It's automatic, isn't it? You get home, back from the supermarket or the store, and pop that pack of tomatoes in the fridge. Maybe it's not tomatoes and its strawberries instead. When you go to eat them later, supposedly fresh from the fridge, they just don't taste right. That's the fridge effect for you.
Because so much of what we buy is kept in giant fridges in the supermarkets, and because the labels say "best stored in a refridgerator", we just shove it in there, no questions asked. Try reading the label closer; best stored in a refridgerator. Well, it's kind of right. If you want strawberries or nectarines to last longer, stick them in the fridge. If you want them to taste better, don't.
Thing is, storing things in the fridge won't ripen them, and certainly won't make them taste any better. Think of the ideal situation; walking out to a nectarine tree or a strawberry plant in your back garden, choosing a perfectly ripe fruit, picking it, then eating it right there - it tastes absolutely delicious and guess what - no fridge in sight.
It is unfortunate however that if you don't store many types of fruit in a fridge, they won't last as long, but there's an easy way around this - eat it! Supermarkets have gotten us into the habit of the large once weekly shop, when in fact there isn't much fresh fruit and veg that you should be buying in quantities large enough to store and use for a week. Fruit is so much better when it's fresh, and loses taste and nutritional value the longer it is stored. Some fruit and vegetables, such as peas or sweetcorn, should be eaten so quickly after harvesting (because the sugars start converting into starches) that they'll never taste as good as they can from a supermarket. Other fruit, such as pears, can actually never be harvested when perfectly ripe because they are just too perishable, but that's another feature on its own.
Tomatoes really shouldn't be stored in the fridge - if they're fresh, they'll last for up to a week in the fruit bowl, and they shouldn't be kept any longer than that. The tart taste of tomatoes is due to a chemical called Linolenic Acid converting to Z-3-Hexenel, and this reaction is disrupted by cold. If you must store tomatoes, bring them out of the fridge for at least an hour before eating to let them warm to room temperature. Tomatoes are disadvantaged to begin with, since they are picked long before they are ripe, when they are still green all over, and then ripened in large ripening rooms using ethylene gas. Even vine ripened tomatoes are treated in this manner.
Some types of citrus fruit in particular suffer from chill damage, including mandarins and lemons and limes. When fruit is chill damaged, the internal structure of the fruit is damaged to the extent that metabolic reactions necessary for ripening can no longer take place, and in citrus fruit at least is typically shown by a dulling of the skin and a drying out of the fruit inside.
Stone fruit are a funny bunch. Some of the best tasting fruit are in this category, yet they aren't treated properly. Chilling injury can occur in soft fruit very easily, when the fruit is subjected to cold temperatures for as little as ten days, and by cold temperatures, about 50°F/9°C, which is warmed than the average home fridge. Storing stone fruit below this temperature when they are near ripe encourages breakdown of the fruit, and results in a nasty, dry, mealy tasting fruit.
Let's not go too far the other way though - some produce benefits from being stored in the refrigerator. Carrots, for example, will not last long at all outside of a fridge; they lose moisture and go wrinkly and dry, and eventually soft (take it from someone who ran a produce department only half chilled and lost a lot of carrots!), and fortunately, not much of the taste is lost in carrots during refrigeration. Potatoes are easy too and you've probably been storing those right for years - a cool room (10-15°C) away from light benefits them the most; don't put salad potatoes in the fridge! It encourages the conversion of starches to sugars, and gives them a very sweet taste; indeed, they can even be part caramelised during cooking because there is so much sugar present, and that's not good for potatoes.
So what causes the ripening process in fruit, and how can we control it to our advantage? Fruit ripening is largely due to a gas, a plant hormone called ethylene. The ripening effects of ethylene have been known for millennia; the ancient Egyptians would gas figs to stimulate their ripening, and the ancient Chinese would burn incense in closed rooms to promote ripening of pears. It wasn't until the turn of the last century, in 1901, that a Russian scientist called Dimitry Nelijubow isolated ethylene as the active ripening agent, and it took until 1934 for it to be discovered that plants produced ethylene.
Ethylene is actually responsible for multiple processes within plants, but we're just interested in ripening. Ethylene is produced from a chemical called ACC; what it stands for isn't really important to us, but what we do need to know is that the reaction increases in the presence of oxygen, hence the use of Controlled Atmosphere for storage which we'll touch on in a moment. The other important thing to remember is, as many biological reactions, this one increases in speed with temperature; therefore, keeping fruit cold, in a fridge for example, slows down the production of ethylene gas, which in turn slows down ripening. Hence why we should store things in a fridge to keep them longer.
So fruit produce ethylene gas, which causes them to ripen, right? Well, kind of - fruit ideally need an external source of ethylene to ripen, that is, they don't really ripen from what they produce themselves. Bananas are good producers of ethylene, which is why it's commonly recommended to place any fruit you want ripening in a paper bag with some bananas. For now, we won't go into the actual ripening process, because it's a bit more technical than this article is supposed to get. If anyone wants the details in a future article, get in touch with us and let us know.
All types of fruit produce different amounts of ethylene, and all types of fruit are sensitive to it and affected by it in different amounts. Some good ethylene producers are apples, bananas, melons, peaches, pears and tomatoes, whilst some sensitive items are broccoli, cauliflower, cabbage, and lettuce.
It is by virtue of our understanding of the ripening process and the part that ethylene plays in it that we can have so many of our fruit and vegetables all year round; a good example is apples. Apples are mostly harvested between the months of August and November in the Northern Hemisphere. In particular, Washington apples from North America are picked in this period. However, they are not available for a few select months; they are in fact available all year round. How can this be? Are apples really stored for months on end? Yes, they are, using Controlled Atmosphere (CA) storage. The Washington apple company has enough space to use CA storage for 121 million boxes of apples; at 12kg a box, that's 1.2 billion metric tonnes of apples, the highest of any growing region in the world.
Controlled atmosphere has been around since the mid 1980s, and basically involves controlling several variables, including the temperature, oxygen levels, carbon dioxide levels and ethylene levels within storage rooms, to control the atmosphere at such a level to inhibit ripening, which is the natural aging process of fruit. It's not an easy process; allowing the oxygen level to go too low causes apples to start to ferment and go brown, while allowing the carbon dioxide level to go too high again causing injury, typically signified by roughening of the skin of the fruit with possible skin staining, and internal browning. However, if it is gotten right, controlled atmosphere storage can significantly extend the storage time of fruit.
In the case of apples, different varieties of apples require harvesting at different stages of ripening, and different quantities of gas for their CA environment; some apples are more tolerant of carbon dioxide than others, yet some have no preference. CA storage has a sister method too, Modified Atmosphere storage, which requires slightly less work than CA storage, since it does not control the atmosphere so rigidly.
Unless we understood the process of ripening, we wouldn't have perfected our supply chain of bananas; there are stories that early shipments of bananas came ashore on ships along with shipments of citrus fruit. The bananas had produced large quantities of ethylene during the voyage, and were ripening naturally, while the citrus, a fruit sensitive to ethylene, had ripened and decayed into a squidgy mess. Nowadays, controlled atmosphere is typically used for shipment of fruit. Fortunately, it doesn't noticeably damage the quality of the fruit, but then again, how many of you have grown and eaten your own bananas?
So, what message are we trying to get across? Simply that in order to experience any fruit or vegetables you buy or grow at their best, you shouldn't store them long, and especially not in the fridge. After all, they aren't called fresh fruit for nothing...