Though at birth the human digestive tract is nearly sterile, that of a child or adult contains a complex mixture of many different bacterial species known as the intestinal microflora. Some of the most famous thoughts in this area were developed by Elie Metchnikoff in the early 1900's. Metchnikoff proposed consumption of fermented milk would improve intestinal health, leading to a longer, healthier life. Since that time, researchers have demonstrated that changing the intestinal microflora can affect well-being and susceptibility to certain acute and chronic diseases.

Q: So, what are probiotics and what do they have to do with Metchnikoff's theory?

A: The term probiotic, introduced in the mid 1950's, is derived from Greek and means "for life." Although a number of definitions exist for this term, probiotics are generally defined as live microorganisms that improve our health or well-being by modifying the intestinal microflora. In most, but not all definitions, the probiotic must survive passage through the stomach, take up residence in the gut and have a positive effect on health. Perhaps the most significant criteria when selecting probiotic bacteria is a demonstrable influence on human health.

Probiotics have been studied for their ability to stimulate the immune system, to prevent diarrhea, to reduce gut colonization by pathogenic bacteria, to exhibit anticarcinogenic activity, and to reduce serum cholesterol, lactose intolerance and allergic reactions to foods. However, not all probiotics have the same potential health benefits. Probiotic effects vary from strain to strain within the same species, and not all strains have been studied to evaluate their potential health benefits.

Although a wide range of microorganisms have been reported to have probiotic activity, the most important to the dairy industry are those referred to as lactic acid bacteria. This group includes a number of species of Lactobacillus and Bifidobacterium.

From a practical perspective, dairy product manufacturers are most concerned with technological criteria in selecting probiotic strains. The ideal probiotic strain is easy to cultivate on an industrial scale; demonstrates good viability during manufacturing and over the shelflife of the food product; does not change the sensory properties of the food product; exhibits bacteriophage resistance; and exerts positive health benefits when consumed.

The generally accepted dose required to achieve a therapeutic affect is approximately one billion live cells per serving. Research has shown probiotic supplements and dairy products containing probiotics do not always contain the number of bacteria stated on the label. As the popularity of these products increases, it will be important to deliver what the consumer expects in order to maintain confidence in the category.

Q: OK, so then, what are prebiotics?

A: Prebiotics are substances that are not metabolized in the human stomach or small intestine. They provide an ecological advantage to a specific group of probiotic bacteria growing in the colon.

Many prebiotics are complex carbohydrates that serve as a specific energy source for probiotic bacteria. Some prebiotic polysaccharides occur naturally in foods, while others are isolated and added to food products as ingredients.

The rationale for consuming prebiotics is that probiotic bacteria already resident in the gut are "given a boost" to assure a healthy microflora and maintain positive health benefits. Since prebiotics are more stable than probiotic microorganisms in a food or beverage, this option is attractive to many food manufacturers. The search for novel prebiotic compounds that promote growth of specific desirable probiotics is ongoing.

Q: When discussing probiotics and prebiotics with my supplier, I have heard the term synbiotic. What does this mean?

A: Rational pairing of a specific probiotic microorganism with a prebiotic that specifically promotes the growth of that same organism often leads to enhanced success in colonizing the gut. This approach is synbiotic.

When designing such products it is important to be sure that the prebiotic-probiotic pair be suitable. That is, the prebiotic must provide an ecological advantage to the specific strain(s) being used in the product. For example, a yogurt manufactured with inulin and a starter culture containing Lactobacillus acidophilus would be considered synbiotic, since it is known that the prebiotic inulin provides an ecological advantage to L. acidophilus. Use of the synbiotic concept allows manufacturers to design products with very specific functional claims.