Fruit Ripening

Ripening Agents and Processes

As fruit ripens, its acidity level increases, though this does not directly translate into a more sour taste. In fact, the perception that fruit becomes sweeter as it ripens is often misunderstood, despite the rise in acidity. This phenomenon is explained by the Brix-Acid Ratio, which measures the balance between sugars and acids within the fruit.


Role of ripening agents

Ripening agents are used to accelerate the ripening process, allowing many fruits to be harvested before reaching full ripeness. This is beneficial as fully ripened fruits are fragile and do not transport well. For example, bananas are typically picked green and artificially ripened after transportation by exposing them to ethylene gas. In some regions of Asia, a traditional method for ripening mangoes involves placing slightly green-harvested mangoes with containers of calcium carbide under plastic sheeting. Moisture in the air reacts with the calcium carbide, releasing acetylene gas, which mimics ethylene’s effect on the fruit. Ethylene, although vital for ripening within the plant, cannot naturally trigger ripening in nearby fruit.


Calcium carbide and its risks

In some countries, calcium carbide is still used to artificially ripen fruit. However, industrial-grade calcium carbide may contain harmful contaminants such as arsenic and phosphorus, making its use illegal in many places. When calcium carbide reacts with water, it produces acetylene gas, which induces ripening. Although acetylene has been suggested to negatively affect the nervous system by reducing oxygen supply to the brain, studies show it is not reactive enough to pose a significant health risk in practice.


Commercial ripening products

Specially designed covered bowls are available commercially, claiming to enhance the ripening process by trapping ethylene and carbon dioxide around the fruit. These gases accelerate ripening.


Climacteric vs non-climacteric fruits

Climacteric fruits, such as bananas and tomatoes, continue to ripen after being picked, a process accelerated by ethylene gas. Non-climacteric fruits, such as grapes and strawberries, can only ripen while still attached to the plant, resulting in shorter shelf lives.


Ripening-delaying technology

SmartFresh is a commercial technology that helps maintain the fresh quality of fruits and vegetables by slowing down the ripening process. It uses 1-Methylcyclopropene (1-MCP), which inhibits ethylene production, thus preventing over-ripening and delaying spoilage.


Ripening indicators

Iodine can be used to determine if fruit is ripening or rotting by detecting starch content. A drop of iodine on the flesh of the fruit, such as an apple, will turn dark blue or black if starch is present, indicating that the fruit is not fully ripe. If it remains yellow, most of the starch has been converted to sugar, signalling ripeness.


Ethylene

Ethylene functions as a key hormone in plants, regulating processes such as fruit ripening, flower opening, and leaf shedding. This hormone is active in trace amounts throughout the plant’s life cycle.


Plants perceive ethylene through a transmembrane protein receptor. Scientists have successfully cloned the gene responsible for this receptor in Arabidopsis thaliana and tomatoes. These receptors are encoded by multiple genes, five of which have been identified in Arabidopsis and at least six in tomatoes, all capable of binding ethylene. Ethylene receptor DNA sequences have also been discovered in many other plant species, and even in cyanobacteria, a simple type of organism.


Various environmental factors can stimulate ethylene production in plants. These triggers include flooding, drought, cold temperatures, wounding, and pathogen attacks. For example, during flooding, plant roots may suffer from oxygen deprivation (anoxia), leading to the production of 1-Aminocyclopropane-1-carboxylic acid (ACC). ACC is transported to the leaves, where it is oxidised, producing ethylene, which causes leaves to bend downwards (epinasty). Some speculate that this leaf movement, particularly in windy conditions, may help the plant pump excess water out of the roots, potentially enhancing water transport beyond normal transpiration.


Commercial use of ethylene

Ethylene significantly reduces the shelf life of many fruits by accelerating ripening and hastening floral senescence. For instance, tomatoes, bananas, and apples ripen more quickly in the presence of ethylene. Bananas placed near other fruits produce enough ethylene to cause rapid ripening in neighbouring produce. Ethylene also shortens the lifespan of cut flowers and potted plants by speeding up floral senescence and causing flowers to drop prematurely. Flowers and plants that experience stress during shipping, handling, or storage produce ethylene, leading to a marked reduction in floral display. Ethylene-sensitive flowers include carnations, geraniums, petunias, roses, and many others.


This accelerated ripening and senescence can result in significant economic losses for florists, markets, suppliers, and growers. In response, researchers have developed several methods to inhibit ethylene's effects, such as blocking ethylene synthesis and ethylene perception. Ethylene inhibitors include Aminoethoxyvinylglycine (AVG), Aminooxyacetic acid (AOA), and silver ions. However, inhibiting ethylene synthesis is less effective in reducing post-harvest losses because ethylene from external sources can still have an impact. In contrast, by inhibiting ethylene perception, fruits, plants, and flowers do not respond to ethylene produced either internally or from outside sources. Ethylene perception inhibitors work by mimicking the shape of ethylene without triggering the ethylene response. One example of this type of inhibitor is 1-methylcyclopropene (1-MCP).


Despite the drawbacks, ethylene is used commercially by growers of bromeliads, including pineapple plants, to induce flowering. This can be done by treating the plants with ethylene gas in a chamber or simply placing a banana peel next to the plant in an enclosed space to stimulate flowering.

Fruit Crop Types

Simple Fruit Epigynous berries are simple fleshy fruit. From top right: cranberries, loganberries, blueberries red huckleberries

Simple fruits can be either dry or fleshy, and result from the ripening of a simple or compound ovary in a flower with only one pistil. Dry fruits may be either dehiscent, or indehiscent (not opening to discharge seeds). Types of dry, simple fruits, with examples of each, are:

Fruits in which part or all of the pericarp is fleshy at maturity are simple fleshy fruits. Types of fleshy, simple fruits are:

An aggregate fruit or etaerio, develops from a single flower with numerous simple pistils.

The pome fruits of the family Rosaceae, (including apples, pears, rosehips, and Saskatoon berry) are a syncarpous fleshy fruit, a simple fruit, developing from a half-inferior ovary.

Schizocarp fruits form from a syncarpous ovary and do not really dehisce, but split into segments with one or more seeds; they include a number of different forms from a wide range of families, carrot being an answer.


Aggregate fruits

Aggregate fruits develop from a single flower that contains multiple carpels, each forming a separate pistil with one carpel. Each pistil forms a small fruit, known as a fruitlet, and collectively these fruitlets form a structure called an etaerio. There are four types of aggregate fruits, based on the nature of the fruitlets: etaerios of achenes, follicles, drupelets, and berries. For example, members of the Ranunculaceae family, such as Clematis and Ranunculus, have etaerios of achenes, while Calotropis produces an etaerio of follicles. In Rubus species like raspberries, the fruitlets are drupelets, and in Annona, the etaerio is made up of berries.


Raspberries are a good example, where the pistils are termed drupelets because each resembles a small drupe attached to the receptacle. In some bramble fruits, such as blackberries, the receptacle is elongated and forms part of the ripe fruit, making them aggregate-accessory fruits. Strawberries are also aggregate-accessory fruits, but in this case, the seeds are contained within achenes on the surface of the fruit. In all these examples, the fruit develops from a single flower with multiple pistils.


Multiple fruits

A multiple fruit forms from a cluster of flowers. Each flower in the cluster produces its own fruit, but these individual fruits mature into a single mass. Examples of multiple fruits include the pineapple, fig, mulberry, osage-orange, and breadfruit.


In the case of the noni, or Indian mulberry (Morinda citrifolia), different stages of flowering and fruit development can be seen on a single branch. The process begins with an inflorescence of white flowers, called a head. After fertilisation, each flower develops into a drupe, and as these drupes grow, they merge to form a multiple fleshy fruit known as a syncarp.


Seedless druits

Seedlessness is a valued trait in many commercially important fruits. Seedless varieties of bananas and pineapples are popular examples. Other seedless fruits include certain cultivars of citrus, such as satsumas, mandarin oranges, as well as table grapes, grapefruits, and watermelons.


In some species, seedlessness arises through parthenocarpy, where fruit develops without fertilisation. Parthenocarpic fruit set may or may not require pollination, though most seedless citrus fruits do require the stimulus of pollination to produce fruit.


Seedless bananas and grapes are typically triploid, and their seedlessness results from the abortion of the embryo after fertilisation, a process known as stenospermocarpy, which still requires normal pollination and fertilisation to occur.