Thursday, August 7, 2014
The Science of Canning: Part I
Case in point: home canning.
Canning season is now upon us, and every year there are one or more stories in the media about safety concerns about food you put up yourself. Not only are these threats of spoiled and dangerous food dramatically overestimated, but a simple knowledge of science will go a long way toward keeping your food safe.
Let's go back to a few principles:
Tonicity: Remember high school biology? If you are like me, you spent a lot of time trying to figure out the concepts of something being "hypertonic" and "hypotonic," probably in relation to red blood cells and the surrounding plasma. It was always confusing, but it's an important concept in home canning.
The concept is this: systems like to be in balance. A cell, which is filled with water, solutes (substances dissolved in the water), and organelles (little cell organs), will try to achieve a balance with the liquid that it lives in. If the cell has more solutes than the surrounding liquid, it will take in liquid until there is a balance. If the cell has more water than the surrounding liquid, it will eject water until there's a balance. If it takes on too much water, it will explode; if it expells too much, it will "dry up" and die.
If this has you scratching your head, stay with me: the implications for home canning are simple. In canning, we are trying to control the presence and growth of bacteria, which are just cells. Often, we do this by playing with the tonicity. For example, if I pickle my cucumbers in a brine, I've put the cucumbers and whatever bacteria are hitching a ride into a solution that has way more solutes in it than the bacteria cells do, because the brine is salty. The brine is hypertonic to the bacteria cells.
We often think that salt "draws the water out" of things. It does, in this very process. And not only does a brine make the cells of the cucumbers release water and get floppy, it also draws the water out of bacteria cells. The bacteria cells quickly reach a state in which they don't have enough interior water to live, and they die. This is why salt and brine have been such an important food preservation method throughout history; salt creates an environment that the bacteria can't easily live in because they expel too much of their own water. Sugar will do the same thing, which is why jams and jellies are so shelf-stable and why we often recommend them as a first canning project.
pH: We also control bacterial growth by changing the pH of the environment inside our canning jars. If you've heard of "high acid" and "low acid" foods, you know that acidity is an important component of keeping foods safe. Most bacteria prefer an environment that is a neutral pH, so plunging them into an acidic environment will kill them or inhibit their growth. This is why we put added lemon juice or vinegar into our home-canned tomato juice and sauce. Tomatoes used to be a very acidic fruit and thus very safe to can, but modern tastes have led to the development of hybrids that are much less acidic. Therefore, we have to boost the acidity (lower the pH) to create an environment that our little buggy friends don't like.
Temperature: Temperature is a very effective way of controlling bacterial growth, and some temperatures will kill bacteria. For example, temperatures above 165 degrees F will kill many bacteria, which is why we use meat thermometers to make sure our Thanksgiving turkeys have reached the right internal temperature.
Heat that doesn't reach 165 but is still above 140 will slow bacterial growth, which is why we keep hot food hot when we serve it. Likewise, freezing food doesn't kill bacteria, but it does slow its growth, which is why frozen food keeps a long time but not forever.
Water bath canning relies on something called "convective heat transfer," which means that it is the transfer of heat due to the movement of fluids. If you process your canning in a water bath canner, you are basically using convection to sterilize the interior of the jar, the head space, and the food. Because this process is never 100 percent effective, we only process high acid foods in water bath canners.
Pressure: Pressure is another force that kills bacteria, and it is one of the most effective around. Like our own bodies, bacterial cells can only withstand a certain amount of pressure before they burst and die. Pressure canning is therefore possibly the very safest way of preserving food, because it creates enough pressure to kill all of the bacteria in the jar and in the food, plus it employs heat to help the job. A pressure canner is basically an autoclave, that machine that your doctor or dentist uses to sterilize medical equipment, and, indeed, a pressure canner can be used to autoclave medical equipment in an extreme emergency.
Antibacterials/Antifungals/Antimicrobials: Finally, some spices have their own antibacterial properties, which is why many preserved foods, particularly those that were developed before pressure canning or modern freezers, are so highly spiced. Some traditional medical lore (used before modern antibiotics and germ theory) even suggests treating wounds and diseases with certain herbs, spices, and plants with antibacterial, antifungal, or antimicrobial actions.
OK, so what do we do with all that? Tune in for my next post, in which I'll introduce you to my theory of how to combine scientific preservation methods to ensure a safe product.
Posted by Jennifer Lorenzetti at 10:25 AM