A brief history of research
From 1920, F.D.Kidd and others discovered the phenomenon of respiratory jump during fruit ripening, and the physiological and biochemical research before and after fruit ripening continued to develop. In 1935, R.Gane proved that ethylene plays an important role in fruit ripening. In 1970s, Fruit and Its Biochemistry, edited by A.C.Hulme, came out, which summarized the basic contents of postharvest physiology and biochemistry for the first time. 1980 "new progress in biochemistry of fruits and vegetables", a symposium of the European phytochemistry society, further collected the research on postharvest physiology and biochemistry since the 1970s, and reported the application of molecular biology concepts and techniques to study postharvest physiology of fruits.
In ancient China, there was knowledge and technology to extend the storage period by using the characteristics of postharvest physiological changes of fruits. Fruit storage was recorded in Zhou Li during the Warring States Period (5th century BC ~ 3rd century BC), and the storage and processing methods of fruits were described in more detail in Qi Yao Min Shu (533 ~ 544) of the Northern Wei Dynasty. After 1970s, with the development of storage and fresh-keeping research, scientific research on fruit ripening physiology began. Soon, the research field expanded, involving postharvest physiology. Great progress has been made in the past 10 years.
research contents
From the beginning of fruit setting, fruits go through the whole life activities of growth, development, maturity, maturity and final senescence, and are generally in the stage of maturity or maturity and final senescence after harvest. Its main physiological activities are as follows:
breathe
The fruit in the test tube is still alive and needs to breathe. The changes of postharvest respiration rate, biochemical changes of respiratory substrates, types of respiration and internal and external factors affecting respiration directly affect the edible time and storage life of fruits (see fruit respiration).
Endogenous hormones and their functions
Ethylene is widely regarded as a mature hormone. There are many kinds of fruits, and their ripening process is closely related to ethylene. But the ethylene content of different kinds of fruits is different. 197 1 year Baur et al. proved that the precursor of ethylene production is ovalbumin. In 1980, Yang Xiangfa also published a diagram of ethylene biosynthesis and its influencing factors (see the process of fruit ripening), which greatly promoted the study of postharvest physiology of fruits. In addition, after fruit harvesting, auxin was degraded by 2,4 dichlorophenoxy and isobutyric acid (CPIBA), while gibberellin and cytokinin decreased degradation, and abscisic acid synthesis increased, which also participated in the biochemical process of postharvest entities. The balance between various hormones plays a major role in regulating fruit respiration and substance metabolism.
Biochemical metabolism
After the fruit leaves in vitro, especially when it enters mature or fully mature fruit, with the change or enhancement of respiration rate, with the participation of endogenous hormone change and balance process, the cell membrane permeability changes, thus changing the metabolic direction, and substrates such as carbohydrates, organic acids and fats are slowly oxidized, accompanied by some synthetic processes, such as the synthesis of various metabolic enzymes, protein and ribonucleic acid (RNA). The energy required for this synthesis process is provided by breathing. The biochemistry of postharvest fruits was deeply studied, and it was gradually realized that hydrolysis and synthesis coexisted and depended on each other during fruit ripening, which was a highly orderly biochemical change process.
Changes in nutritional composition
There are starch, sugar, organic acids, organic salt protons or ions (such as K+, Cl-, 1 199 1, phosphate, Ca2+, Mg2+, etc. ), alcohols, esters, flavonoids, glycosides, phenols, amino acids, amides and protein. With the harvest of fruits, starch decreased, sugar increased, acid was oxidized and aromatic substances increased in most fruits. The speed of this process is related to the edible period and the best edible period of the fruit. Studying the effects of storage environmental conditions and respiration on the decomposition and synthesis of nutrients can create necessary good conditions for storage and preservation.
Moisture change
Water is an indispensable substrate for metabolic activities of organisms, and it is also the most important factor to maintain the fresh state of fruits. Generally, the water content of fruits is as high as 80% or more, and the water supply of plants cannot be obtained after in vitro harvesting. There are often pores, lenticels, calyx holes and channels formed by wounds (fruit stalk ends) on the fruit surface, and water is still transpiration and diffusion through these channels and epidermal cells. Water loss not only causes direct weight loss, but also causes cell expansion pressure, tissue wilting and appearance atrophy. Water deficit stimulates the increase of endogenous ethylene, abscisic acid (ABA) and other hormones in tissues, and accelerates aging. Too much water in the environment is conducive to microbial reproduction and infection, water absorption and fruit cracking. It is an important content to study the way of fruit dehydration, the nature of surface coverage, the physiological effect of water shortage and the influence of external temperature, humidity and airflow speed to control fruit dehydration, ensure normal life activities and keep fresh.
Storage disease
Postharvest physiology research also involves pathology, including diseases during postharvest storage: microbial infection and physiological disorders and their relationship, forming a new field of postharvest pathology. Postharvest physiological disorders are mostly caused by inappropriate environmental factors, such as chilling injury (see chilling injury during fruit storage), carbon dioxide poisoning, lack of oxygen, nutritional imbalance, and accumulation of toxic substances caused by abnormal metabolism in the body, such as apple bitter pox, tiger skin disease, citrus dry water and so on.
Pre-harvest factors
Because cultivation measures, water and fertilizer management and nutritional status have great influence on postharvest physiological activities and quality of fruits, even under the same storage conditions, their effects are often different. Pre-harvest and post-harvest life activities of fruits are a whole, involving maturity, maturity and senescence. Therefore, the harvest time has an important influence on the physiological and biochemical process, storage and quality of fruits after harvest.
environmental factor
The harvested fruit is an independent system, which is easily influenced by external environmental conditions. People usually change the post-harvest environmental conditions to accelerate or delay the ripening and aging process of fruits, so as to improve the quality and prolong the storage period. Temperature is one of the most important factors, and all metabolic activities and biochemical reactions are affected by temperature. Lowering the temperature in a certain range can weaken metabolism, which is the basic principle of general fruit refrigeration. Different varieties have different critical temperatures, below which metabolic disorder will occur, resulting in chilling injury: humidity affects the water balance of fruits and directly affects the quality and flavor, so lack of water will lead to changes in cell metabolism; The concentration of oxygen and carbon dioxide in the environment significantly affects the respiratory rate and metabolic pathway. Proper reduction of oxygen and increase of carbon dioxide concentration will inhibit respiration and ethylene production and prolong storage time, which is the basic principle of modified atmosphere storage. Therefore, studying the influence of environmental factors has a direct effect on developing new preservation technologies.