Fruit ripening is a feature of nature, a biochemical process by which produce becomes acceptable for consumption and bestows its nutritional benefits when consumed. One of the consequences of ripening is that fruit becomes softer and sweeter to the taste which helps its appeal. One of the reasons for such sweetness is that during ripening starch is converted to sugars such as fructose. From the sensory perspective the fruit is ripe when its colour has become appealing, has full flavour and aroma.
Fruits are classified according to their ripening behaviour. They may be defined as:
Climacteric fruits are those which show a dramatic increase in the rate of respiration during the ripening process. Ripening often occurs when they are exposed to the gas ethylene which promotes ripening. Typical fruits are bananas, apples, figs, papaya, guava, pears, plums, tomatoes and mango.
These are fruits which do not ripen further once they have been harvested.
Non-climacteric fruits are those such as grapes, cherry, strawberries, watermelon, pineapple and the citrus fruits. These do not show a dramatic increase in the rate of respiration during their ripening. They are not affected by the presence of ethylene.
To obtain a full ripeness and flavour, these fruits are harvested once they are fully ripe rather than picking before they are known to be ripe.
What Changes Can Be Expected During Ripening
Various parts of the fruit and a number of molecules are affected. These include:
- cell walls are weakened
- starch degradation
- changes in organic acids
- changes in colour as pigment distribution is altered
- production of flavouring compounds
- production of vitamin C (ascorbic acid)
- changes in phenolics, amino acids and proteins
- increases in respiration
- changes in water transpiration
- alteration in ethylene evolution rate
The Process Of Ripening
An unripe fruit such as a green banana produces ethylene which is the simplest carbon compound to contain a C-C double bond. It induces or turns on various genes involved in enzyme synthesis which are involved in the degradation of various parts of the fruit so making it ready for ripening. Various biochemicals are affected during ripening – starch conversion has already been mentioned but chlorophyll, various acids, pectins and proteins are all involved. The affect on the ripening fruit is to induce the creation of more ethylene which positively induces further changes in ripening.
The Biochemical Synthesis of Ethylene
The amino acid methionine is converted to adenosylmethione, catalysed by the enzyme AdoMet Synthetaste using ATP. A second synthetase (ACC Synthase) converts this molecule to ACC (amino cyclopropane carboxylic acid). The third enzyme ACC Oxidase converts ACC to ethylene in the presence of oxygen.
The control of ethylene production is highly important in delivering fruit of suitable quality to the retailer.
Changes To Cell Walls
The cell walls in fruit are rich in polysaccharides such as pectin. These are broken down catalysed by the action of enzymes. These polysaccharides become more solubilized as they are broken down. There is a loss of neutral sugars such as galactose and arabinose. The enzymes which cause cell wall break down are:
Control Of Ripening
Methods to control ripening are very important in managing the supply chain of fruit. Any improvements in extending or reducing shelf-life are important. How many instances have we tried to eat a ready-to-eat avocado only to find it is as hard as stone.
One particular method is to regulate the temperature.
If the temperature is increased, the rate of ripening increases. So storage at lower temperatures – just above freezing helps delay this particular process. Storage at chill temperatures is important immediately as soon as the fruit is harvested. It reduces the rate of respiration and drops ethylene production.
If the fruit is stored below the optimum for storage then it suffers cold injury and produces spoilage.
A very important chemical used to block the action of ethylene is 1-MCP (1-methyl cyclopropene).
There are some excellent reviews which should be consulted regarding fruit ripening (Brady, 1987). One covers the influence of ethylene in this process (Lelièvre et al., 1997).
Brady, C. J. (1987). Fruit ripening. Annual Review of Plant Physiology, 38(1), pp. 155-178 (Article)
Lelièvre, J. M., Latchè, A., Jones, B., Bouzayen, M., & Pech, J. C. (1997). Ethylene and fruit ripening. Physiologia Plantarum, 101(4), pp. 727-739 (Article)