Detailed Explanation of Pear Storage Characteristics and Fresh-keeping Techniques
Pear is one of the three major fruits in our country, and its output ranks first among the three major fruits. Its deep-processing volume is limited, and its rot rate is high. In order to extend market supply, storage and fresh-keeping should be done well in combination with its variety characteristics, reducing urban biological waste and providing a guarantee for generating income in the pear industry.
I. Biological Characteristics
Pears are composed of fruit, core, and seeds. The growth process of pear fruit follows an S-shaped growth curve. In the early stage, cell division and embryo growth are dominant, and volume increases slowly. In the later stage, the fruit grows rapidly. The increase in the 40 to 50 days before harvest is about 10 times the growth amount of the early stage.
According to the hardness or softness of the flesh after ripening, pear fruits can be divided into two major categories: hard-flesh type and soft-flesh type. Hard-flesh type varieties such as Yali pear, Cili pear, Xuehua pear, Dangshan pear, Korla fragrant pear, etc. From ripening to senescence, this type of fruit has no obvious respiratory climacteric period, always remains crisp, tender, and juicy, and is relatively storage-tolerant. Soft-flesh type fruits, such as Bartlett pear, Jiuwu pear, Jingbai pear, Zimu pear, Hongxiao pear, etc., have coarse and hard flesh at harvest, with a raw, bland, or sour-astringent taste, but after-ripening makes the fruit soft. This type of fruit has an obvious respiratory climacteric period. After harvest and pre-cooling, it should immediately enter storage at about 0°C.
II. Storage Characteristics
1. Tolerance to low-temperature storage: Experiments have shown that low-temperature cold storage is an effective measure for pear fresh-keeping. It can reduce respiration intensity, reduce losses of sugar and vitamin C, and delay after-ripening. At the same time, low temperature slows down the harm from pathogenic bacteria and provides basic conditions for pear fruit to maintain fresh and crisp quality. Different pear varieties have different storage temperature requirements. The temperature for long-term storage and fresh-keeping is roughly between -1 and 2°C. Most pears cannot go below -1°C, otherwise chilling injury will occur. For example, Pingguo pear stored at 20°C for 4 months loses edible value, but if stored at about 0°C, the storage period can reach as long as nine months with good quality.
2. Tolerance to relatively high humidity storage: The water content of pears is generally about 90%. Because the epidermal tissue is not well developed, they quickly lose water and wilt after harvest under natural conditions. Relatively high relative humidity is a necessary condition for pear fresh-keeping. The speed of water loss among pear varieties is also related to the number and opening of stomata on the fruit itself, the thickness of the wax layer, and the amount of pectin. Generally, the relative humidity of ordinary storage rooms should be kept at 90% to 95%, and cold storage should preferably be kept at 85% to 95%.
3. Gas composition: Pears are relatively sensitive to carbon dioxide. High concentrations of carbon dioxide can cause physiological metabolic disorders in pears and lead to physiological diseases, causing flesh browning. For example, when the carbon dioxide concentration is greater than 2.81%, Pingguo pear is prone to carbon dioxide injury (that is, the peel and flesh appear brown, smooth, and cooked-like); when oxygen content is insufficient (less than 13.5%), it is prone to honey disease. Therefore, during controlled atmosphere storage, the concentrations of carbon dioxide and oxygen in the bag or room must be controlled. For most pears, suitable storage gas composition is oxygen content of 12% to 15% and carbon dioxide content of less than 1%. A few pears are rather special. For example, Yali pear in an environment of 5% to 7% oxygen and carbon dioxide can effectively inhibit and reduce the incidence of black heart disease, and the storage effect is relatively good.
III. Storage and Fresh-keeping Techniques
1. Timely harvest without damage. The suitable harvest period can be determined according to variety characteristics and the length of the storage period. If harvested too early, the fruit is not yet mature, the sugar and acid contents are both low, and there are few components available for respiratory consumption, so it is not storage-tolerant; if harvested too late, the flesh cells have already begun to senesce, the fruit senescence process accelerates, and it is also not storage-tolerant. Only by accurately judging the harvest period of pear fruit can long-term storage be achieved. For varieties of the white pear and sand pear systems, harvest when the fruit surface shows its inherent color, the flesh changes from hard to crisp, the seed color turns brown, and the fruit stalk easily separates from the fruit spur; for varieties of the European pear and Qiuzi pear systems, which have obvious after-ripening changes, appropriate early harvest is acceptable, that is, harvest when the fruit size has basically become fixed, the green color of the fruit surface gradually fades, the seeds have not yet turned brown, and the fruit stalk easily separates from the fruit spur.
During picking, basket (box) loading, loading and unloading, and transportation, try to avoid injuries such as rubbing, bumping, pressing, piercing, and striking, reduce the number of times of pouring baskets (boxes) and repacking, and handle them gently. For some pears, the fruit must retain its stalk at harvest. The harvest time is best in cool weather or every morning.
2. Grading, bagging, and packaging. After harvest, pears should undergo preliminary selection and grading, and cracked fruit, diseased and rotten fruit, deformed fruit, pierced fruit, and overripe fruit should be removed, so as to achieve same quality at the same price and standardization of commodities.
Pear fruit generally does not use the form of individual fruit packaging with plastic film or cartons lined with plastic film, because this will make the carbon dioxide gas volume too high and easily cause browning of the flesh and core. Generally, 0.01 to 0.02 mm thick polyethylene small bags are used for individual fruit packaging. This not only has an obvious fresh-keeping effect, but also does not cause carbon dioxide injury to pear fruit, and its natural loss rate and rot rate are both low. It is a simple, economical, and practical treatment measure. After individual fruit packaging, the pear fruit is placed into cartons, evenly placed in two layers, with thin cardboard separating them in the middle, or cardboard grids are placed inside the carton, one fruit per grid, placed in two layers above and below, with separation in the middle for a better effect. Holes are also punched in the sides of the carton to achieve ventilation and fresh-keeping effects.
3. Pre-cooling. After harvest, pear fruit still carries out vigorous respiration and evaporation, decomposition, and consumption. At the same time, after being harvested from the field, pear fruit also releases a large amount of field heat, which will rapidly raise the ambient temperature around the harvested pear fruit, accelerate ripening and senescence, and obviously reduce freshness and quality. Therefore, within a relatively short time after harvest, pear fruit must be cooled to 3 to 4°C. After harvest, pear fruit should be transported as soon as possible to a pre-cooling room that has been thoroughly disinfected and whose room temperature has already been lowered to the required temperature, and placed separately by batch and grade. The pre-cooling temperature is set at 0 to 3°C, and the purpose of pre-cooling can be achieved in 1 to 2 days. However, for a few pears, the pre-cooling temperature cannot drop too quickly. For example, Yali pear is relatively sensitive to temperature. After harvest, it needs to be pre-cooled at 10 to 12°C and slowly cooled to 0 to 3°C for storage. If it is immediately placed under 0°C storage after harvest, physiological injury will occur, causing browning of the core and flesh.
4. Storage management
4.1 Temperature management. Stable low temperature can effectively inhibit the respiration intensity of pear fruit, reduce metabolism to the minimum, and thus delay senescence. At the same time, low temperature slows down the harm from pathogenic bacteria and provides basic conditions for pear fruit to maintain its fresh and crisp quality. In order to better understand the temperature conditions in various parts of the warehouse, thermometers should be placed in different representative locations. During storage, try to keep the warehouse temperature stable and reduce large fluctuations in temperature. The cold storage temperature should be kept at 0±1°C. Under general conditions, it is ideal for the air temperature fluctuation range of the cold storage not to exceed 2°C, and for the fruit temperature fluctuation range not to exceed 0.5°C.
4.2 Humidity management. Pear fruit itself contains a relatively large amount of water, which determines the relative humidity level during storage. Stable and suitable humidity can maintain the fresh, crisp, and tender state of pear fruit for a long time. In order to maintain or increase the humidity in the warehouse, it can be solved by sprinkling water on the floor, spreading wet sawdust, or using wet sacks. If the humidity is too high, place dry sawdust or quicklime in the warehouse, or open windows for ventilation. Place hygrometers at different locations in the warehouse so as to accurately grasp changes in humidity and do a good job in humidity regulation.
4.3 Gas composition management. When using controlled atmosphere storage, be sure to pay attention to the concentrations of oxygen and carbon dioxide, especially keeping the carbon dioxide concentration below 2%, and the oxygen concentration at 12% to 15% is suitable. When controlled atmosphere storage is not used, use 0.01 to 0.02 mm thick polyethylene film for sealed individual fruit packaging, or ventilate regularly to regulate the gas composition and concentration in the warehouse and prevent the occurrence of carbon dioxide injury in pear fruit.
4.4 Appropriate ventilation. Whether it is a cold storage or a natural ventilation storage, sufficient ventilation volume is necessary. It can carry away the heat load in the warehouse, remove ethylene, ethanol, and carbon dioxide gas produced during the physiological metabolism of the fruit, and supplement appropriate oxygen. It can also prevent uneven temperature in the warehouse and regulate the level of humidity. However, during ventilation, attention should be paid to the outside temperature to determine the ventilation time, and the wind speed should be reduced to prevent large fluctuations in the temperature range inside the warehouse.
4.5 Regular inspection. Regularly inspect changes in the color, aroma, taste, and shape of pear fruit, and whether there is any occurrence of off-flavors, deterioration, rot, disease, or rodent damage. Do a good job in cleaning the warehouse, recording temperature and humidity, and opening doors and windows for ventilation and air exchange.