, depending on the variety, however, it is not very reliable when the temperature differences between day and night are not very marked as in tropical regions where for example, oranges do not develop their characteristic orange color. On the other hand, when temperature differences are very large, the color change occurs before the fruit matures physiologically, which can lead to harvesting completely colorful but physiologically immature fruits. When the color change is not reliable, it is recommended to use as an indicator of maturity the percentage of juice, Brix degrees and total solids/acidity ratio.

In conditioning and packaging centers the fruit is washed, brushed, disinfected, waxed, selected, classified and packed, and when necessary it is desverdized to give it a better presentation.

When fruits are going to be subjected to refrigerated de-nesting treatments for more or less long periods or when they remain more than 24 hours before their selection and packaging, it is recommended to pre-treat them with fungicides to prevent their infection. This treatment should be performed within 48 hours.

De-greening is normally applied to citrus fruits, mainly oranges and mandarins when despite having reached the maturity required by the trade, their color remains partially or totally green which is unattractive in certain markets. The treatment consists of subjecting the fruit to a flow of Ethylene of 2 to 5 PPM in desverdizing chambers, between 20°C and 22°C with humidity of 90% to 95% and air velocity of 14 to 20 m /minute. The_CO2 content should not exceed 0.2%. It is necessary to prevent the fruits from remaining longer than necessary in the chamber, since Ethylene accelerates aging and this limits the commercial life of the fruits. After de-veraging, it is recommended to let the fruit stand for at least 12 hours before passing it to the sorting and packing line.

Depending on the target market, citrus fruits can be stored for a short time at room temperature and for longer storage periods they must be stored in refrigeration. Packaged fruit can be stored for several weeks or months at temperatures of 3°C to 8°C, however, grapefruits should be kept between 10°C to 15°C to avoid cold damage with relative humidity of 85% to 90%. Temperatures close to 0°C cause damage to most citrus fruits.

TABLE NO. 1 GUIDE OF RECOMMENDED TEMPERATURES AND HUMIDITY FOR THE STORAGE OF SOME FRUITS AND CITRUS FRUITS (Temperatures in °C)

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