One of the first exercises assigned in art classes is to paint something in the ‘Still Life’ genre—very often a bowl of fruit. But that title brings a wry smile to the faces of those involved in the study or application of the science of postharvest physiology. To them, the fruit in the bowl is hardly ‘still’. Rather, “fruits and vegetables are living tissues and continue to respire and transpire after detachment from the plant.”1
In other words, although already harvested, there are myriad physiological and biochemical processes going on inside the fruit, some of which actually increase fruit palatability and quality, others lead to a decline in fruit quality. Postharvest physiology aims to understand and manage these in such a way as to maximize a fruit’s quality and storage life.
Fruit are complex entities however, and each type of fruit responds differently to its surrounding environment—no one postharvest handling recipe fits all.
Bananas, mangoes, apples, papaya, tomatoes, apricots and European pears are all classified as climacteric fruit (their respiration rate increases sharply at maturity), which in practical terms means they can be picked unripe because the ripening process can proceed after harvest. This is provided they have been picked ‘mature’, i.e. have attained maximum size and weight. Inside these fruits, postharvest ripening processes include the initial generation of some ethylene (C2H4), a hydrocarbon gas, which in turn triggers more ethylene production and a series of complex ethylene-related biochemical processes generating desirable flavour, aroma, sweetness, texture, and thus improved eating quality. While postharvest physiologists have now described many of these complex processes, there are still gaps in our knowledge of the sequence of events. Externally, fruits change from green to either yellow/orange (bananas, pears, mangoes) and/or to orange (apricots, papaya), or to red (tomatoes). Both flavour and aroma components (generally volatile esters and alcohols) continue to accumulate right up to full ripeness.
In contrast, grapes, oranges, cherries, strawberries, pineapples, lychees, and blueberries are all classified as nonclimacteric fruit (they maintain a consistent, low respiration rate at full maturity), and should be allowed to ripen on the plant, before being harvested. But even in these fruits which will not ripen further once picked there are some aesthetic and palatability-improving processes that continue after harvest, e.g. pineapples will soften, and the green skin of pineapples and oranges will turn yellow in the presence of ethylene.2
Once the fruit has attained peak ripeness, deteriorative processes soon predominate. So far, postharvest physiologists have successfully developed a number of techniques to retard or inactivate these processes, including controlling temperature (cold storage), light, relative humidity, air pressure (hypobaric storage), and levels of ethylene, oxygen, and carbon dioxide in the surrounding air (controlled/modified atmosphere systems). However, avenues for further improvement are clearly not exhausted yet, with some recent surprising, even counter-intuitive, discoveries. For example, while it has long been known that many fruits will last much longer in cold storage than at warmer temperatures, periodic removal from cold storage—a procedure known as ‘intermittent warming’—actually improves storage life of at least some fruits, not lessens it. The mechanisms at work inside the fruit that make this procedure effective are still a mystery.3
Suffice to say, there’s a lot happening in a bowl of fruit! And just as no-one would deny the glass or china bowl (not to mention the artist’s ‘still life’ painting) must have had a capable and intelligent maker,4,5 how much more capable and intelligent was the Maker of fruits, with their researcher-challenging complex internal physiology and biochemistry! And in fruits with viable seeds,5 there exists the potential to generate whole new plants ‘after their kind’, just as the Maker originally programmed them to do (Genesis 1:11–12), producing many more fruits multiple times—whereas no glass or china bowl or man-made machine can ever multiply itself. No wonder the Bible says that those who deny the Maker—their Maker—are “without excuse” (Romans 1:20).
References and notes
- Sinha, N., Sidhu, J., Barta, J., Wu, J., and Cano, M. (Eds), Handbook of fruits and fruit processing, 2nd Edition, Wiley-Blackwell, Ames, Iowa, USA, 2012, p. 88. Return to text.
- It should be noted that categorization of some fruits (e.g. muskmelon, guava, certain plum cultivars) as climacteric or nonclimacteric can be problematic, especially as our knowledge has increased in recent years. As a modern textbook points out, “current evidences indicate that the classical classification between climacteric and nonclimacteric fruit is probably an oversimplification”, adding to the complexity of this whole subject. Ref.1, p.13. Return to text.
- One suggested factor is that warming allows toxic (to the fruit) chemicals that have accumulated in the fruit during chilled storage to be metabolised, but research has not confirmed this. Rees, D., Farrell, G., and Orchard, J. (Eds), Crop postharvest: Science and technology—Perishables, Blackwell Publishing Ltd, Oxford, UK, 2012, p.14. Return to text.
- Of course a machine could have made the bowl, but an intelligent engineer had to design the machine; it did not make itself. Return to text.
- With the whole creation being “in bondage to decay” since Adam sinned (Genesis 3:14–19, Romans 8:19–22), some mutations have damaged or (even) destroyed the genetic information that codes for seeds. We see seedlessness or unviable shrivelled seeds in many commercial horticultural crops, e.g. dessert bananas, some citrus fruit, and certain table grapes. These varieties survive because man, seeing seedlessness as a desirable trait, propagates these varieties vegetatively, i.e. without seeds, as when one plants potato pieces to get more potatoes. The cost of fruit production can be higher for seedless varieties, however—e.g. chemicals normally released by the developing seeds that promote fruit enlargement have to be sprayed manually onto certain seedless grapes in order to achieve acceptable fruit size and yield. See also, Batten, D., A vase of flowers—by special arrangement, Creation 31(4):56, 2009; creation.com/flower-vase. Return to text.