For this reason, these bacterial sugar robbers, Leuconostoc mesenteroides, are considered by U.S. growers and processors to be the greatest cause of cane deterioration.

     Fortunately, ARS researchers in New Orleans, Louisiana, are finding ways to give sugar growers and processors the upper hand in the ongoing battle against Leuconostoc. Already, ARS chemist Gillian Eggleston, who works at the agency's Southern Regional Research Center, has uncovered simple technologies for alleviating the burden of these costly bacteria--and Louisiana factories are eating them up.

Dismal Dextrans

     Like most microbes, Leuconostoc bacteria don't need much coaxing when it comes to capitalizing on their favorite food source.

     "Any time sugarcane is cut, injured, or damaged," says Eggleston, "Leuconostoc are there, ready to invade." They seize on damage inflicted by temperature extremes--from the burning of cane that's done to ease harvest to the freezing weather that occasionally hampers Louisiana, the northernmost cane-growing region in the world.

     Cane is also vulnerable just after it's been cut. In the humid, dog days of late summer and early fall, just-harvested cane may sit for several hours in fields before it's loaded onto trucks and shuttled to the factory. It may even have to wait in the factory yard before it's crushed.

     "And while it's not especially common, the combination of a sudden freeze followed by an especially warm and humid thaw-out period can spell disaster for cane," says Eggleston. Just as roadways suffer cracks and potholes due to weather extremes, sugarcane is also prone to fissure-like wounds caused by widely swinging temperatures. Always the opportunists, Leuconostoc bacteria invade these broken-tissue areas to access dead tissues and sugars.

     As they feed, the bacteria turn cane's simple sugars into clunky compounds that are chemically much different from sucrose. While most of this activity is occurring on a minute scale, growers do have one red flag signaling a bacterial invasion: Patches of crimson-stained plant tissue, often found along the cane plants' vulnerable bamboo-like joints, indicate that the cane is deteriorating.

     One bacterial byproduct is dextran--a viscous polysaccharide that represents huge headaches for processors. Because of its bulky, unwieldy structure, dextran makes it harder for factories to process cane. It's also a bitter pill to swallow, economically.

     For factories, the more dextran there is in cane, the less sucrose there is for turning into sugar. There are also penalties to contend with--mostly from the refiners who clarify raw sugar until it takes the shape of fine, white crystals.

Leuconostoc species are epiphytic bacteria that are wide spread in the natural environment and play an important role in several industrial and food fermentations. Leuconostoc mesenteroides is a facultative anaerobe requiring complex growth factors and amino acids (Reiter and Oram 1982; Garvie 1986).

Most strains in liquid culture appear as cocci, occurring singly or in pairs and short chains, however, morphology can vary with growth conditions; cells grown in glucose or on solid media may have an elongated or rod shaped morphology. Cells are Gram positive, asporogenous and non-motile.

A variety of lactic acid bacteria (LAB), including Leuconostoc species are commonly found on crop plants (Mundt et al 1967; Mundt 1970). L. mesenteroides is perhaps the most predominant LAB species found on fruits and vegetables and is responsible for initiating the sauerkraut and other vegetable fermentations (Pederson and Albury 1969). L. mesenteroides starter cultures also used in some dairy and bread dough fermentations (Server-Busson et al. 1999).

Under microaerophilic conditions, a heterolactic fermentation is carried out. Glucose and other hexose sugars are converted to equimolar amount of D-lactate, ethanol and CO2 via a combination of the hexose monophosphate and pentose phosphate pathways (Demoss et al 1951; Garvie 1986; Gottschalk 1986). Other metabolic pathways include conversion of citrate to diacetyl and acetoin (Cogan et al 1981) and production of dextrans and levan from sucrose (Alsop 1983; Broker 1977).

Viscous polysaccharides produced by L. mesenteroides are widely recognized as causing product losses and processing problems in the production of sucrose from sugar cane and sugar beets (Tallgren et al. 1999). The first observation of the production of polysaccharide "slime" from sugar, dates to the earliest days of the science of microbiology; Pasteur (1861) attributed this activity to small cocci, presumably Leuconostoc species. Commercial production dextrans and levans by L. mesenteroides, for use in the biochemical and pharmaceutical industry, has been carried out for more than 50 years (Alsop 1983; Sutherland 1996).

Dextrans are used in the manufacture of blood plasma extenders, heparin substitutes for anticoagulant therapy, cosmetics, and other products (Leathers et al 1995; Sutherland 1996; Alsop 1983; Kim and Day 1994). Another use of dextrans is the manufacture of Sephadex gels or beads, which are widely used for industrial and laboratory protein separations (Sutherland 1996). Currently, L. mesenteroides has significant roles in both industrial and food fermentations.

Leuconostoc mesenteroides is a bacterium associated with the sauerkraut and pickle fermentations. This organism initiates the desirable lactic acid fermentation in these products. It differs from other lactic acid species in that it can tolerate fairly high concentrations of salt and sugar (up to 50%sugar). L. mesenteroides initiates growth in vegetables more rapidly over a range of temperatures and salt concentrations than any other lactic acid bacteria. It produces carbon dioxide and acids which rapidly lower the pH and inhibit the development of undesirable micro-organisms. The carbon dioxide produced replaces the oxygen, making the environment anaerobic and suitable for the growth of subsequent species of lactobacillus. Removal of oxygen also helps to preserve the colour of vegetables and stabilises any ascorbic acid that is present.

Lactic acid fermentation is used throughout the world to produce speciality foods:

• Western world: yogurt, sourdough breads, sauerkraut, cucumber pickles and olives
• Middle East: pickled vegetables
• Korea: kimchi (fermented mixture of Chinese cabbage, radishes, red pepper, garlic and ginger)
• Russia: kefir
• Egypt: laban rayab and laban zeer (fermented milks), kishk (fermented cereal and milk mixture)
• Nigeria: gari (fermented cassava)
• South Africa : magou (fermented maize porridge)
• Thailand : nham (fermented fresh pork)
• Philippines : balao balao (fermented rice and shrimp mixture)