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Extracellular Microbial Polysaccharides

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Extracellular Microbial Polysaccharides Reprinted from t00d Technology. © 1974 by Institute of Food Technologists. 3499 Purchased by U.S. Dept. of Agriculture for Official Use. Extracellular Microbial Polysaccharides New hydrocolloids of interest to the food industry ALLENE JEANES o THE POLYSACCHARIDE HYDROCOLLOIDS crobiology, production, characterization and applica­ (designated polysacolloids by Stoloff, 1958), used tra­ tions (both actual and proposed) of extracellular mi­ ditionally for many purposes in foods and food pro­ crobial polysaccharides (Jeanes, 1966,1968,1973) and cesses. have been obtained from plants, both land and the prognosis for their potential in the food industry aquatic. An additional newly developing source of (Glicksman, 1969). Reviewed here are some of the polysacolloids of demonstrated or apparent suitability basic research and the associated industrial develop­ for use in foods is based on the biosynthetic capabil­ ments on extracellular microbial polysacchariries as re­ ities of nonpathogenic microorganisms. Research at lated to the interests of the food industry. the Northern Regional Research Laboratory has aug­ The extracellular microbial polysaccharides that have mented awareness of the great numbers of microbial been most studied are members of several classes rep­ species, nonpathogenic to man, that produce polysac­ resentative of the neutral and polyanionic types. Re­ charides both extracellularIy and abundantly. Our re­ search on the polycationic type is just beginning search also has extended cognizance of the diversity of (Jeanes, et aI., 1971). The polysaccharides now in the polysaccharide composition and structure, and has industrial production are dextran, which is a neutral established that the properties may be not only un­ glucan, and xanthan, a polyanionic heteropolysaccha­ usual but also amenable to practical utility. ride. These biopolymers and a few others of their re­ Interest in this new source of hydrocolloids stems spective types are the subjects for further consider­ from several considerations which are: 1) Successful ation here. performance of several of these microbial products based on their distinctive properties, 2) general suit­ DeXTRAN ability for ingestion without adverse effects because the constituent natural sugar residues and their pattern of The dextran now produced in the United States, Can­ glycosidic linkages permit either digestion and metab­ ada, Sweden, and elsewhere is synthesized from su­ olism or inertness and noncaloric effect, 3) economic crose by the extracellular enzyme, dextransucrase, from feasibility resulting from the extracellular occurrence Leuconostoc mesenteroides strain NRRL B-512 (F). of these biopolymers and their production in good (Unless stated otherwise in this discussion, the term' yields from low-cost substrates by fermentation proce­ "dextran" designates the product from this specific dures, 4) the need to supplement supplies of the nat­ strain. Dextrans from other strains differ in structure ural plant gums, and 5) the various advantages of do­ and properties.) Cell-free culture filtrates containing mestic products over foreign imports. the enzyme may be used; the resultant solution con­ Several comprehensive reviews have detailed the mi- tains only the dextran product, the enzyme and resid­ ual nutrients from the culture filtrate, and the by­ product, fructose. Partial depolymerization of this "native" dextran, which has molecular weight in the approximate range of 30-90 million, and subsequent fractionation, permits production of a series of lower molecular weight products which are marketed for pharmaceutical and industrial uses (Jeanes, 1966). O-Chain This dextran is composed exclusively of a-D-gluco­ pyranosidic residues, 95% of which are linked only through carbons 1 or 1 and 6; 5% carry branches pre­ dominantly one or two glucose units long attached at carbon 3 positions (Fig. 1), Tests on metabolism of NRRL B-512 (F) dextran have been made mainly on the clinical-size fraction of M w 75,000 ± 25,000. When infused intravenously to man or animals, metabolism is complete (Terry et aI., 1953) ; when ingested, it serves as a source of liver gly- Fig. I-DEXTRAN from Leuconostoc mesenteroides NRRL B­ 512(FJ has a fleXible uniformly linked backbone structure and THE AUTHOR is a Research Chemist, Northern Regional Re­ infrequent branches predominantly one or two glucose residues search Laboratory, Agricultural Research Service, U,S. De­ long. partment of Agriculture, Peoria, Illinois 61604. 34 FOOD TECHNOLOGY-MAY 1974 Extracellular Microbial Polysaccharides... Table I-VISCOSITY VS. CONCENTRATION relations of A-Y·6272 native dextran from Leuconostoc mesenteroides NRRL B-5121F) 8·1973 Deacetyla!!!L"''':'....Sodium Alginate. 20,000 I- at 25°C, 3.84 sec-I. /8"" .... 0 High Viscosity .... x Dextran concentration (%) Viscosity (cp) • ... ••0 ...·8.1973 ... / 10.0001- ~~..... Native /8.1459 1 13 X" ... 2 80 I ..... I- ~'i 3 220 x/ 4 300 '" f~1 ,/'" 8·3225 Deacylated 5 400 .~'" l:' .......-" c:> J fli,---·.,_~A- .S­ . f =: .W .t~ / '.-.. Y·2448 c:.>'" f;l/ cogen IBloom and Wilhelmi, 1952). Comparison of /A ---- .•.---- 1.0001- u~ products of M" 75,000 and 10 X 10" in feeding tests .-= i!x '"c:> ~ with rats showed digestibility to be 90% and 86%, re­ ... JJj--A_A spectively, and other observations were essentially alike ;;;:'" I- i: (Booth et aI., 1963). There is no scientific basis for expecting significant difference in digestibility of par­ lI.',1 tially degraded and native dextrans. Breakdown in the 8·512F Dextran intestinal tract is determined by whether dextranase is present and the structure (degree of branching) of the L/~ dextran, and not by molecular size. Dextranases occur rl II in intestinal mucosa of man and other mammals IDahl­ 1000 0.5 1.0 2.0 3.0 qvist, 1961, 1963), in the colon as a product of certain Concentration. Percent IWI. Polymer /Wt. Solution) of the normal flora (Sery and Hehre, 1956) and in Fig. 2-VISCOSITY CONCENTRATION CURVES for microbial various tissues, especially spleen, liver and kidney polysaccharides and an alginate. (Wells-Brookfield Micro Vis­ (Ammon, 1963). cometer [Cone and Plate, Model RVT], 3.84 sec-I (I rpm), For practical use in foods, costs favor native dextran. 25°C.J Action by Food and Drug authorities in the United States IFDA), however, has been based on the tests with fractions of partially degraded dextran and the lack of adequately supported petitions for approval of native dextran. Dextrans of average molecular weight 10 Polymth'"d' below 100,000 were included among GRAS products (FDA, 1960) until their proposed deletion because of B-U59 12448 disuse (FDA, 1973b). In Canada, also, approved use B-3225 H'I'" has been restricted to molecular weight below 100,000. 11401 0",,1 Dextran NRRL B-512IF) dissolves readily in water B1913 Hat,Y! to give clear solutions which are stable to sterilization ~ by heat and to freezing and thawing. Viscosity­ 1.0 concentration relations are shown in Table 1 and com­ Gum So<§. ,<s pared with those of polyanionic polysaccharides of similar molecular weight in Figure 2. In order to com­ I.... 'pare the flow behavior of dextran with that of 1% solu­ '" tions of other polysacolloids under our test conditions ~ (Jeanes and Pittsley, 1973, 15% concentration was ~ 0.1 necessary IFig. 3). In terms defined for this system ..c:'" IPatton, 1969) the dextran solution shows plastic char­ on'" acteristics closely similar to those of 1% locust bean , gum. The same similarity was observed by Szczesniak , and Farkas (1962) and related to mouthfeel charac­ '....-8·1973 teristics. The influence of concentration on these fun­ VNative damental flow characteristics has been considered by 0.01 both Szczesniak and Farkas (1962) and Patton (1969). \.1401 We found that dextran solution at 5% concentration showed essentially Newtonian-type flow. A number of other well-defined characteristics of this dextran are bases for uses in the food industry as de­ tailed in specific examples by Jeanes (1967) and Glicksman (1969). The notorious role of dextrans as 100 1000 nuisances in the cane sugar industry attest to general Shear Stress. dynes/cm. 2 ~rystallization 1bility to interfere with of sugar solu­ Fig. 3-SHEAR STRESS-SHEAR RATE-VISCOSITY relations for tions. Aqueous solutions of the B-512(F) dextran dry aqueous dispersions of microbial polysaccharides and plant gum to water-soluble, cohesive films. "Crystalline" regions controls. Concentrations were I % except as indicated. IWells­ that would diminish solubility do not develop during Brookfield Micro Viscometer [Cone and Plate, Model RVT], 19ing of the native material in the presence of mois- 25°C.J MAY 1974-FOOD TECHNOLOGY 35 Extracellular Microbial Polysaccharides 0 dard; confirmation 01 effective date of order listing xanthan gum as an optional ingredient. Federal Register 36 (168) Part 25: 17333. Au­ gust 28. Food and Drug Administration. 1973a. Certain cheese products; order listing xanthan gum as an optional ingredient. Federal Register 38(49) Part I: 6883. March 14. Food and Drug Administration. 1973b. Generally recognized as safe (GRAS) substances no longer in use. Proposal for removal from GRAS list. Federal Register 38(71): 9310. April 13. Glicksman, M. 1969. "Gum Technology in the Food Industry." Aca­ demic Press, New York. Harada, T., Masada, M., Fujimori, K., and Maeda, 1. 1966. Produc­ tion of a firm, resilient gel-forming polysaccharide by a mutant of Alcaligenes faecalis var. myxogenes 10C3. Agri. BioI. Chern. 30: 196. Jeanes, A. 1966. Dextran. In "Encyclopedia
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