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Figure 1. Structure of prebiotics. Shown (upper panel, left-to-right) are short chain fructooligosaccharides (FOS), containing two, three, or four fructose units linked 0-1,2 and with a terminal glucose. On the far right, longer chain FOS are shown, where n can equal up to twenty or more fructose units. The lower panel shows two forms of galactooligosaccharides (GOS), with galactose units linked 0-1,4 (left) or 0-1,6 (right), both linked to terminal glucose units.

Box 4—1. Probiotics and Prebiotics (Continued)

a naturally-occurring plant polysaccharide (consisting of fructose units, linked (3-1,2 with a terminal glucose residue) that can be used in its intact form or as a mixture of partially hydrolyzed fructooligosaccharide (FOS) molecules.The latter can also be synthesized from sucrose via a transfructosylating enzyme that adds one, two, or three fructose units to the sucrose backbone.

Another type of prebiotic oligosaccharide that has attracted considerable attention are the galactooligosaccharides (GOS). These oligosaccharides are built from lactose via addition of galactose residues by p-galactosidases with high galactosyltransferase activity. Galactooligosac-charides are arguably the most relevant prebiotics being used in foods, since the GOS molecules closely resemble the oligosaccharides found in human milk.These human milk oligosaccharides (which also exist in milk from other species, but usually at lower levels) are now widely thought to be responsible for the bifidogenic properties associated with human milk. In fact, it had long been suggested that there was something in human milk that promoted growth of bi-fidobacteria (the so-called "bifidus" factor) and that this factor accounted for the dominance of these bacteria in the colon of nursed infants.That infants fed mother's milk suffered fewer intestinal infections and were generally healthier than formula-fed infants provided circumstantial evidence that having a greater proportion of bifidobacteria (and perhaps lactobacilli) in the colon would be desirable, not just for infants, but for the general population as well.

References

Fuller, R. 1989. Probiotics in man and animals. J.Appl. Bacteriol. 66:365-378.

Gibson, G.R., and M.B. Roberfroid. 1995. Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J. Nutr. 125:1401-1412. Lilly, D.M., and R.H. Stillwell. 1965. Probiotics: growth-promoting factors produced by microorganisms. Science 147:747-748.

Reid, G., M.E. Sanders, H.R. Gaskins, G.R. Gibson,A. Mercenier, R. Rastall, M. Roberfroid, I. Rowland, C. Cher-but, and T.R. Klaenhammer. 2003. New scientific paradigms for probiotics and prebiotics. J. Clin. Gastroenterol. 37:105-118.

Shortt, C. 1999. The probiotic century: historical and current perspectives. Trends Food Sci. Technol. 10:411-417.

Table 4.2. Commercial probiotic organisms used in dairy products1.

Organism

Supplier or source

Lactobacillus acidophilus NCFM

Danisco, Madison,WI, USA

Lactobacillus acidophilus SBT-2062

Snow Brand Milk Products,Tokyo, Japan

Lactobacillus casei strain Shirota

Yakult,Tokyo, Japan

Lactobacillus casei F19

Arla Foods, Skanderborgvej, Denmark

Lactobacillus fermentum RC-14

Urex Biotech, London, Canada

Lactobacillus gasseri ADH

Danisco, Madison,WI, USA

Lactobacillus johnsonii KA1 (NCC 533)

Nestle, Lausanne, Switzerland

Lactobacillus plantarum 299v

Probi, Lund, Sweden

Lactobacillus reuteri SD2112 (ATCC 55730)

Biogaia, Stockholm, Sweden

Lactobacillus rhamnosus GR-1

Urex Biotech, London, Canada

Lactobacillus rhamnosus GG (ATCC 53103)

Valio Ltd., Helsinki, Finland

Lactobacillus salivarius UCC118

University College, Cork, Ireland

Bifidobacterium longum SBT-2928

Snow Brand Milk Products,Tokyo, Japan

Bifidobacterium longum BB536

Morinaga Milk Industry, Zama City, Japan

Bifidobacterium breve strain Yakult

Yakult,Tokyo, Japan

'Adapted from Reid, 2001 and Sanders, 1999.

'Adapted from Reid, 2001 and Sanders, 1999.

be labeled as "directly set" to denote this manner of acidification—and often lack the flavor of the fermented versions. They certainly lack the nutritional benefits that may be contributed by live active cultures.

The function of lactic acid bacteria in the manufacture of cultured dairy products is quite simple—they should ferment lactose to lactic acid such that the pH decreases and the isoelectric point of casein, the major milk protein, is reached. By definition, the isoelectric point of any protein is that pH at which the net electrical charge is zero and the protein is at its minimum solubility. In other words, as the pH is reduced, acidic amino acids (e.g., as-partic acid and glutamic acid), basic amino acids (e.g., lysine and arginine), and partial charges on other amino acids become proto-nated and more positive such that at some point (i.e., the isoelectric point), the total number of positive and negative charges on these amino acids (as well as on other amino acids) are in equilibrium.

For casein, which ordinarily has a negative charge, the isoelectric point is 4.6.Thus, when sufficient acid had been produced to overcome the natural buffering capacity of milk and to cause the pH to reach 4.6, casein precipitates and a coagulum is formed.Along the way, the culture may also produce other small organic molecules, including acetaldehyde, di-acetyl, acetic acid, and ethanol. Although these latter compounds are produced in relatively low concentrations, they may still make important contributions to the overall flavor profile of the finished product. The culture may also produce other compounds that contribute to the viscosity, body, and mouth feel of the product (see below). The choice of culture, therefore, is dictated by the product being produced, since different cultures generate flavor and aroma compounds specific to that product (Table 4-3).

The actual manufacture of cultured dairy products requires only milk (skim, lowfat, or whole, or cream, depending on the product) and a suitable lactic starter culture. However, the process is not quite so simple, because de-

Table 4.3. Organisms used as starter cultures in the manufacture of fermented dairy products.

Product

Organisms

Yogurt

Streptococcus thermophilus Lactobacillus delbreckii subsp. bulgaricus

Buttermilk

Lactobacillus lactis subsp. lactis Lactobacillus lactis subsp. cremoris Leuconostoc lactis Leuconostoc mesenteroides subsp. dextranicum

Sour Cream

Lactobacillus lactis subsp. lactis Lactobacillus lactis subsp. cremoris Leuconostoc lactis Leuconostoc mesenteroides subsp. dextranicum

Kefir

Lactobacillus kefiri Lactobacillus kefiranofaciens Saccharomyces kefiri

fects associated with flavor, texture, and appearance are not uncommon. As will be described later, the most frequent and serious problem in the manufacture of many of these products, especially yogurt, is syneresis.

Syneresis is defined as the separation (or squeezing out) of water from the coagulated milk. To many consumers, the appearance of these pools of slightly yellow-green water (which is actually just whey) from the top of the product is considered unnatural and objection-able.Thus, to minimize syneresis problems, and to improve the body and texture of the finished product, manufacturers perform several steps whose purpose is to enhance the water binding capacity of the milk mixture. First, the milk solids are increased, either by adding dry milk powder or by concentrating milk. Second, the milk mixture is heated well above ordinary pasteurization conditions to denature the whey proteins, exposing more amino acid residues to the aqueous environment. Finally, most manufacturers have incorporated stabilizers, thickening agents, and other ingredients into the formulation to further reduce syneresis. However, a few countries, France, in particular, prohibit many of these additional ingredients and instead rely on other means to reduce syneresis (discussed below).

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