Reveal the current situation and development prospects of apple postharvest pre-cooling technology
【Funds】: Shaanxi Province Apple Industry Transformation and Upgrading Key Technology R&D and Industrialization Demonstration Technology Major Project "R&D of New Technology for Postharvest Commercialization Treatment and Efficient Cold Chain Logistics of Apples" (2020ZDZX03-05-01); Shaanxi Fruit Industry Center "Apple Precooling Technology Demonstration and Promotion Project" (ZCZX2023-CS-107).
Tang Yu et al
Pre-cooling refers to a commercial treatment measure that quickly removes the heat in the field and lowers the temperature of the apples to a suitable temperature for storage after fresh apples are harvested and before storage or distribution. Apples are harvested with vigorous respiration and a lot of field heat, which will have a great impact on the maintenance of fruit quality if not eliminated in time. Pre-cooling can reduce the respiration rate of apples, inhibit fruit metabolism, slow down nutrient consumption, prolong the shelf life, reduce the risk of anaerobic respiration during storage and transportation, and effectively reduce the condensation caused by uneven humidity caused by vigorous respiration of fruits in the storage environment. Pre-cooling can effectively control the reproduction and infection of pathogens, and significantly reduce fruit decay during storage. In addition, pre-cooling helps to reduce the refrigeration load of the storage cold storage, so that the stored fruits can reach the ideal and stable low temperature storage state more quickly.
The apple storage and preservation technology system has realized the deep integration of post-harvest treatment, storage and preservation, and cold chain logistics, showing a high degree of systematization and scientificity, and pre-cooling is an important link that needs to be strengthened. In the following, according to the application status of Apple's pre-cooling technology, combined with the phased results of the project, the key points of operation of the key links of pre-cooling technology are proposed, as well as some suggestions for the further application and promotion of pre-cooling technology.
1 Application status of Apple pre-cooling technology
1.1 Apple pre-cooling technology is mainly used in the storage of early-maturing and late-maturing varieties Early maturing apple varieties have relatively poor storage resistance, fast senescence and short storage period, and their harvest time is in the high temperature season, so pre-cooling treatment is very important. In order to ensure the quality of fruits and shelf life in the later stage of storage, scientific pre-cooling must be used to extend the shelf life, reduce losses and increase efficiency. Therefore, the application and promotion of pre-cooling technology focuses on early-maturing and late-maturing apple varieties that require long-term storage.
1.2 Apple storage enterprises lack the driving force for the promotion of post-harvest pre-cooling technology Most enterprises do not have special apple pre-cooling equipment, and only a small number of large enterprises are equipped with pre-cooling storage, and the overall pre-cooling rate of apples in the process of post-harvest storage and transportation is low. There is a lack of standardized pre-cooling management in production practice, and most enterprises choose direct storage and refrigeration during the fruit harvesting season, and the harvesting and storage operations are often relatively primitive and extensive, resulting in high fruit damage rate and quality decline in the later stage of storage. In addition, a large number of late-maturing Fuji varieties with good storability are stored in the enterprise, and the proportion of long-term storage is relatively small. For Fuji apples stored in the short and medium term, the positive impact of pre-cooling on their product quality and value improvement did not show any results. At the same time, producers and distributors do not have enough awareness and attention to the importance of pre-cooling for long-term storage fruit quality maintenance. In this case, in order to pursue benefits and economic returns, enterprises are not highly motivated to increase pre-cooling processes and special equipment.
The utilization rate of mobile cold storage in the field is low. In 2022, Yichuan, Fuxian, Luochuan, and Huangling in Shaanxi Province used the rural revitalization bridging funds to purchase mobile cold storage, which was equipped with pilot village committees, e-commerce companies, fruit enterprises, cooperatives, etc. However, our investigation found that the use efficiency of this apple harvesting mobile cold storage is low, mainly because the mobile cold storage needs three-phase electric power supply, and most orchards have no power supply conditions; Once the sales task is completed, some mobile cold stores are left idle and can only be used for the next fruit season. On the whole, mobile cold storage is in the problem of "enterprises do not want to use it, fruit farmers cannot afford to use it, and village collectives do not use it well".
1.3 The experience of apple post-harvest processing technology needs to be summarized The survey of apple refrigeration enterprises in northern Shaanxi and Shandong found that the experience of apple pre-cooling and warehousing management of some enterprises is very worthy of summary and promotion. For example, some companies improve the cooling efficiency of apples through the method of batching storage of ordinary cold storage. Shaanxi Huasheng Modern Agriculture Group Co., Ltd. does not have a special pre-cooling storage, mainly uses a high-power compressor to refrigerate and has good thermal insulation performance, and uses ordinary cold storage to pre-cool according to the principle of pre-cooling;Huangling County Tianzhuang Town Hongchang Fruit Industry Co., Ltd. does not have a special pre-cooling storage, adopts the method of natural pre-cooling, and transports the apples back to the reservoir area in time after harvesting, releases the field heat in the channel or loading and unloading platform in the reservoir area, and enters the warehouse in batches within 24 h at the latest, and does a good job of environmental control during the storage period to extend the apple supply period;Fuxian Luping Fruit Industry Co., Ltd. 2023 The annual cold storage is renovated into a pre-cooling storage, which runs well; the fruit farmers in Zhuojia Village, Qiulin Town, Yichuan County, insulate the walls of their vacant houses, install refrigeration equipment, and build a small cold storage at the door of their homes. Qixia Defeng Food Co., Ltd. standardizes warehousing management, It is stipulated that the fruit must be stored within 48 h after harvesting, and the daily storage volume shall not exceed 15% of the storage capacity, and the storage shall be completed in about 7 days, and the fruit storage quality is good. These experiences in post-harvest treatment provide a practical basis for proposing standardized pre-cooling technologies.
2 Key technical points of pre-cooling based on investigation and practice
Based on the investigation and analysis of the current situation of the apple industry and the practical experience of storage enterprises, we put forward suggestions for the operation of apple pre-cooling, which mainly include the technical points of key links such as pre-cooling preparation, fruit harvesting, grading and packaging, warehousing, pre-cooling and storage.
2.1 Preparation before pre-cooling The apples used for storage should not be covered with nitrogen fertilizer within 20 days before harvesting, no irrigation within 10 days before harvesting, and no harvest within 3~5 days after rain. Clean and disinfect the storage body before entering the warehouse. Prepare for cooling in advance for the cold storage to prevent the temperature drop from damaging the storage body.
2.2 Fruit harvesting In order to improve the storage quality of apples, the time from harvest to storage should be shortened as much as possible (no more than 24 hours is appropriate). In order to improve the efficiency of pre-cooling, it is required to harvest the fruit in a period of low temperature, such as harvesting from 10:00 after the dew is dry every morning, it can be directly packed into a box for pre-cooling, if it is harvested after 10:00, the fruit needs to be placed under the canopy in the field, on the cold storage platform or in the cold storage corridor for ventilation overnight, natural cooling, and pre-cooling before 8:00 a.m. the next morning. After the natural cooling process, the fruit temperature can be reduced by 5~10 °C, and the pre-cooling time can be shortened by 12~24 h.
2.3 Grading and packaging Apples used for storage need to be initially graded in the field when harvested, and then packed into well-ventilated, strong and pressure-resistant large wooden boxes, iron baskets or plastic boxes to be transferred to pre-cooled storage; The permeability of pre-cooled packaging is high, and the total area of the box vent is not less than 5% of the surface area of the box. Pre-cooled packaging is recommended without a plastic film bag lined with an inner line.
2.4 Warehousing in batches In order to achieve the purpose of rapid cooling, it is necessary to control the amount of warehousing. The one-time storage volume of cold storage pre-cooling does not exceed 20% of the storage capacity, and the general stacking density does not exceed 50 kg/m 3. When stored in large wooden boxes or large iron baskets, the stacking density can be increased by 10%~20%. According to the specific situation of the storage enterprise, multiple cold stores can be opened at one time for pre-cooling, and the apples can be stored in one cold storage after the pre-cooling is over, which can speed up the storage time. The stacking density of the special pre-cooling storage and the transformation of the warehouse can be increased accordingly, and the general storage capacity should be less than 60% of the storage capacity, and the stacking density is generally not more than 160 kg/m 3.
2.5 Pre-cooling condition control Pre-cooling should be completed within 48 h, and the core temperature should reach the end temperature of pre-cooling. Theoretically, the suitable temperature for refrigeration of apples is the pre-cooling termination temperature, and in order to improve the turnover efficiency of the pre-cooling storage, the pre-cooling termination temperature can be slightly higher than the suitable storage temperature. Different varieties have different sensitivities to low temperatures, and the pre-cooling endpoint and cooling strategy are set according to the differences of varieties. Most apple varieties are suitable for storage at a temperature between -1~0 °C. Varieties that are sensitive to low temperature should be stored at a temperature of 2~4 °C. Fuji series, Marshal series, etc., can reduce the storage temperature to -1~0 °C by rapid cooling method; For some low-temperature-sensitive varieties, the method of slow cooling (gradient cooling) can be used to reduce the storage temperature to the appropriate temperature, such as honey crisp (2~4 °C), Venus gold (0~2 °C), Ruixue (0~2 °C), etc.
3 Suggestions for the application and promotion of Apple pre-cooling technology
3.1 Transform the existing cold storage One or more of the existing ordinary cold storage of the enterprise shall be transformed according to the standard of pre-cooling storage. If the power of the original compressor is large, in the case of meeting the refrigeration demand of the existing cold storage, it is only necessary to increase the number of fans and carry out thermal insulation and airtightness treatment on the wall according to the actual situation, and the transformation cost is small. If the original compressor is old and the power is not enough, it is necessary to increase the refrigeration equipment, so that the cost of transformation is relatively large.
According to our survey, some fruit enterprises in Shaanxi have gradually recognized and begun to pay attention to apple pre-cooling. For example, at present, Shaanxi Wang Zhangjiao Agricultural Development Co., Ltd., Fuxian Lvping Fruit Industry Company, Hongjia Company, etc., are operating or building new pre-cooling storage, and some enterprises have improved quality and efficiency through the application of pre-cooling technology. Fruit enterprises in northern Shaanxi have received financial and policy support from the local government in the application and promotion of apple pre-cooling technology.
3.2 Construction of shared pre-cooling storage in the relatively concentrated area of the fruit warehouse or storage trading market to build or renovate the pre-cooling storage for the use of storage enterprises without pre-cooling storage to solve their pre-cooling needs. For example, Luochuan Fruit Investment Company has rebuilt three pre-cooling warehouses of 200 t/room, which can not only be used by its nine shareholding enterprises, but also provide fruit pre-cooling services for other enterprises in Luochuan apple wholesale market. Yinong Apple Professional Cooperative (Zhou Fangmin) transformed all the existing 1 000 t ordinary cold storage into pre-cooling storage, providing fruit merchants and fruit enterprises with pre-cooling services before post-harvest warehousing and pre-packaging after delivery.
3.3 Make full use of ordinary cold storage pre-cooling Storage enterprises without special pre-cooling facilities can start the refrigeration system of ordinary cold storage in advance after apple harvesting, adjust the storage temperature to about 0 °C, and then put apples into the warehouse in batches and in small quantities, and reduce the temperature of the core as soon as possible in the shortest possible time. Compared with the traditional cold storage pre-cooling method, although the pre-cooling time of the same batch of apples is longer, the efficiency is lower, and the effect is poorer, its storage quality is still significantly better than that of apples without pre-cooling.