Detailed Explanation of Key Points for Storage and Fresh-keeping Techniques of Peaches, Plums and Apricots

Peach, plum and apricot (including cherry) fruits all contain a hard stone and belong to the drupe category of fruits, sharing common characteristics in fruit development and postharvest physiology. It is precisely because the fruit contains a hard stone that a double-S growth curve appears during growth. Peach, plum and apricot fruits have high respiration intensity; all have a respiration peak, so they all belong to the climacteric type of fruit, which determines that they have basically similar storage, transport and fresh-keeping technical measures. However, as tree species and varieties differ, the storage, transport and fresh-keeping techniques adopted also differ. Peaches and plums

Generally divided into early-, mid- and late-maturing varieties, and early-maturing and late-maturing varieties differ greatly. The early-maturing Chunlei peach can mature in early May in Shandong, while the late-maturing winter peach only matures in November. Early-maturing plums mature in early June in Shandong, while Black Gem matures at the end of October. But the maturity periods of other fine peach and plum varieties are relatively concentrated between July and August. Apricots and cherries have a relatively small difference between early and late maturation, and their maturity periods are relatively concentrated, so the phenomenon of "rotting in peak season, shortage in off season" inevitably appears, which requires

improving the storage, transport and fresh-keeping technical level of various varieties, extending the market supply period, and increasing fruit growers' income.

1. Postharvest physiology and storage characteristics of peach, plum and apricot

After peach fruit harvest, the activities of pectinase, cellulase and amylase in the fruit tissue are very strong, which is the main reason why peaches soften and spoil very easily at room temperature after harvest. Especially for honey peaches, the respiration intensity rises rapidly after harvest, 1 to 2 times higher than that of apples, and they soften within 1 to 2 days under room temperature conditions. Low temperature, low oxygen or high carbon dioxide can inhibit the activity of these enzymes, so harvested fruit should be cooled immediately and promptly enter a controlled atmosphere state to maintain their firmness and quality.

Peaches are more sensitive to temperature than other fruits. After harvest, the respiration intensity of peaches is strongly inhibited under low temperature conditions, but chilling injury easily occurs. The freezing point temperature of peaches is -1.5°C, and chilling injury easily occurs below 0°C for a long time. The time and degree of chilling injury occurrence are related to temperature. According to research, peaches sometimes suffer chilling injury at 7%, 3°C to 4°C is the peak of chilling injury occurrence, while near 0°C it is instead slight. In peach fruits that suffer chilling injury, the cells all thicken, the fruit becomes corky, the flavor becomes bland, the flesh hardens, the flesh or vascular bundles turn brown, and the peach stone cracks; some varieties become bitter or produce off-flavors after chilling injury, but different varieties have different chilling injury symptoms.

Peach fruit is very sensitive to carbon dioxide, and carbon dioxide injury occurs when the carbon dioxide concentration is higher than 5%. The symptoms of carbon dioxide injury are brown spots and decay on the peel, browning of the flesh and vascular bundles, little fruit juice, hard flesh, and abnormal flavor, so attention should be paid to maintaining suitable gas indicators during storage.

The surface of peach fruit is covered with fuzz. Most of the fuzz connects with the epidermal stomata or lenticels, which increases the transpiration surface of peaches by more than ten times or even a hundred times, so peaches lose water very rapidly after harvest under exposed conditions. Generally, under conditions of 70% relative humidity and 20°C, left exposed for 7 to 10 days, the fruit's water loss will exceed 50%. After water loss, the fruit shrivels and softens, and in severe cases loses its commercial value.

The postharvest physiological characteristics of drupe fruits such as plums and apricots are similar to those of peaches.

2. Storage conditions for peaches, plums and apricots

1. Storage temperature The suitable storage temperature for peaches, plums and apricots is 0°C to 1°C, but long-term storage at 0°C easily causes chilling injury. At present there are several methods for controlling chilling injury. One method is the intermittent warming method, that is, peaches are first stored at -0.5°C to 0°C for 15 days, then warmed to 18°C and stored for 2 days, then transferred to low-temperature storage, and so on repeatedly. Another method is to treat harvested fruit with two temperatures, first storing at 0°C for 2 weeks, then storing at 5°C (in the United States, in order to prevent peaches from developing chilling injury, at 0°C the gas concentrations are controlled at O2 1% and CO2 5%. During controlled atmosphere storage, every 3 weeks or 6 weeks the controlled atmosphere peaches are warmed once, then returned to 0°C; after 9 weeks of storage at 0°C they are taken out of storage, placed at 18°C to 20°C to ripen, and then sold. This method extends storage life by 2 to 3 times compared with ordinary refrigeration). Intermittent warming can reduce the respiration intensity of peaches, lower ethylene release and alleviate chilling injury; at the same time, the temperature increase is also beneficial to the volatilization and metabolism of other harmful gases.

2. Storage environment humidity When storing peaches, plums, apricots and cherries, the relative humidity should be controlled within the range of 90%-95%. If the humidity is too high, it easily causes rot and aggravates the symptoms of chilling injury; if the humidity is too low, it causes excessive water loss and weight loss, affecting commercial quality and thereby causing undue economic losses.

3. Gas composition Under controlled atmosphere conditions with O2 concentration 1% and CO2 concentration 5%, the storage period of peaches can be doubled (under the same conditions of temperature, humidity and other conditions). For plums, O2 concentration 3%-5% and CO2 concentration 5% is the suitable controlled atmosphere state. But it is generally believed that plums are extremely sensitive to CO2, and long-term high CO2 increases the rate of fruit apex cracking. The most suitable gas composition for controlled atmosphere storage of apricots is O2 concentration 2%-3% and CO2 concentration 2%-3%. The suitable controlled atmosphere gas composition for cherries is O2 concentration 3%-5% and CO2 concentration 10%-25%. Cherries tolerate high CO2 concentrations, so high CO2 treatment is also used during transport to inhibit their respiration intensity and maintain freshness.

3. Pre-storage agricultural measures for peaches, plums and apricots