The magical mystery of an apple’s sleep
It might have been an apple that Snow White bit into that sent her into a death-like sleep but, this notion of suspended-animation is not the stuff of fairytales nor science-fiction stories.
Apples, one of the most popular fruit commodities on earth, and a R6.3 Billion industry in South Africa, are nowadays available throughout the year because of on-going improvements into prolonging the life of the apple by reducing the temperature and limiting the amount of oxygen it receives and scrubbing, or removing, the ripening hormone, ethylene. Too much heat, oxygen or ethylene rushes maturation and the apple eventually dies of old age. Too much cold or too little oxygen damages the living fruit and it might senesce or age and expire. Getting the balance right is what agricultural scientists continue to focus on.
Henk Griessel is Tru-Cape Fruit Marketing’s Quality Assurance Manager. His primary job is to ensure the integrity of the fruit so that the consumer has a good eating experience when biting into a Tru-Cape apple. As Tru-Cape Managing Director, Roelf Pienaar, often says: “Ours is a factory without a roof,” reminding us all that fruit production is entirely in the hands of nature but that doesn’t mean that man doesn’t play a significant role in the process.
Griessel explains: “There are typically three stages during which an apple is picked. These, determined by the rate at which starch converts into sugar, tell the grower if the fruit is ready. Fruit that is intended for Controlled-Atmosphere storage (CA) is picked at the stage which we call the pre-climacteric optimum. This stage is when the fruit is mature but before ripening has been initiated. The major difference between apples, pears and, say citrus or grapes, is that apples and pears ripen after picking whilst grapes, for instance, are picked ripe. These characteristics of apples and pears make it possible for us to store the fruit for long periods. Ripening is initiated by several factors and is mostly under the control of the ripening hormone, ethylene. To retard the whole process we reduce the temperature to just above freezing, take away most of the oxygen and scrub the ethylene. By doing this we are, basically, placing the fruit in suspended animation; it lives but we have retarded the ripening and ageing process so much that we can store it for three times the normal length. This is similar to what is depicted in science-fiction movies where they put the space crew under conditions that prevent them from ageing while en route to Mars.
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“The retarding of ripening has a long history. In the mid-1800s people saw that transporting vegetables in train wagons supplied with ice resulted in vegetables staying fresh much longer. They had kerosene lamps on the trains, before electricity, and wherever they had one of these lamps fruit and vegetables senesced much faster. The scientists of those days determined the beneficial effect of low temperatures and the negative effect of burning kerosene which gave off the ripening hormone, ethylene. So, the first stage was building cold storage or Regular Atmosphere stores (RA). These stores could keep the product cold and extended its life by about 50%. “Then came a period whereby a group of scientists discovered the effect of taking away most of the oxygen, called Controlled Atmosphere or CA. We in South Africa were the first in the world to build these gas-tight stores at Molteno Trust in Elgin in the 1934 South Africa and a global leader in this field.
Controlled Atmosphere (CA) became quite popular in the late ’70s and ’80s when most of the farms built these stores. Apples could then be successfully stored for eight to 10 months.
“The last bit of this progression came in the late 90s with the advent of determining stress by shining lights of a certain wavelength on a few fruit. The logic was that if we could reduce the oxygen even more without damaging the fruit, then we might be able to store fruit for even longer. Dynamic Controlled Atmosphere was born (DCA). We now monitor when the fruit in the room becomes stressed by recording stress to the chloroplast, the small structure where the green pigment is situated. If the chloroplast is stressed, the wavelength of the light it reflects changes and we know the fruit is starting to experience stress. We counter this by upping the oxygen concentration slightly until the stress is no longer there. This is real science fiction in action! By doing this we can now store fruit for about three-times its normal life with no detrimental effect.
“As we harvest apples between February and May, any local fruit on the shelves from June to January will have been in CA storage,” Griessel comments.
“The storage science behind delivering the best eating experience is fairly complex. Depending on which market, for example, the fruit is intended for, it will be taken out of CA storage and placed into RA storage. And, linking back to the Snow White fairytale, Griessel says that it was probably the ethylene gas released from the lamps that the seven-dwarves used that woke her rather than a kiss from a prince.” Griessel jokes.