In this article we will discuss about:- 1. Meaning of Fruit Ripening 2. Factors Affecting Fruit Ripening 3. Hormonal Regulation 4. Controlled Ripening 5. Artificial Fruit Ripening.
Meaning of Fruit Ripening:
There are several developmental phases through which the fruit passes and fruit ripening is one of them. In fact, ripening begins moment the growth of the fruit is completed. Fruit maturity is a stage of fruit harvesting while fruit ripening is a stage of fruit consumption.
The fruit ripening is associated with many visible changes in the colour, the flavour and the aroma. Thus, the fruit is ready for eating purposes. Fruit ripening is a type of ageing and many people prefer to call it “fruit ageing” than fruit ripening. In many fruits the ripening occurs after picking or the process is hastened after picking. Ripening processes are of degradative nature.
Studies in recent years have shown that several biochemical processes must occur sequentially. However, these processes may not be linked with each other.
Factors Affecting Fruit Ripening:
In the following some of the important factors affecting fruit ripening are described:
i. Climacteric:
The visible changes in the fruit leading to ripening are accompanied by a rapid increase in respiration. This process is called climacteric and is distinctly visible in many fleshy fruits like apple, banana, apricots, papaya, tomato etc. However, fruits like figs or cherries do not show climacteric.
This does not mean that the non-climacteric fruits always have low rate of respiration. Some of the compound fruits in fact have high activity of respiration. In general climacteric fruits are rich in carotenoids whereas non-climacteric fruits contain anthocyanins. In apple once the climateric begins the free fructose disappears from the cytoplasm due to phosphorylation.
Simultaneously there is a change in tonoplast permeability which presumably permits movement of fructose from the vacuole to the cytoplasm. Thus, there is an increased respiration. The alternative explanation is that the rate of respiration is regulated by ADP. Thus respiration rate in low if ATP/ADP ratio is high.
The climacteric rise in respiration results from a high energy requirement in the initial stages of fruit ripening. The respiration is enhanced when ATP is split and level of ADP rises. Tomato fruits when sprayed with 2, 4- DNP are prevented from ripening.
One of the factors inducing increased respiration is natural un-couplers of oxidative phosphorylation. Climacteric fruit extracts did act as un-couplers of oxidative phosphorylation. The present thinking is that increased respiration may be attributed to high energy requirements in ripening.
ii. RNA Metabolism:
Tracer studies have shown that in several fruits increased RNA synthesis accompanies fruit ripening. Most of the evidence is based on assays of the rate of incorporation of RNA precursors and indicates that RNA synthesis includes mRNA and is enhances during early part of climacteric rise.
In picked up apples about 50% RNA increased at the initiation of the climacteric increase. When the climacteric is high the increase in its synthesis does not occur.
In general, new synthesis of RNA seems to be essential for the ripening process. Pears sprayed with Act.D did not ripe. The rise in the RNA concentration is followed by an increase in the protein content because of new synthesis. Indeed the synthesis of new proteins is essential for the ripening of many fruits.
When the mature, unripe banana and pears were sprayed with cycloheximide, ripening was inhibited. This was especially so when it was administered during early stages. It is assumed that enzymes involved in ripening were synthesized during the early stages.
iii. Enzymes:
Changes in the pattern and activities of several enzymes are reported during fruit ripening. In general, several hydrolytic enzymes increase. These include polygalacturonase, cellulase, pectin methyl esterase, etc. Some of the enzymes soften the fruits and bring about changes in taste as well. The sweetness in several fruits is caused by breakdown of starch into sugar. Sometimes fruits abound in free fatty acids.
However, the importance of several enzymes in ripening of fruit is not clear. This category includes lipidase and peroxidase. It is believed that these enzymes may be involved in the biosynthesis of ethylene. Sometimes different isozymes are associated with fruit ripening.
Increase in chlorophyllase, lipase causes breakdown of chlorophyll and free fatty acids, respectively. Similarly increased lipoxidase is also reported. Large increase in acid phosphatase activity parallels the climacteric in mangoes. In several fruits enzymes of glycolysis, oxidative processes—HMP shunt and citric acid cycle also increase.
iv. Pigment Formation:
Fruit ripening is also accompanied by dramatic changes in its colour e.g., in tomato following sequence of colour changes are observed:
Green—White—Yellow—Orange—Red
The red colour is due to lycopene. Carotenoid formation occurs when chloroplast is converted into chromoplast. However, not in all the cases the change in fruit colour is associated with the formation of carotenoids.
On the contrary in many fruits anthocyanin is synthesized during ripening as in apple. The present thinking is that synthesis of carotenoids and anthocyanin in ripening fruits is regulated by phytochrome system.
v. Effect of Potassium Nutrition on Fruit Ripening:
In tomato fruit increased potassium (K+) nutrition causes an increase in the concentration of organic acids, in particular citric and malic acids. It may be recalled that tomato is a climacteric fruit so that the pre-climacteric respiration minimum is followed by a peak during which the rate rises by 110—250%.
When the plants are supplied with high concentrations of K they have reduced rate of respiration especially during the climacteric phase. There is great accumulation of oxaloacetic acid (OAA) which is also increased by K application.
This increase is due to the oxidation of malate by malate dehydrogenase and can be inhibited by malate and succinate oxidation by tomato fruit mitochondria. The rate of endogenous concentration of OAA could be controlled by the rate of transamination with L-glutamate through the action of GOT.
Fruit ripening is also retarded by osmotic water intake and by washing out of some unidentified substances. Besides the climacteric respiration, other characteristic metabolic pathways can be seen. For instance, in ripening mango fruits aspartate and glutamate decrease, while α-aminobutyrate increases.
Together with changes in enzyme activities, the following metabolism of aspartate and glutamate must occur:
This metabolism indicates that the most significant amino acids are decomposed. This may partly explain why protein synthesis ceases during ripening.
Hormonal Regulation of Fruit Ripening:
As many as five types of plant hormones are known to regulate fruit ripening. In recent years occurrence of IAA in fruits has been demonstrated beyond doubt. While young seeds are the main site of IAA synthesis, in the mature fruit it is synthesised in the fruit flesh. In fact auxins slow down fruit ripening except in some cases where they may quicken.
Perhaps auxins prevent ethylene formation in fruits. Obviously auxins must be degraded endogenously through series of enzymes like IAA—oxidase, etc. to control fruit ripening. Moment the auxins are degraded the fruit tissue becomes sensitive to ethylene.
Very little is known about the endogenous cytokinin content and its metabolism in fruits. On the basis of their function in the leaves, they possibly contribute in keeping the protein and chlorophyll content constant.
The effect of gibberellins in a way is comparable to auxins and cytokinins. Most studies have been done on oranges where GA inhibits degradation of chlorophyll/and delay carotenoids accumulation. Thus pigment formation is delayed. Similarly banana fruits sprayed with GA do not undergo yellowing even though other processes occur normally.
In large number of fruits, before the ripening is ultimately achieved there is accumulation of ABA (Fig. 25-2). Perhaps this phytohormone regulates fruit ripening. In apples after a week of harvesting ABA content increases many times. ABA concentration is very high in the inner part of the green fruit flesh of tomatoes.
It may be mentioned that tomatoes ripen in a centrifugal direction and as the process progresses the relationship is reversed. Thus in ripened part ABA level falls down. In the following diagram (Fig. 25-3) a relationship between phytochrome, ABA and lycopene content of ripening tomatoes is given.
It will be observed that with the red light illumination of tomatoes, ABA content rises several-fold in first few days and then declines. The present thinking is that ABA triggers lycopene synthesis.
Ethylene is an important hormone concerned with ripening. Fruits fail to ripen in the absence of ethylene. It is shown that ethylene probably brings about the climacteric. Similarly, non-climacteric fruits once treated with ethylene also show increased respiration. Perhaps difference between climacteric and non-climacteric fruits may be due to ethylene production. Ripening can be induced only when auxin is degraded by IAA oxidase, etc.
In view of the reported effect of ethylene in altering the proportion of individual tRNA species, ethylene may be regulating translation of mRNA and thus initiate ripening. In tomatoes, exogenous application by ABA enhances ethylene production.
Whether ABA induces ethylene synthesis in vivo is not clear. Light is also shown to induce ethylene formation. For instance, red light induces ethylene formation while FR slows it. Obviously the phenomena of fruit ripening appear to by a set of highly complex physiological events.
It may be stated that ethylene formation in plants is not exclusively induced by light. It is also produced when a tissue is injured, or diseased or due to physical and chemical stresses. Even action of some metal ions e.g. Cu++ and Ca++ causes ethylene formation. Most studies are available in tomato.
In the following scheme a possible relationship between phytochrome and some hormones in fruit ripening has been elucidated:
The above scheme provides tentative relationships between various components though precise relationships between various components though precise relationship of ABA and ethylene is not well understood. There are reports that ethylene causes increase in ABA level and the latter hormone might initiate fruit ripening by stimulating ethylene production.
Flesh Softening:
During ripening there is breakdown of insoluble protopectin into soluble pectic compounds. The process is enzymes mediated. No detailed mechanism of softening is known. During ripening there is shortening of the polymer chain length, demethylation of carboxyl groups and deacetylation of hydroxyl groups.
All these affect cell wall consistency through change in the bonding with associated cell wall constituents e.g., cellulose, hemicellulose.
Carbohydrates:
Most climacteric fruits possess starch as a storage reserve. This is broken down into soluble sugars due to enzymes. Thus fruit attains sweetness.
Loss of Astringency:
In some fruits which are unripe, there is abundance of tannins of low molecular weight (polyphenols) which react with proteins e.g. banana or sapota. When eaten they give astringent taste. With ripening, tannins polymerise into large molecules and lose their capacity to react with protein. Instead they get trapped in the cell.
Organic Acids:
Sourness of fruits is due to organic acids. The taste is determined by the ratio of sugars and acids. With increased ripening, the total activity decreases. However, in banana, the acids increase on ripening.
Aroma and Flavour:
Ripe fruits have intense aroma and flavour. Aroma is due to the volatile chemical compounds which are enzymatically synthesised and emitted. These volatile compounds are esters and lactones, alcohols, acids, aldehydes, ketones, acetals, phenols, ethers, etc.
Controlled Ripening:
Harvesting does not indicate the end of a fruit life. Several of the fruits can be successfully stored up to several weeks by controlling mechanical injury, transpiration, respiration, decay and physiological breakdown. Several physiological and chemical agents are employed to slow down metabolic rates in fruits.
By refrigeration of fruits, storage period is enhanced. It helps in two ways: slowing down respiration due to low temperature and checking microorganisms development. Temperature also influences endogenous ethylene production.
Recently controlled atmosphere (CA) storage is used in collaboration with refrigeration. These processes maintain high quality of fruits. The technique is affectively used in storing apples, citrus, etc. The CA is affected by increasing CO2 in the atmosphere or reducing O2 levels.
Similarly, some fruits are stored under low pressure. It is a new approach in the long-term storage of fruits. In this method, ethylene evolved is removed, and the partial presence of oxygen is lowered. This slows down the ripening.
Studies at the Bhabha Atomic Research Centre, Mumbai have demonstrated the potential of low- dose gamma irradiation for retarding ripening in mango, papaya and banana. Irradiation also increases pigmentation.
Sometimes fruits are dipped in wax emulsions or plastic films. Even treatment with GA retards ripening.
Artificial Fruit Ripening:
Ethylene is currently used commercially to induce ripening in mangoes, tomatoes, banana, and even degreasing citrus fruits. Temperature affects the process of artificial ripening with ethylene. This gas merely removes chlorophyll and unmasks yellow and orange pigments.
In some fruits, there is synthesis of these pigments also. In fruits with pronounced climacteric, 0.1-100 ppm ethylene is effective when applied in the pre-climacteric stage. There are several sources of ethylene (ethrel, CPTA). Sometimes acetylene and carbon monoxide are also used for artificial ripening of bananas and mangoes.
Hot water dip treatment of mangoes enhances ripening and colour development. This also lessens microbial growth. The ripening is independent of maturity of fruit. In order to have characteristic taste, only optimal mature fruits should be artificially ripened.