Good and Bad Bacteria

• Ordinary milk is a nutritional powerhouse, but it doesn't have much staying power. At room temperatures, it can spoil or grow startling quantities of bacteria in a matter of just a few hours. However, some of those bacteria have the paradoxical effect of keeping the milk safe to eat. They consume the milk's sugars and transform them into acids, thickening the milk and making it inhospitable to most other bacteria. Certain strains of Lactobacillus and Staphylococcus, which thrive at relatively high temperatures, are responsible for giving yogurt its distinctive tang and texture.

Plain and Greek Yogurt

• Yogurt thickens because, as the milk grows more acidic, its proteins begin to congeal, just as they would if they were cooked. Yogurt made from whole, natural milk is relatively soft, but in Greece and other Balkan countries the milk is sometimes concentrated first by boiling it down. This gives it a higher concentration of proteins, and the acidity sets this type of yogurt to a thicker, richer texture. Modern commercial yogurt achieves a similar result by adding dried milk powder, which also raises the yogurt's protein levels. Lower-cost brands sometimes thicken the yogurt further with gelatin or other stabilizers, though this technique is less authentic.

In a Salad

• Either type of yogurt is a natural complement to cucumbers, either when shredded for tzatziki or when sliced or diced for a salad. Most recipes are relatively simple, augmenting the natural flavors of the vegetable and yogurt with a handful of fresh herbs or small quantities of freshly ground pepper, cumin, coriander and similar spices. Fresh dill, fennel fronds and mint are especially compatible with yogurt and cucumbers, and any of the three can be added to your salad alone or in combination.

Preparing the Cucumbers

• If you're serving the salad right away, you can simply toss the ingredients together and set your bowl on the table. There's little time for the cucumbers to release their juices, and Greek yogurt is thick enough to absorb the extra juices. If you're preparing the salad ahead of time, it's helpful to take a few extra steps to compensate for the cucumbers' tendency to ooze moisture. Slice or dice your cucumbers and place them in a colander, then sprinkle them lightly with salt. Cover them with a plate and set a can of food on top, to act as a weight. Let the juices drain for 30 to 60 minutes, then rinse and dry the cucumbers and prepare your salad.