http://www.finecooking.com/article/the-science-of-pickles
The fermented pickling process begins when vegetables are submerged in a salt brine-basically a mixture of salt and water, but it can include flavorings like spices, herbs, garlic, or chiles. Practically any fresh vegetable can be pickled (other than fragile leafy greens like spinach and lettuces), but the most common candidates are crisp and moist, such as cucumbers, peppers, and okra. The brine should cover the vegetables at all times to limit their exposure to oxygen, thereby inhibiting the growth of fungus and mold. To keep the vegetables submerged, they’re weighted down or packed tightly into jars so that they can’t float to the surface.
The brine’s salinity level depends on what you’re pickling and the results you’re going for. Crisp leafy vegetables, such as cabbage, are fermented into sauerkraut or kimchi at 1% to 2% salinity, which allows different bacteria and a variety of flavors to develop as acidity increases. The most typical brine strength for home pickling is between 5% and 6% salinity, which is slightly less than the 7% to 8% full-strength brine used to preserve lemons and olives. Half-sours, a popular Jewish-deli-style cucumber pickle, are brined at around 3.5%.
Within a day or two of starting a batch of pickles, you’ll see bubbles of carbon dioxide gas in the liquid surrounding the vegetables, indicating that the lactobacillus is thriving and the brine has started to acidify. To allow the gas to escape, pickling containers are usually left uncovered or covered with a porous fabric, like a kitchen towel, to keep out debris. The amount of time you allow pickles to actively ferment depends on how sour you want them. Relishes and half-sour cucumbers usually ferment at cool room temperatures for about a week, but you can start tasting them to check on their progress after 4 days. Full-sour cucumbers ferment for about 2 weeks, and sauerkraut or kimchi for a month. When the pickles have reached their desired degree of acidity, they should be refrigerated to slow the fermentation. They’ll continue to sour, but they won’t spoil.
During home pickling, what can go wrong?
The brine gets cloudy. Mild cloudiness (caused by small floating bits of spices or vegetable) is nothing to worry about. You can keep your brine clearer, though, by avoiding table salt, which contains anticaking agents that don’t dissolve and can make brine appear cloudy. Use pickling salt, kosher salt, or unrefined sea salt.
Also, the minerals in hard water can cause cloudiness. If you have very hard water, boil it for 5 minutes, let it cool undisturbed for 8 hours, then slowly pour off the top half of the water, leaving the cloudy layer of sediment at the bottom of the pan.
The pickles become discolored. Avoid aluminum, brass, iron, copper, and zinc cookware, containers, and utensils, which can react with the acids developed during fermentation, causing off colors ranging from blue to pink. Instead, use nonreactive plastic, ceramic, enamel, glass, or stainless steel. Discoloration may also be caused by spoilage microbes, mold, and yeast (more on this below). Garlic that appears green or blue is safe to eat, but pink pickles may indicate yeast growth and should be discarded. When in doubt, throw it out.
Mold, yeast, and rotten smells develop. To remove undesirable microbes, rinse vegetables and containers in warm water. During fermentation, shield your pickles from microbes in the air by keeping them submerged in the brine. To ensure that the pickles stay submerged, you can weigh them down with a small glass jar filled with water, or use a small weighty object wrapped in plastic. These steps help prevent aerobic bacteria, yeasts, and molds from consuming the lactic acid in your pickles, which can reduce the brine’s overall acidity, resulting in off aromas as unwanted bacteria break down proteins and fats in the pickles. You can also keep mold and yeast at bay by fermenting pickles at a cool room temperature of about 65°F, since warmer temperatures may increase the likelihood of spoilage.
The finished pickles are limp. Start with very fresh vegetables; less-than-fresh vegetables won’t get any crisper after pickling. If you’re pickling cucumbers, avoid seedless English ones, which are rich in an enzyme that makes pickles soften during fermentation. Be sure to trim the blossom ends from all cucumbers because that’s where the softening enzymes reside. You can also add grape leaves, cherry leaves, or oak leaves to the brine, as these leaves contain astringent tannins that inhibit the softening enzymes.
If you’re cooking the vegetables before pickling, don’t overcook them. Cooking will help soften dense vegetables like beets, but as heat ruptures the cell walls in vegetables, liquids are released, causing vegetables to lose their turgidity. When pickling juicy vegetables like cucumbers, raw is the way to go for the crispest results. Using unrefined sea salt or pickling salt in your brine also encourages crispness. These salts contain calcium and magnesium, which reinforce naturally occurring pectins in the vegetables’ cell walls, keeping the pickles nice and firm.