Fruit storage and refrigeration

Today, our pace of life, food needs, and suppliers make fruit storage ripening necessary to satisfy the customer. To achieve this, the use of refrigeration and gas mixing processes is vital.

When entering a shopping centre where fruits and vegetables are sold, we automatically look for those with the best ripening point or close to this condition. However, we do not always consider all the process and technology that implies being able to obtain products at their point.

Ethylene is a non-toxic, flammable, colourless gas with a characteristic odour and sweet taste. It is also a natural hormone of plants, which actively complies with their growth, development, maturation, and ageing. It is very important for ripening some fruits such as bananas, tomatoes, papayas, melons, pineapples, and citrus. Despite this, it can also be very harmful since it immediately accelerates the ageing process, thus reducing the quality of the product and, therefore, its shelf life. First of all, it is important to know the nature of the fruit to consider its ethylene content, its metabolic route, how it is synthesized and where it acts.

Ripening process:  Fruit ripening is linked to complex transformation processes of its components. When harvested, 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 (CO 2 ) and water. These processes are important because they influence changes produced during storage, transport and marketing. The phenomena highlighted during maturation are respiration, sweetening, softening, changes in aroma, colouration and nutritional value.

Respiration: The intensity of respiration of a fruit depends on its degree of development. It is measured as the amount of CO 2 (mg) released from each kilogram of fruit per hour.

After reaching the minimum maturation, there are fruits in which the respiratory intensity increases again until reaching a maximum value, called the climacteric peak; these fruits are called “climacteric fruits”.

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

Taste: When ripening begins, the content of carbohydrates and the typical sweetness of ripe fruits increase. The acids decrease, and the sour taste disappears, giving way to a mild one.

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

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

Colour changes: The transition usually goes from green to another colour 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 colour variation also indicates chemical changes, such as in mangoes due to increased carotene content, while dyes such as anthocyanins are activated by light.

Nutritional value: In general, fruits lose vitamin C when they ripen on the tree and during storage; in this case, the loss depends largely on the temperature, being much lower the closer it is to 0°C.Other elements such as pro-vitamin A are sensitive to contact with the oxygen in the air, so peeling, chopping, and fruit smoothies should be made just before consumption.

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

How do I know what type of fruit I will handle for optimal results?

It is always necessary to know the products with which you will work. During their handling, storage and refrigeration, some specific characteristics require special care related to temperature, relative humidity, etc. In the case of fruits, refrigeration and air conditioning services providers need to know the varieties of fruits. For this text, there are two large groups: climacteric and non-climacteric.

Climacteric fruits, such as tomatoes, are initially green and change to shades characteristic of their variety as chlorophyll decreases as they ripen. During respiration, oxygen (O 2 ) decreases, and carbon dioxide (CO 2 ), Ethylene, starch, soluble solids, and ascorbic acid increase.

Another fruit of this type is the banana, in which carotenoids and xanthophylls increase when chlorophyll decreases. The amount of dry matter, starch and hemicellulose decreases and gives rise to higher sugar content. As maturation increases, the banana transpires like other climacteric fruits, so the atmosphere where it is is important since O 2 decreases and CO 2 increases.

If this type of fruit is left on trees, it ripens more slowly than when harvested (due to a substance on the branches). A study on avocados showed that they take months to mature on the tree, while when harvesting them, this process takes between 3 and 4 weeks. Also, their respiratory intensity increases when they mature, and their storage period depends on this.

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, maturation will continue. Otherwise, the respiratory intensity decreases and maturation remains static. For example, if Ethylene is applied to the green pepper on the plant, it matures, turning red, but if it is outside the plant, it will remain green, changing only if it is previously somewhat reddish. Some products mature on the tree or plant, and others do not; For example, if we cut melons before they are ripe, they will never ripen regardless of the amount of Ethylene applied.

Fruits and vegetables breathe when they are on the plant when they are cut, taking in O 2 and giving off CO 2 and sweating (losing moisture). The difference between the two states is that the flow of sap and other nutrients that compensate for losses due to respiration is maintained when attached to the plant. In contrast, the product must be kept in its reserves when separated from the plant. In suitable conditions, they lose properties. Here, the conditions achieved by merchants and suppliers take on greater importance so that the vegetables reach the consumer in optimal conditions.

How does temperature affect enzyme 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, normal maturation will not occur, being irregular.

The lower limit for enzymatic inactivity is between 0°C and 2°C, but at this temperature, the water in the product freezes, causing it to expand and affect the tissue cells. When the tissue thaws, it does not reabsorb the water because the cells are damaged, and thus the texture is modified. Therefore, a high or very low temperature is not convenient. The ideal is to stay a little above the freezing temperature of the fruit to store it longer and prolong its shelf life.

When there are non-climacteric fruits, the cold delays deterioration. In climacteric fruits, the beginning of ripening is delayed, and if they are kept at a low temperature for a long time, Ethylene must be applied (for a longer time) to make them ripen. The optimum range for organoleptic maturation is between 10°C and 30°C, the optimum being 20°C. There is a benefit when the temperature is lowered, but factors such as maturation, storage life, cold damage, etc., must be considered.

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

Storage conditions

RH

. Low relative humidity: implies product dehydration, wilting, weight loss, etc.   

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

This variable is important in terms of freshness; it allows the fruit to have a better appearance or promotes the formation of mould or other undesirable characteristics.

When there is no adequate or balanced control of relative humidity, it can fall into two parts: low, which implies dehydration of the product, wilting and weight loss, among others, or high, whose tendency is to allow and 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 consider whether the product supports it. Some exceptions must have a relative humidity greater than 95%, such as nuts, bulbs, lettuce, celery, etc.

Air renewal

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

In cases where the said flow has a higher speed than necessary, it affects the product with loss of water and, therefore, weight. Then, the care in the control of cooling air flows must necessarily be levelled 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 detrimental to the quality of the fruit, a point where air filtering and control must be taken care of.

Banana ripening, storage and refrigeration

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 colour depending on the cultivated variety.

For its preservation, the storage warehouses must be cooled before the entry of the product, whose temperature must drop to 13°C as soon as possible.

The green-ripe fruit can be kept from one to four weeks, depending on the management conditions and state of maturity. Once ripe, the plantain has a good shelf life beyond two to four days, depending on ambient temperature. Likewise, it must remain stored at 14°C, with a relative humidity of 90% to 95% for green-ripe bananas and 85% if it is ripe. Temperatures of 11°C to 12°C will cause damage due to cooling. Ripe-green fruit is slightly more sensitive to cold than ripe fruit. A few hours of exposure at 10°C may result in the shell colour’s dullness, while 12 hours at 7°C are enough to affect the eating quality of this fruit.

The maturation process can be induced and accelerated through the external application of Ethylene in special maturation chambers. The process lasts 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 damage known as “cooked” occurs, an effect that results in fruits with softened pulp.

The ambient relative humidity must be maintained at 95% to 98% below these. The fruits are more sensitive to staining with simple friction, deteriorating the quality and presentation of the product.

Tomato Ripening, Storage, and Refrigeration

Proper temperature, humidity, air circulation, ventilation, and Ethylene are required for proper ripening.

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

It is recommended to ripen tomatoes as soon as possible, avoid “holding”, and delay ripening. Tomatoes will respond better and ripen evenly when external Ethylene is used 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 completely red.

One of the causes of the loss of taste of tomatoes is the cold. If the tomato flesh temperature is outside the acceptable range, visible internal damage to tomato appearance and decline in flavour can occur. Therefore, it is crucial to ensure that the pulp temperature is always above 12.5°C during all the maturation steps. When shipping mixed loads at a lower temperature, some form of protection such as a plastic cover or insulation should be used to maintain the temperature of the fruit.

Ripening, storage and refrigeration of papaya

To deliver this fruit in its optimum quality, it is recommended that freshly packed and pre-refrigerated be stored between 9°C and 10°C. At this temperature, papaya has a very slow ripening process. Transportation must be at this temperature using either a trailer, container, or refrigerated ship cell. When the papaya has arrived at its destination, the temperature can be increased to speed up its ripening. If the temperature is below 18°C, the colour change and ripening are relatively slow.

The storage and handling of the fruit should not go below 10°C to avoid burns and lack of consistency once it reaches 100% colour. Avoid abrupt and long changes in temperature as this results in hard fruit.

A closed storage room is ideal for controlling the temperature and, consequently, the ripening process. The ideal relative humidity for preserving papayas is between 80% and 95% to avoid dehydration of the fruit since if it dries, 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 between 40 and 60 hours.

To reach a maturity level of 75% in general, a uniform and controlled temperature are essential to ensure gradual and controlled ripening while the fruit is in storage and guarantee maximum product quality. For example, if 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. Please do not allow the fruit to be overripe, decreasing its shelf life and flavour.

Another point to consider is that papaya produces Ethylene, and its sensitivity is high, so care must be taken with hybrid 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 flavour and constitution of this fruit will last approximately 6 days later, its flavour will decrease, and the texture will change until it becomes corrugated.

Melon Ripening, Storage and Refrigeration

Cantaloupe melons are harvested for ripeness, not 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 colour of its skin is typically grey to dull green when the fruit is not commercially ripe, uniformly dark green when commercially ripe and light yellow when fully ripe for consumption. When 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 may be reduced. Generally, they can spend 12 to 15 days as normal post-harvest life within the optimal temperature range. Temperatures lower than outside this range can sometimes lead to chilling injury during short-term storage or transportation.

The high humidity of 90% to 95% is essential; they maximize post-harvest quality and prevent desiccation. Water loss can be significant through damaged or battered areas of the fruit. Extended periods in humidity above the optimum range or condensation can encourage mould growth on the surface.

This type of melon is moderately sensitive to ambient Ethylene, and over-ripening can be a problem during distribution and short-term storage.

Honeydew melons are also not harvested for size. Its 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” colour (available colour of the skin or shell, not its greenish or yellowish tints) of the fruit, which changes from greenish to cream with yellow tints.

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

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

Ripening, storage and refrigeration of the pineapple

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

To cool pineapples, forced air systems are recommended to allow the temperature to drop quickly, eliminating the heat from the field. Rapid cooling prevents weight loss and wilting. Special care must 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 its degree of maturity because the less mature, the more susceptible it is. The damage begins at 6°C and manifests itself with brown spots on the surface and crown of the fruit. Part of the pulp turns brown, acquiring an unpleasant aroma and flavour. It is worth mentioning that at 7°C, the pineapple will last for 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 there are leftovers?

Once the pineapple has been chopped, to preserve it, it must be peeled, cut, placed in plastic bags and frozen directly. Approximately 10 hours before consumption, it is left to thaw. Thus, the pineapple retains its nutritional properties (including its fibre and vitamin C content) and its delicate sweet flavour and allows you to make the most of this fruit.

Ripening, storage and refrigeration of citrus

Citrus fruits are non-climacteric fruits; if they are cut immature, their flavour and sweetness will not improve. They will not continue to ripen after harvest, so they should not be picked green. It is very important to harvest them when they are physiologically mature. That is when they have reached their maximum development and a good relationship between the concentration of sugars and acidity. In general, a change in skin colour can be a good indicator of maturity. The fruit is ripe when the colour of the skin changes 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 colour. On the other hand, when temperature differences are very large, the colour change occurs before the fruit is physiologically ripe, leading to the harvesting of fully coloured but physiologically immature fruit. When the colour change is not reliable, it is recommended to use the percentage of juice, Brix degrees and total solids/acidity ratio to indicate maturity.

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

When the fruits undergo refrigerated DE greening 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 infection. This treatment must be carried out within 48 hours.

DE greening is normally applied to citrus fruits, mainly oranges and mandarins. Despite having reached the maturity trade requires, their colour remains partially or green, unattractive in certain markets. The treatment subjects the fruit to a flow of Ethylene from 2 to 5 PPM in DE greening chambers, between 20°C and 22°C, with humidity from 90% to 95% and air velocity from 14 to 20 m/minute. The CO 2 content should not exceed 0.2%. It is necessary to prevent the fruits from remaining longer than necessary in the chamber since Ethylene accelerates ageing, limiting the commercial life of the fruits. After DE greening, it is recommended to let the fruit rest for at least 12 hours before passing it to the selection and packing line.

Depending on the destination market, citrus fruits can be stored for a short time at room temperature, and for longer storage periods, they must be stored under refrigeration. Packed fruit can be stored for several weeks or months at 3°C to 8°C. However, grapefruits should be kept at 10°C to 15°C to avoid chilling injury at 85% to 90% relative humidity. Temperatures close to 0°C cause damage to most citrus fruits.

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

Product Temperature (°C) RH (%) Approximate storage life

Guava 8 to 10 90 2 to 3 months

lime 8.5 to 10 85 – 90 1 to 4 months

green lemon in general 10 to 14 85 – 90 2 to 3 weeks

overall coloured lemon 0 to 4.5  

85 – 90 2 to 6 months

European green lemon  

11 to 14 85 – 90 1 to 4 months

Yellow European Lemon0 to 10 85 – 90 3 to 6 weeks

Mexican lemon 8 to 10 85 – 90 3 to 8 weeks

Mango 7 to 12               

90 3 to 6 weeks

Tangerine 4 90 – 95 2 to 4 weeks

Cantaloupe 7 to 10 85 – 90 3 to 7 weeks

Orange 3 to 9 85 – 90 3 to 12 weeks

Avocado 7 to 12 85 – 90 1 to 2 weeks

Papaya 7 to 13               

85 – 90 1 to 3 weeks

green pineapple        

10 to 13 85 – 90 2 to 4 weeks

ripe pineapple 7 to 8 85 – 90 2 to 4 weeks

coloured banana        

13 to 16 85 – 90 20 days

Green banana 12 to 13 85 – 90 1 to 4 weeks

Watermelon 5 to 10 85 – 90 2 to 3 weeks

Pink grapefruit 10 to 15 85 – 90 6 to 8 weeks

Grape – 1 to 0 90 – 95 1 to 4 months