Functionality of probiotics in milk and juice.

The symptoms of many gastrointestinal (GI) disorders and diseases can potentially be alleviated and prevented by consuming probiotic foods. Probiotics are currently defined as 'live microorganisms which when administered in adequate amounts confer a health benefit on the host' and they are typically represented by the bacterial genera <i>Lactobacillus</i> and <i>Bifidobacterium</i>. Good viability is a prerequisite for optimal probiotic functionality and therefore probiotic foods should contain high enough levels of the specific probiotic strain(s) throughout storage and during consumption. This review focuses on two specific food carriers for probiotics, namely milk and juice (vegetable or fruit). Probiotic strains typically grow poorly in milk owing to their low proteolytic activity, inability to utilize lactose, or because of special needs for certain growth factors missing in milk. Probiotics, especially bifidobacteria, usually survive better in non-fermented milk than in fermented milk. This is mainly due to the higher pH values of non-fermented products compared with fermented ones. Several <i>in vitro</i> studies have shown that milk or milk components can protect lactobacilli and bifidobacteria against low pH and also against bile. The protective effect of milk has also been shown <i>in vivo</i>, when the survival of differently formulated probiotics through the human GI tract was studied. Many probiotic strains are able to grow and maintain their viability in vegetable juices, but their growth and viability in fruit juices is hampered by the presence of organic acids and low pH. Depending on the strain and its susceptibility to organic acids, low pH and possible antimicrobial compounds present in fruit juices, a satisfactory stability in fruit juices can, however, still be achieved. All food applications have some kind of limitation regarding the viability and stability of probiotics. Therefore, special attention has to be paid to the selection of the suitable probiotic strain for a specific food application or to enhance the survival of a specific strain selected for the product.