How to keep grapes fresh? Grape preservation methods

Many people like to eat grapes, right? How do we keep these fresh grapes fresh? Do you know?

How to keep grapes fresh?

Grape is one of the five major fruits in my country, and its storage and preservation are crucial to the development of China's grape industry. This paper compares and analyzes the grape storage and preservation technologies in production and research through the grape harvest period, common pathogens, common diseases, and gas conditioning preservation, low temperature storage, radiation treatment storage, coating preservation, and chemical agent preservation.

Grapes, known as "crystal pearls", have attracted much attention and favor due to their bright color, juicy taste and rich nutrition. At present, the world's grape cultivation area has reached 1000 million hectares, with an annual output of nearly 6000 million tons, accounting for 20% of the world's total fruit production. In fact, since the 1980s, my country's grape production has developed rapidly. In 1999, the cultivation area reached 223 million hectares, making it one of the five major fruits in my country, along with apples, citrus and bananas, of which more than 65% are used for fresh consumption.

Grapes are thin-skinned and juicy, making them difficult to store and transport. The main problems during storage are rotting, threshing, and dry stems. It is estimated that around the world, about 27% of grapes are lost every year due to post-harvest rot. Therefore, in recent years, there have been many studies on grape storage and preservation technologies at home and abroad, and various new technologies have also been applied, such as refrigeration, ice temperature storage, radiation storage, new preservatives, and controlled atmosphere storage.

The respiration of grapes is non-climacteric. There is no accumulation of starch-like substances during the ripening process. After harvest, the sugar content no longer increases, but is only consumed, and the quality of the fruit tends to deteriorate. Therefore, it is very important to judge the best harvest period for grapes. Most standards are: fruit sugar content 16%-19%. Acid content 0.6%-0.8%, sugar-acid ratio 20-35:1, total pectin to soluble pectin ratio 2.7-2.8:1.

Timely precooling after harvest can reduce water loss, browning, threshing and softening of the fruit stems, and to a great extent inhibit the growth and reproduction of fungi. However, grapes have weak resistance to air, so in order to extend the shelf life of grapes, corresponding anti-corrosion measures must be taken to prevent the grapes from being invaded by pathogens during storage and preservation, causing rot and mildew.

1. Common pathogens of grapes after harvest

Studies have shown that common pathogens that cause grapes to rot during storage, transportation and sales after harvest include: Botrytis cinerea, Rhizopus, Aspergillus niger, Penicillium, Alternaria, etc. Among them, gray mold caused by Botrytis cinerea is a devastating disease of fresh-table grapes, because the fungus can still grow and reproduce under low temperature conditions, and grapes have weak resistance to it. Therefore, in order to extend the shelf life of grapes, corresponding antibacterial and antiseptic measures must be taken to prevent grapes from being attacked by pathogens during storage and preservation, causing rot and mildew.

Main diseases that are prone to occur during storage and their prevention measures

3.1 The pathogen of grape gray mold is a gray-green Botrytis spore. Fruits and stalks are easily infected during storage, and it is the main disease of grapes in the later stage of storage. In the early stage of the disease, the lesions are round, sunken, with clear boundaries, brown or yellow-brown, and obvious cracks at the infection points. The skin will fall off the lesions when lightly pressed. Soon, the fungus forms gray conidia through cracking, causing the fruit to rot completely, and the fruit surface is covered with gray mold layer and has a bad smell. After being infected, the stalks turn black, and black blocky sclerotia appear in the later stage. The pathogen invades the inside of the fruit through the lenticels during the flowering and ripening periods, which is a latent infection. Therefore, prevention should be strengthened during the nutritional growth and development period of grapes, one of which is to spray fungicides. In addition, sulfur dioxide treatment, or biological agent treatment at a low temperature of 3-5℃, and film bag packaging can achieve better results.

3.2 Penicillium disease is caused by Penicillium, which is the most common disease in storage. The pathogen forms light brown, water-soaked circular or semicircular depressions on grape berries, but there is no obvious boundary, the interface shrinks, and the fruit softens until the inside of the berry becomes a transparent pulp. The initial stage of the disease is a white mold layer, which turns blue when fruiting bodies or spores are formed, and has a musty smell. It is mainly infected through wounds. The prevention method is to minimize mechanical damage and lower the storage temperature.

3.3 Black mold, also known as black spot, is caused by the fungus Psoralea corylifolia. It can penetrate the healthy epidermis and is a disease that is difficult to control during long-term storage at low temperatures. This pathogen forms brown or dark brown rot around the stem end of the grapes. The disease starts from the stem end and spreads to the fibrous tissue or the cluster. The infected grapes will fall off. It is common in the storage and preservation of grapes. It can be prevented by spraying pesticides during the growth and development period of the grapes, or pre-storing at low temperatures after harvest.

3.4 Drying and threshing Grapes tend to lose water and shrink during storage, and the spike stalks and fruit branches dry up, causing threshing. This is mainly because the relative humidity of the storage environment is low, causing the fruit to lose water and the fruit stalks to shrink. You can use a few layers of paper or a layer of film bag to line the packaging to retain moisture and keep the relative humidity above 90%. In addition, select grapes with lignified spike axes and fruit stalks and thick fruit clusters as storage fruits.

Grape varieties and storage resistance There are many grape varieties in my country. Those medium and late-maturing varieties with thick skin, hard flesh, high sugar content and waxy and powdery covering on the fruit surface are resistant to storage, such as Longan and Purple Rose Fragrance; Hebei Xuanhua's Plum Fragrance, Heilongjiang's American Red and Red Perfume (Catawba) and other varieties have good storage resistance; Black Betty, Kyoho, Sunset Red, Black Aolin and other varieties have medium storage resistance; white grapes are easily scratched and browned during transportation and storage; some varieties, such as Seedless White, have berries that are very easy to fall off or the fruit stalks break, and are more difficult to store.

2. Harvesting and pre-harvesting management

The respiration of grapes is non-climacteric, and there is no after-ripening process after harvesting. If climatic conditions permit, the harvesting period should be delayed as much as possible. The later the grapes are harvested, the higher the sugar content, the thicker the skin, the stronger the toughness, the better the color, the more fully formed the fruit powder, and the more resistant the fruit is to storage. Watering must be stopped 7-10 days before the grapes are harvested to increase the sugar content in the grapes. If a large amount of water is poured before harvesting or it is cloudy and rainy, the grapes are easy to rot, which is not conducive to storage. Grapes should be picked in the early morning and evening when the weather is clear and the temperature is low. When harvesting, carefully cut off the bunches, remove the diseased and insect-infested fruits and inferior fruits on the bunches, and put them into boxes, baskets and other containers lined with 3-4 layers of packaging paper. Fill them tightly to prevent shaking, cover the fruit boxes with several layers of paper and then cover and seal them. If they are stored on the spot, stack the grapes in a cool place first for pre-cooling, so that the grapes cool down as the temperature drops, and store them in the cellar only when there is a slight frost on the open field.

(III) Storage methods and management

The storage temperature of grapes should be between -1 and 0°C. The higher the sugar content, the lower the freezing point. Most grape varieties will not freeze at -2°C. Even after a slight freeze, the grapes can still return to a fresh state. Grapes require a higher relative humidity. A relative humidity of more than 90% is better. High humidity conditions can reduce water loss in grapes and keep the grapes fresh. Grapes are usually stored in cellars (or caves) or indoors in the production area, and can generally be stored until the Spring Festival. Grapes can also be stored in mechanical cold storage until May of the following year, or even extended to July.