Food preservation

As a kid, I found the kitchen a rather mysterious place, filled with exotic implements like the bizarre “colander,” the ominous “deep-fat fryers,” and the straight-out-of-the-mad-scientist’s-laboratory “pressure cooker,” as well as bizarre ingredients like “bouillon,” “baker’s chocolate” (real chocolate’s evil twin), “paprika,” “cloves,” and something called “pectin.”

Both the pressure cooker and pectin mostly came into play this time of year, when my mother would undertake strange rituals involving fruits and vegetables which I never really understood until…well, until this week, actually, when I decided to write this column.

Two things cause fruits and vegetables to spoil: microbes (bacteria and molds) and the plant’s own enzymes, ripen fruits and vegetables in the first place but produce off-tastes, destroy vitamins, and give the produce that all-too-familiar brown, bruised color.

Canning destroys bacteria and stops the workings of plant enzymes through heat. All bacteria must be killed, because canned food is stored at room temperature: any bacteria that aren’t killed may continue their growth as the food cools and cause spoilage.

Highly acidic foods such as tomatoes and most fruits require immersion for only about 30 minutes in a bath of boiling water, because bacteria don’t grow well in acidic environments. However, most vegetables are much less acidic, so they must be treated more severely–which is where the pressure cooker comes in. Putting water under pressure raises its boiling point so you can heat it hotter: in canning, vegetables are usually cooked in a pressure cooker at 116 degrees Celsius for 30 to 90 minutes. The high heat is necessary to destroy a bacterium called Clostridium botulinum, which produces the deadly nerve toxin that causes botulism. It can grow in airless cans (it doesn’t require oxygen), and its spores have survived boiling at ordinary temperatures for as much as five hours.

Canned food frequently lacks flavor; the same high temperatures that make it safe to eat after months on the shelf also destroy some of the taste. But nobody could accuse preserves, jams and jellies of lacking in taste; not surprising, since the active ingredient in making them is sugar.

We think of sugar as promoting decay, especially in teeth, but in fact a highly concentrated sugar solution surrounding a bacterium draws water out of the cell, dehydrating it to death. Sugar also helps preserve fruit’s shape and texture by interacting with parts of the fruit’s cell walls, firming them up, even though the sugar draws water out of the fruit’s cells just as it draws water out of bacteria.

One of the chemicals the sugar reacts with in the cell walls is the one that puzzled me in Mom’s kitchen: pectin. Boiling fruit releases this pectin, which then creates the gel that is an integral part of jams and jellies: its long, string-like molecules bond together in a fine mesh that traps water.

Grapes and most berries have lots of their own pectin, but others, including apricots and peaches, need extra to make a satisfactory gel. The pectin Mom used was probably extracted either from apple cores or from the white layer between the peel and meat of citrus fruits. (In fact, simmering lightly pared lemon slices with the fruit you’re turning into jam or jelly is one way to add pectin.)

Unfortunately, pectin only gels under certain conditions. In water, pectin molecules all have the same negative charge, and so repel each other. Acids reduce this charge, but pectin molecules still have trouble bonding because then they’re more likely to bond with water than with other pectin molecules. Sugar makes gelling possible by grabbing all the water molecules for itself.

Fruits are full of acids and sugars, so thickening a preserve with pectin should be easy, right? Wrong. The precise balance among pectin, acid and sugar that’s needed is very delicate: a pH between 2.8 and 3.4, a pectin concentration of 0.5 to 1 percent, and a sugar concentration of 60 to 65 percent are optimal, but unless you’re cooking in your high school’s laboratory, you probably can’t measure any of those with any great ease, so jams and jellies are almost as likely to come out watery or too stiff as they are just the way you want them.

Which makes what Mom did in the kitchen even more mysterious, because somehow, she seemed to come up with perfect jams and jellies every time…or at least, that’s the way I remember it.

I guess when it comes to matters of the kitchen, experience as important as science.

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