Preservation of fruits and vegetables

1, the definition of the respiration of fruits and vegetables, methods and breathing patterns

In the storage of fruits and vegetables, the main reproduction of life activities is respiration. The essence of respiration is a slow biooxidation-reduction process through many intermediate reactions with the participation of a series of specialized enzymes. Respiration is the process of gradually oxidizing and decomposing complex organic substances in cell tissues into simple substances, which eventually become carbon dioxide and water, releasing energy at the same time.

The respiration of fruits and vegetables is divided into two types: aerobic respiration and hypoxic respiration. Breathing in a normal environment (ie under conditions of sufficient oxygen) is called aerobic respiration. The sugar and acid in the body are fully decomposed into carbon dioxide and water, and the heat energy is released. The following formula can be used:
C6H12O6+6O2→6CO2+6H20+674 kcal

Hypoxic breathing (or anaerobic respiration) is known as hypoxic breathing in the absence of oxygen in fruits and vegetables. In this state, sugars and acids in the body cannot be sufficiently oxidized to produce didecene carbon and intermediates such as acids, aldehydes, and ketones. It can be expressed by the following equation:
C6H12O6→2CO2+2C2H5OH+28 kcal

Aerobic respiration and a small amount of anoxic respiration are physiological functions that fruits and vegetables themselves have during storage. A small amount of anoxic breathing is also a manifestation of the adaptability of fruits and vegetables, so that the fruits and vegetables can maintain their life activities in the event of temporary hypoxia. However, long-term severe hypoxic breathing will destroy the normal metabolism of fruits and vegetables.

Breathing types of fruits and vegetables can be divided into breathing and no breathing.

(1) Respiratory transition type: also known as respiratory peak type. The respiration intensity of such fruits and vegetables rose to the highest value at maturity and then decreased. In this stage of respiratory transition, the fruit's flavor quality is best followed by deterioration. Therefore, the period of respiratory respiration is actually a transitional period from fruit ripening to excessive aging. Tomatoes, net melons, apples, pears, bananas, etc., belong to this type.

(2) No respiration type: it can be divided into gradually decreasing type of breathing and rising type of late breathing. A. Decreased breathing, refers to the fruit in the mature period, the respiratory intensity gradually decreased, there is no respiratory peak. Such fruits include oranges, cherries and grapes. B. Late-rising type refers to the gradual increase in respiratory intensity at the late stage of fruit ripening, with no downward trend. Such fruits include persimmon, peach and strawberry.

2, the difference between hot and cold breath in the fields of fruits and vegetables

Before and after fruit and vegetable picking, the heat temporarily stored in fruits and vegetables due to factors such as sunlight and temperature is called field heat. Most of the energy released during the respiration of fruits and vegetables is emitted in the form of heat outside the body. This heat is called respiratory heat. Field heat and respiratory heat are the two sources of heat that should be overcome when fruits and vegetables are stored at low temperatures. The difference between the two is as follows: First, the heat source is different, the field heat is from the fruits and vegetables, and the heat source of respiration is within the fruit and vegetable; Second, the treatment method is different. Usually, pre-storage and pre-cooling methods are used for field heat, and the respiratory heat must be controlled from the breath. Strength and improving the storage environment are two aspects.

3. Factors influencing moisture loss of fruits and vegetables and measures to prevent wilting

Keeping fresh fruits and vegetables is largely to maintain moisture. The phenomenon of water loss during storage of fruits and vegetables is unavoidable because the respiratory metabolism of fruits and vegetables consumes some moisture. In addition, due to various factors, it also caused part of the water to evaporate. The internal factors affecting the moisture loss of fruits and vegetables include the chemical composition of the fruit and vegetable tissue structure, such as different species and varieties, fruit maturity, peel thickness, wax layer thickness, cell gap, and cell fluid concentration; external factors such as storage environment temperature, relative humidity, Light, wind speed, etc. will affect the evaporation of water.

When the water vapor pressure of air in fruits and vegetables storage environment is lower than the surface water vapor pressure, the moisture of fruits and vegetables will be evaporated, the cellular turgor pressure will be reduced, and fruit and vegetables will produce wilting phenomenon. Dehydration of more than 5% in general shows a loss of state, the surface shrinkage, gloss faded, cell gaps increased, the organization became sponge-like. Citrus, cucumber, radish, etc. are easy to see this phenomenon. The wilting causes the appearance of fruits and vegetables to be damaged, the quality is reduced, the loss is increased, the normal respiration is affected, the activity of the enzyme is promoted, the senescence of the tissue is accelerated, and the inherent resistance to storage and resistance of the fruits and vegetables is greatly weakened. Therefore, in the preservation of fruits and vegetables, excessive evaporation of water must be prevented to prevent the wilting of fruits and vegetables. Its methods are A, to strengthen the pre-cooling treatment, to minimize the product temperature and store temperature "temperature difference" after storage, B, to strengthen the storage temperature control, to ensure the appropriate relative humidity required for fruits and vegetables. C, control the air charge speed, can also promote plastic film packaging technology.

4. Prevent sweating of fruits and vegetables during storage and transportation

Fruits and vegetables are often seen in the storage of condensation on the surface of the product, this phenomenon is called "condensation" (commonly known as sweating). Condensation creates favorable conditions for the rapid propagation and growth of microorganisms, especially fruits and vegetables that are damaged by machinery and are more likely to cause rot. The cause of condensation is due to the temperature of the storage environment falling to the dew point temperature, which causes excessive water vapor to precipitate out of the space and form a bead of water on the surface of the object. If the temperature continues to fall below 0° C., frost forms.

A large pile of fruits and vegetables is sometimes dew, because of the large stack, not easy to ventilate and ventilate. The temperature inside the stack is higher than the surface temperature, and the air temperature inside the stack is also high. The temperature difference in the relatively warm storage inside the stack is unstable, and when the temperature suddenly drops, the condensation phenomenon is also likely to occur. The greater the temperature difference between inside and outside, the easier it is to dew.

In order to prevent the condensation of fruits and vegetables during storage and transportation, all good insulation conditions of the storage and transportation yard are required; during storage and transportation, a stable low temperature is maintained; when the ventilation is conducted, the temperature difference between inside and outside should not be too large. Generally speaking, the temperature difference exceeds 5°C. Condensation phenomenon; storage and transportation of fruits and vegetables should not be too thick, too large, pay attention to good ventilation within the heap.

5, cold damage and control measures for fruits and vegetables

Fruits and vegetables show physiological and metabolic incompatibility at low temperatures above 0°C, known as "cold damage" or "cold damage." In the storage of fruits and vegetables, cold damage occurs when the temperature is lower than the proper temperature for storage of the variety. Such as the sweet pepper storage temperature is 7-8 °C, if less than 5 °C damage; Similarly, bananas can not be less than 12 °C. Fruits and vegetables grown in the tropics, subtropics, or in summer or early fall have poor adaptability to low temperatures. In the case of long-term low temperature conditions of 0°C, they are prone to cold damage. Fruits and vegetables grown in the north or mature in autumn and winter, such as apples and Chinese cabbage, are suitable for storage. The temperature is low and it is not prone to cold damage.

After the fruits and vegetables were chilled, the tissues became dark, brown and shrinking, and the appearance of pitted stripes and odors. Some fruits and vegetables with thin skin and soft skin are prone to scum-like plaques.

Control measures: A, variable temperature storage. According to different fruits and vegetables varieties tolerate the limits and time of low temperature, find the most suitable storage temperature to avoid cold injury. B, temperature regulation. High storage temperatures in general help prevent chilling, which is due to reduced evaporation of water. C, gas control. If the oxygen concentration in the environment is too high or too low, it will affect the occurrence of chilling injury. To avoid chilling, the oxygen concentration should be 7%. At the same time, a certain concentration of carbon dioxide inhibits cold damage. D. Breeding of low-tolerance varieties is a fundamental measure that requires long-term efforts. In addition, the gradual cooling of fruits and vegetables and improving the maturity of fruits and vegetables can also reduce the sensitivity to chilling injury.

6. To prevent freezing damage of fruits and vegetables during storage

Damage caused by freezing of fruits and vegetables is called freezing injury. It refers to the phenomenon of physiological disorders and tissue necrosis produced by fruits and vegetables at temperatures below the freezing point of fruits and vegetables.

There are two kinds of freezing injury during storage: First, the absolute temperature of the storage environment is too low; second, due to hot and cold, the temperature difference is too large. For example, the red banana apple has an ice point of -3.3°C and a cold spot of -4°C. Guoguang apple has an ice point of 02.7-3.4°C and a fruit heart freezing point of -2.4-3.3°C. Therefore, when the apple fruit is placed for a long time - 3 °C -5 °C environment, it is prone to frost damage.

In the winter season, cold protection materials were not installed in time at Kumen and the air hole. There is no proper distance or no cover for the cold storage fan outlet, which is the common reason that the operating department freezes fruits and vegetables. For this reason, during the storage period, especially before and after the “March 9th,” the custodian took timely notice of climate change and took corresponding measures.

7. Ripening and aging of fruits and vegetables

Maturity generally refers to the growth and shaping of fruit (or nutrient storage organs of vegetables), the expansion of cells at the end, the volume and weight no longer increase, showing the stage of the characteristics of the variety. This stage can be completed in the tree, but also in the storage period, the length of time depends on the variety of fruit and vegetable varieties, cultivation and storage conditions.

Aging generally refers to the end of changes in the ripening stage of fruits and vegetables, the disintegration of tissues, and the tendency of cells to collapse.

Maturity and aging are a continuous process of transition. They are different stages in the process of life. There is no difference between the two, and there is no absolute gap. Fruits and vegetables that have grown into maturity will mature.

8, after the ripening effect of fruits and vegetables

After fruit and vegetable picking, there is a process of ripening by itself. This is the “after-ripening effect”. For transportation or storage, some fruits and vegetables need to be harvested in advance. Its purpose is to extend the storage and storage period through its own after-ripening effect. It is also possible to take measures (such as low temperature, gas conditioning, etc.) as necessary to suppress the ripening process and achieve long-term storage. If you need to mention the morning market, the use of vinyl can promote ripening of fruits and vegetables. Some fruits, such as pears, must be cooked later to be better eaten. Fruits that generally belong to the peak of breath have obvious after-ripening characteristics.

9. Suitable harvesting of fruits and vegetables

The harvest time of fruits and vegetables is mainly determined by the maturity of the organs of fruits and vegetables, but it is also related to post-harvest use, market distance and storage conditions. Generally, the long-distance transportation is harvested earlier than the local sales. Raw materials processed by pots and preserves should be harvested as early as possible, while raw materials used for processing juices, fruit wines, and jams should be fully matured and harvested.

According to the different uses of fruits and vegetables, people divide the standard of harvesting maturity into: storage and transportation maturity, food maturity, processing maturity and physiological maturity.

10. Judgment of maturity of fruits and vegetables

Determine the maturity of fruits and vegetables should be comprehensive analysis of various factors to determine. Usually judged by the senses and fruit growth period, and refer to other aspects. Usually judged from the following aspects.

(1) Color: In general, the fruit is green before it matures, and when it matures, the green color decreases, and the background and appearance gradually appear. It can be used as a collection mark according to the degree of the appearance of the inherent color of the variety.

(2) Hardness: As the fruit maturity increases, the hardness of the fruit decreases. Therefore, the degree of fruit ripening can also be identified based on the degree of change in fruit firmness. Commonly used fruit hardness tester.

(3) Content of major chemical substances: The changes in the content of certain chemical substances such as starch, sugar, and acid in fruits and vegetables, and the ratio of sugar to acid in fruits are related to maturity. The period of taking can be determined by measuring the content of these chemicals.

(4) Growth period: Under normal climatic conditions, all kinds of fruits and vegetables must mature after a certain number of days. Therefore, the maturity of suitable harvest can be determined according to the growth period.

(5) Plant growth status: Some underground stems, bulbous vegetables such as wolfberry, ginger, onion, etc., were harvested when the ground part began to wither yellow, and had the best resistance to hiding.

(6) Others: As the color of seeds, the formation of fruit powder on the surface of fruit, the thickness of waxy layer, the process of peak fruit respiration, the hardening of the nucleus, and the ease of detachment of fruit stems, etc., can be used as signs of fruit and vegetable maturity.