Ripening is a process in fruits that causes them to become edible. The fruit becomes sweeter, less green and softer. During the process of fruit ripening, changes in texture, colour, flavour and aroma occur in addition to alteration in levels of vitamins and antioxidants taking place. Setting up suitable parameters to ripen any fruit is critical. The major elements that are vital to any ripening programme are maturity, age of fruit, storage temperature, time of ripening process, airflow, ethylene supply and control of CO2 and O2. In general, the fruits are being ripened by various traditional or artificial methods using calcium carbide, smoking, ethrel dipping, diluted ethylene glycol spray and ethylene generators. Among these methods, usage of ethylene by ethylene generator for fruit ripening is safe and falls within the category of food substances that, when used for the purposes indicated, in accordance with good manufacturing practice.
Introduction
India’s diverse climate ensures availability of all varieties of fruits and vegetables. India is the second-largest producer of fruits after China, with a production of 44.04 million tonne of fruits from an area of 3.72 million hectare. The major fruits grown in India are mango, banana, citrus, guava, grape, pineapple and apple. Apart from these, fruits like papaya, sapota, annona, phalsa, jackfruit, ber, pomegranate in tropical and sub-tropical group and peach, pear, almond, walnut, apricot and strawberry in the temperate group are also grown in a sizeable area. Plants provide minerals and vitamins which humans cannot produce including non-essential micronutrients that have been linked to the promotion of good health (Ali.Z.M, Lasan.H., 2004; Payasi et al., 2005 and Rahman et al., 2008).
Ripening is a term which is used to encompass many processes which can occur simultaneously or can be associated or independent of each other (Alley E. Watada, 1986). In this process, fruits become more palatable by making them sweeter, less green, and soft, physiologically and commercially. They are the most significant phase in their life. Fruits are being ripened by various traditional or artificial methods of fruit ripening, it has been observed, mostly to meet consumers’ demand and other economic factors. However, in the recent years, artificial fruit ripening has been considered a matter of concern and the effect of artificial ripening has become questionable because of various health related issues (Fattah, and Ali,2010; Siddiqui et al., 2010; and Mehnaz et al., 2013). The need for artificial fruit ripening is often encountered when fruit-sellers offer fruits to customers before due season. In order to avoid economic loss, fruit sellers sometimes prefer collecting fruits before they are fully ripe, and artificially ripen fruits before selling to the consumers. (Vila, 2003).
Though the demands of the consumers are met to a great extent with the help of these ripening agents, it is important to investigate any possible health hazards that are associated with them. Most of the ripening agents are toxic and their consumption can cause serious health problems, such as heart disease, skin disease, lung failure and kidney failure (Hakim, 2012; Chace, 1934). Scientists have also reported that regular consumption of artificially-ripened fruits may cause dizziness, weakness, skin ulcer and heart related diseases (Nagel, 1989). In addition, these ripening agents may contain different chemicals as impurities which are also toxic for human health. To address the increasing health related concerns, different countries have issued and implemented different acts and laws to control or to prohibit the production, sale and distribution of artificial fruit ripening (PFAAR, 1955). The purpose of the study is to address the safe protocol for quick ripening, health related issues associated with artificial ripening, and report current research findings of the food value assessment of artificially ripened fruits.
Chemical Agents Used for Artificial Ripening
The process of fruit ripening is chiefly regulated by a gaseous plant hormone called ethylene (Thompson.A.K et al., 1987). It is produced naturally within the fruits which initiates the process of ripening. There are multifarious uses of many ripening agents to release ethylene in order to speed up the ripening process. Chemicals like ethanol, methanol, ethylene glycol, Ethephon, calcium carbide are used to ripen fruits and vegetables artificially (Goonatilake, 2008). There ripening process is given below:
Calcium Carbide
Calcium carbide is the chemical which is colourless when pure, but black to greyish-white in colour otherwise with slight garlic like odour. It is used for the production of acetylene gas. But now a days this process is widely used by Indian farmers or fruit vendors for ripening fruits like mango, banana, papaya, plums, chiku, apples, avocados, melons, peaches, and pears.
The moisture in the air reacts with the calcium carbide to release the gas acetylene, an analogue of ethylene, which acts as an artificial ripening agent.
CaC2 + 2 H2O ? C2H2 + Ca (OH) 2
It is said to have the same effect as ethylene. However, acetylene is not nearly as effective for ripening as is ethylene, and acetylene is not a natural plant hormone like ethylene. It is found that acetylene liberated from calcium carbide has been found to initiate ripening and improve colour development of Dashehari mangoes (Lustre.A.O et al., 1976). The fruits ripened with this method are overly soft and less tasty. All the calcium carbide used in India is of industrial grade and acetylene, when generated from calcium carbide can contain toxic impurities such as traces of arsenic and phosphorous, which can be quite harmful for the health and can lead to various ailments (Saraswathy et al., 2007; Rahman, 2008 and Koning, 1994 ).
Smoking
One of the traditional methods followed for centuries for ripening in many parts of the country is smoking. It appears to accelerate the process of ripening of green and raw banana due to presence of acetylene and ethylene in smoke. In practice, fruits are placed inside a large copper vessel and few incense sticks are lighted inside and then vessel is covered with lid for 24 hours. The fruits ripened by this method have less consumer preference due to cuts, bruises, microbial infection and poor appearance (Sarananda et al., 1990).
Diluted Ethylene Glycol
Ethylene glycol (C6H6 O2) is a colourless, odourless and sweet tasting liquid, which is used in the present day as coolant, antifreeze and so on. Ethylene glycol’s chemical structure contains the gas ethylene. Ethylene reacts with hydrogen peroxide to produce the agent ethylene glycol. A quite a serious problem in usage of ethylene glycol when it is swallowed, there is evidence to support that it is poisonous and can cause kidney failure. Hence ethylene glycol can be diluted with water and skimpily applied to the peel of the fruit only and not injected in liberal amounts to the inside of the fruit. The treated mangoes using 20% ethylene glycol and 80% water were manipulated ripened on the seventh day (Ruchitha Goonatilake, 2008).
Ethrel/Ethephon
Ethrel is the commercial name for Ethephon (2- choloroethanephosphonic acid). It is used for enhancing postharvest ripening. Ethephon is often considered better than calcium carbide because pineapple, banana and tomato treated with 1,000 ppm of ethephon required less time for ripening (48, 32 and 50 h, respectively) than other treated fruits as well as compared with the nontreated fruits. The fruits ripened with ethephone have more acceptable colour than naturally ripened fruits (Rahman et al., 2008) and have longer shelf life than fruits ripened with CaC2 (Medlicott et al., 1987). Ethephon is decomposed into ethylene, bi-phosphate ion and chloride ion in aqueous solution (Singal et al., 2011)]. The released ethylene further fastens up the ripening process. It has major disadvantage that it is applied to the fruits by spraying or dipping in its solution (Pendharkar et al., 2011). It only increases a step in handling and can cause danger of microbial spoilage (IbtissamAbdalla Mohamed-Nour et al., 2010; Madamba, 1977). There is another method of using ethrel. It is really strongly acidic when in solution with water its pH is made to rise above 5 the ethephon molecule spontaneously hydrolyses, liberating ethylene (Saraswathyet al., 2007).
Ethylene generators
Ethylene is Generally Recognised as Safe (GRAS) by the United States Food and Drug Administration (FDA) and falls within the category of food substances that, when used for the purposes indicated, in accordance with good manufacturing practice. Ethylene can be produced commercially but it is not safe to use ethylene filled cylinders due to its explosive nature at high concentration (27,000ppm). Thus safer method to produce ethylene should be used.
Catalytic generators are used to produce ethylene gas, simply and safely. In ethylene generators liquid produces ethylene when heated in the presence of a catalyst. These generators are now widely used for supplying ethylene in ripening room. The liquid, a proprietary product, comprises ethanol and agents that assist in catalysing its dehydration (Saraswathy et al. 2008). Ethylene sensors can be used to precisely control the amount of gas. It is also possible to precisely control RH and CO2, and even gas the fruit without the ripener being onsite.
C2H5OH C2H4 + H2O
Suggested Practices for Ripening Process of Selected Fruits
Banana
Ripening rooms are very important for proper and efficient banana ripening. The room must be air tight, and properly insulated, with adequate refrigeration capacity, heating equipment and air circulation system. Bananas are very sensitive to temperatures. The chilling effect will occur, if fruit is subjected to below 13oC. It causes the peel to have a smoky, dull gray appearance. Cooked bananas result from excessively high temperatures; avoid temperatures above 18°C. The peel will have a brown to orange appearance. The fruit may be soft and have a short shelf life. For best ripening results, humidity should be 85 to 95%. For safe ripening of banana raise pulp temperatures to at least 14°C. Apply 100 - 150 ppm ethylene for a minimum of 24 hours during the initial phase of the ripening cycle. Fruit that is less mature may take an additional 24 hours of ethylene application (Kader Adel and Beth Mitcham, 2015).
Mainly colour changes in banana ripening are based on peel colour rather than pulp colour (Fig 1). Hence colour of banana peel has been used in assessment of banana ripening stages. Commercial standard colour charts are available in which 7 stages of peel colour were reproduced and translated to a numerical scale where Stage 1=all green, 2= green with trace of yellow, 3= more green than yellow, 4= more yellow than green, 5= yellow with trace of green, 6= full yellow, 7= full yellow with brown spots.
Fig. 1 Stages in Banana Ripening
According to colour chart, in terms of peel colour slight difference occurs among the advanced stages of maturity i.e. stage 5, 6 and 7 as compared to initial stages (Tapre et al., 2012).
There are other factors that cause poor ripening, like inadequate humidity, immature fruit, ethylene levels and release of carbon dioxide. Carbon dioxide concentrations above 1% will retard ripening, delay the effects of ethylene and cause quality problems. Therefore, it is recommended to vent rooms by opening the doors for 20 minutes every 12 hours, after the first 24 hours of ripening. In commercial scale, the venting methods are either by automatic fan or "flow-though" (constant) ventilation.
Citrus
At harvest, the peel colour of citrus fruits is sometimes greener than desired. Although eating quality is not improved, all types of citrus, including lemons, grapefruit, oranges, and tangerines, can be degreened by exposure to 1-10 ppm ethylene for 1-5 days. This process stimulates removal of chlorophyll and exposure of yellow or orange pigments. The temperature in which degreening should occur is typically in the range of 20 to 30°C. In general, the exact temperature, ethylene level and exposure time that should be used for each type of citrus and cultivar varies according to maturity, variety and properties (http://postharvest.ucdavis.edu/).
Mango
Mango is the most important fruit covering about 35 per cent of area and accounting for 22 per cent total production of fruits in the country. Early season mangoes may take longer to ripen. Store and ship mature green mangoes at temperatures of about 13°C. Before ripening begins, the fruit pulp temperatures should be raised to 20 to 22°C. Once the fruit pulp temperature stabilises, apply 100 ppm ethylene for a minimum of 24 hours. Actual time of exposure to ethylene is determined by the maturity of the fruit; a slight change in fruit colour indicates that the mangoes are producing ethylene and the external ethylene source is no longer needed. Humidity is very important and it should be maintained at 90-95%.
Carbon dioxide will build up during ripening. If no automatic ventilation system is in place, then be sure to vent the room approximately every 12 hours by opening the doors for 20 minutes even while applying ethylene. The actual CO2 level must be kept below 1% for proper ripening.
After ethylene application, keep pulp temperature at 18 to 22°C until desired level of ripeness is attained (typically 5 – 9 days). Then store ripened mangoes at 10 to 13°C in a high humidity environment for up to one week (Kader and Mitcham, 2015).
Papaya
The degree of maturity will indicate whether papayas should be exposed to ethylene. Papayas which are fully mature at harvest should not be ripened with ethylene if they are to be stored for an extended period of time. Papayas of minimum commercial maturity will benefit from a treatment of ethylene with an improvement in texture and colour. Depending upon desired shipping time, bring the pulp temperature to the range of 20 to 25°C. Apply 100 ppm ethylene for 24-48 hours (actual time of exposure to ethylene is determined by the maturity of the fruit; a yellowing of fruit colour indicates that the papayas are producing ethylene and the generator is no longer needed). Maintain humidity at 90-95% to prevent shrinkage during ripening. Vent the rooms by automatic ventilation or by opening the doors for 20 minutes every 12 hours to flush out carbon dioxide and bring in oxygen. Once the desired level of ripeness is attained, reduced the temperature to 10°C for partially-ripe papayas or to 7°C for ripe.
Plum
Plums, if harvested at a mature stage, will ripen properly without exogenous ethylene application. Ethylene application to fruit harvested at a lower, minimum maturity will help to ripen the fruit more uniformly without speeding up the rate of ripening. However, for the slow ripening plum cultivars, exogenous application of ethylene (100 ppm for 1-3 days at 20°C) is needed for even ripening. Actual time of exposure to ethylene is determined by the maturity of the fruit; a decrease in fruit firmness indicates that the plums are producing ethylene and the external application of ethylene is no longer needed (Crisosto et al., 2014).
Tomato
For ripening of tomato maintain 100-150 ppm of ethylene until a “breaker” or Stage #2+ is reached, usually 24-48 hours will take depending on temperature and maturity. The temperature and relative humidity ranges for ripening were 18° to 21°C and 85 - 95%. Sufficient air circulation is needed to provide even pulp temperatures (Fig. 2) throughout the ripening room and continuously vent Carbon Dioxide from the ripening room (lib.store.yahoo.net/).
Fig. 2 Flow through Ventilation for Ripening Room
Mature green tomatoes will ripen by this method. The external source of ethylene in the ripening room that triggers the fruit to release its own ethylene. However, immature tomatoes will erratically respond to external ethylene and possibly result in poor quality or delayed ripening. On an average, fruit ripened at 17.8° to21.1°C to a breaker stage can then be stored for more than two weeks at 12.5°C until a full red (stage #6) colour is reached. The fan size required for aircirculation is calculated as
Fan size CFM = Volume of the room, cubic feet/Time, min
The air exhaust should be to the outside of the building. The general optimum ripening conditions for fruit is given in Table 1
Table 1 Optimal ripening conditions for fruit ripening
|
Temperature |
18 to 25oC |
|
Relative humidity |
90 to 95% |
|
Ethylene concentration |
10 to 100 ppm |
|
Duration of treatment |
24 to 74 hours depending on fruit type and stage of maturity |
|
Air circulation |
Sufficient to ensure distribution of ethylene within ripening room |
|
Ventilation |
Require adequate air exchange in order to prevent accumulation of O2 which reduces effectiveness of C2H4. |
Conclusion
Once fruits are ripened by safe ethylene treatment they require quick marketing and careful handling to minimise bruising. If delays cannot be avoided, the ripe fruits should be cooled to their minimum safe temperature and kept at that temperature until ready for retail display. Removal of ethylene and decreasing oxygen concentration to the 3-5% range can be useful supplements to maintaining the optimum temperature and relative humidity for delaying further ripening and deterioration of partially ripe fruits. Sometimes fruits are treated with CaC2 to get uniform yellow colour. Consumption of CaC2 treated fruits causes harm to consumers. Hence awareness on ripening process, storage practices and safety is important for successful marketing.
