day, she never got to finish what was perhaps the 18th century version of one of today's most popular dairy snacks -- curds and whey. We know it as yoghurt.

Although yoghurt has been around for thousands of years, its current popularity has nothing to do with antiquity but a public perception that it is a "health food.'' The focus for the health claim is the bacteria which is added during yoghurt production. The bacteria, Lactobacillus bulgaricus and Streptococcus thermophilus, ferment, or culture, ordinary milk into a tart, creamy gel. This gel contains solids, or curds, and liquid, or whey.

This fermenting process was used as long ago as 2000 BC by Middle Eastern civilisations to preserve milk, and since then yoghurt has been a staple in European and Middle Eastern diets. In fact, the ancient Assyrian word for yoghurt was lebeny which meant life.

Even today, the population of some countries, like Bulgaria, consume great quantities of yoghurt in the belief that it increases longevity.

Claims that yoghurt is a healing agent are largely industry led. While there is no conclusive scientific proof to such claims, it is true that yoghurt has nutritional benefits. It is a good source of calcium, vitamins and other minerals.

For those with an intolerance for lactose, yoghurt is easier to digest than regular milk because enzymes from the yoghurt break down some of the lactose during the fermenting process. There is also some preliminary evidence that, once eaten, the bacteria continue to help break down lactose in the intestinal tract.

Even though yoghurt is acknowledged as a nutritious snack, consumers should be aware that not all yoghurts are the same. Indeed, a quick browse along a supermarket dairy case reveals a dizzying array of yoghurts. There are more than 30 different flavours and a number of textures on the market.

How, then, do you decide which one is right for you? Quite simply, you must read the labels carefully, particularly if you're on a low-fat diet.

Yoghurt may contain significant levels of fat or have very little fat. It may be swimming with live bacteria or it may have none at all. It may be plain or sweetened with natural or artificial flavours.

Some 85 per cent of American yoghurts are sweetened with fruit or flavoured with other ingredients like coffee, vanilla or lemon. Others are sweetened with artificial sweetener.

The fat in yoghurt made from whole milk is what makes it creamy, while the low-fat and non-fat yoghurts often contain ingredients such as milk solids or gelatin to produce a creamy consistency. Eight ounces of yoghurt provides between 110 and 300 calories, depending on the milk-fat content and flavourings.

To make their products last longer on store shelves, manufacturers may heat-treat their yoghurts after fermentation. While this doesn't affect the calcium content, it does kill the live cultures -- the very ingredient many people expect in a yoghurt.

If the manufacturer heat-treats the yoghurt after fermentation, the label must say "heat treated after culturing.'' If the yoghurt contains living bacteria, the label may say "active yoghurt cultures,'' "living yoghurt cultures,'' or "contains active cultures.'' Don't be confused by a label that says "made with active cultures.'' All yoghurts are made this way, but only the brands that are not heat treated after fermentation retain their living cultures.

While the US Food and Drug Administration has established certain description regulations, others are just commonly agreed between manufacturers. This adds up to confusion for the consumer, but here is a brief guideline to "yoghurt lingo'' to help you understand the labels: Sundae or fruit-on-the-bottom: In this version, fruit is found on the bottom, so that if you turned it upside down it would look like a sundae. Whey -- the liquid part of the yoghurt -- may float on top of the solid curds. Just stir them together for a smooth, creamy snack.

Swiss or custard: This is when fruit and yoghurt are pre-mixed together in the container. A stabiliser, such as gelatin, is added for firmness or body. In some custard-style yoghurt products, the fruit is placed on top of the yoghurt to make a marbled effect as it settles to the bottom.

Liquid yoghurt: Here, fruit and yoghurt are blended into a drinkable liquid.

Low-fat and non-fat: The FDA recognises three standardised yoghurt products.

Yoghurt has at least 3.25% milk. Low-fat yoghurt is made from low-fat milk or part skim milk and has between 2 and .5% milk fat. Non-fat yoghurt is made from skim milk and contains less than .5% milk fat.

Heat-treated after culturing: This means the yoghurt has been heated after culturing to increase shelf life. The process also kills living yoghurt cultures.

Contains active yoghurt cultures: Yoghurt labelled with this phrase has not been heat-treated after culturing and contains living yoghurt cultures.

Made with active cultures: All yoghurts are made with active cultures. Only those which are not heat-treated, however, retain these on the grocery shelf.

An increasingly popular variation on the yoghurt theme is frozen yoghurt. Like its namesake, it must have two types of bacteria, L. bulgaricus and S.

thermophilus, added during processing. Other than that, frozen yoghurts may bear little resemblance to their unfrozen counterparts.

For one thing, frozen yoghurts are often very sweet. Today's frozen yoghurts taste more like desserts because they're not fermented as long as non-frozen yoghurt. Therefore, they're less acidic and taste sweeter.

Frozen yoghurts may have lesser amounts of living bacteria, depending upon the way manufacturers make them. Some ferment the milk in the traditional way and then freeze it, while others make a kind of ice-milk mixture and add the bacteria later. As a result, the number of live culture organisms in frozen yoghurt varies greatly between brands.

Words like "low-fat'' and "non-fat'' may not be consistent between brands of frozen yoghurt because no standards of identity have been established. Even without standards, however, frozen yoghurt products can provide a creamy dessert, often low in fat, that mimics the flavour and texture of ice cream.

This article is based on information provided by the US Food & Drug Administration.