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This week, I’d like to start by thanking readers for their feedback. It lights up my Sundays, every week, and gives me food for thought.
A recent question from Sandeep Khanna, for instance, inspired this column. Why do different foods spoil differently, he asked? Well, let’s take a look.
First, what does “spoiled” mean? The answer to this varies wildly from person to person, as anyone who has been watched in disgust as they eat a soft banana knows.
Some loss of freshness is inevitable, of course, as produce makes its way from plants and fields to us. In debates over whether a food item is “too bad” or “okay enough” to eat, the concern most people are expressing is over microbial contamination.
There are many ways that microbes get at food.
Most thrive in warm temperatures, which is why refrigeration keeps foods fresh for days, and freezing keep most foods fresh for weeks or months. Many microbes require water to be present; this is why it is crucial to keep provisions such as grains and legumes dry.
Some microbes need oxygen, some are killed by oxygen, and others can survive with or without it, based on their circumstances. This is one reason for the varied kinds of food packaging in use today.
Red meat, for instance, is typically wrapped in modified-atmosphere packaging (MAP), where an inert gas such as carbon-dioxide is sealed in with the food, to keep aerobic microbes from multiplying. Packaged raw vegetables are similarly flushed with nitrogen, for the same reason.
All the microbes we dread in food come from three families: yeast, mould and bacteria.
Yeasts prefer a liquid environment and colonise foods that are high in sugar. These microbes are present in the air, in soil, on the skin of fruits. They react quickly with sugar and can rapidly cause fresh fruit juices to spoil.
One can detect yeast contamination by a shift in odour, colour and flavour, but mainly from the accumulation of carbon-dioxide, which causes bubbles to form and packages to swell.
Yeast thrives at room temperature. It is eliminated at 60 degrees Celsius. This is one reason packaged juices are pasteurised, then packed in sterile, airtight containers.
Mould is also formed by airborne spores. This filamentous fungus is a vital agent of decomposition in the wild, helping recycle the remains of dead plants and animals into nutrient sources for plants. Unfortunately, it has a sort of anti-Midas touch, and decomposes anything it grows on — living things, healthy vegetables.
It flourishes on damp, solid substrates that are not overly acidic or alkaline. It can thus grow on breads, cheeses, citrus fruits. Some mould species are capable of growing in the refrigerator (as we all know). Once the airborne spores find a friendly landing strip, they take three to 10 days to become a visible mass.
Moulds grow in different colours, including orange, red and pink, in addition to the common white and green. As they grow, they burrow deep into the food, so that where we may only see a coloured spot, if we cut into the item, a root system will emerge. It is best to take no risks and simply throw mouldy foods away.
Finally, bacteria. When vegetables in the fridge start to get soft and slimy, that is the work of bacteria that was already present on the skin of the produce at the time of refrigeration. As they grow, they slowly degrade the pectin that gives such produce its firm texture.
Incidentally, some of the worst food contaminators come from a very rare bacterial species, Clostridium botulinum, that can survive higher processing temperatures than most others. Even a trace of contamination by this strain can make humans very sick and even cause death.
Canned-food companies are hyper-vigilant about this. For well over a century, they have followed a series of steps called HACCPs (Hazard Analysis Critical Control Points), to prevent this contamination and ensure the safety of such foods. Home canners are more likely to encounter this problem, but wherever a canned or hermetically sealed cooked food may come from, if it looks swollen or is filled with gas or bubbles, discard it without hesitation.
That’s it for my broad overview of how food spoils. I hope it serves as a useful guide.
As for the argument about the two-day old sambar in the fridge being “too old”, I’m afraid I cannot help you there. If it looks fresh and tastes fresh, we’re just going to have to eat it.
(To reach Swetha Sivakumar with questions or feedback, email upgrademyfood@gmail.com)
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