Tropical Tuber Crops

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5. Value added products from starch

   Starch can be converted to a number of useful products having application in food and industry. The production of liquid glucose and dextrins are well documented. Non-conventional products need to be developed to ensure a high value addition to the crops. Some of these have been attempted and the results are presented.

   1. Starch adhesives

      The simple starch adhesive was made by dissolving starch in dilute alkali with continuous stirring and chemicals like urea and borax were added to improve tack. Finally a small quantity of formalin was added as preservative. Use of Carboxymethyl cellulose and /or sodium silicate enhanced the viscosity. Use of glycerol did not improve the properties. A dry adhesive was made by the following technique. Starch containing 35% moisture was mixed thoroughly with 0.1 part of phosphoric acid and 1 part of urea. The moist mixture was dried under vacuum to 7% moisture level and mixed with 0.5% tri-calcium phosphate. The slurry was heated to 125°C with vigorous stirring and maintained at this temperature for 1 hr. The yellow product when mixed with 4 parts water and boiled, gave a transparent paste, which did not settle or become thick. Anti-fungal and antibacterial agents and stabilisers were added. This paste can be conveniently used as remoistening gum. Starch paste was also made by dissolving the starch in calcium chloride solution. The stability could be improved by adding some glycerol.

      Adhesives were prepared using cassava starch with polyvinyl alcohol (PVA). PVA was dissolved in cold or hot water, pregelatinised starch was added to give a ratio of starch:PVA 1:1 and stirred thoroughly for 2 hours until a uniform paste was obtained. To this material a plasticiser was added to maintain the uniformity of the paste. An antimicrobial agent (either copper sulphate crystals or 40% formalin) was introduced to ensure good storage life for the sample. The samples were tested for their efficiency in the following systems : paper and paper; paper and cardboard; plywood and hardwood; wood and wood ; ceramic and wood and ceramic and ceramic. The procedure adadpted for testing the samples was based on the one used at Forest Research Institute, Dehradun The paste was applied to the surfaces of the two materials and they were placed together and a weight of 2 or 3 Kg was kept on the sample for 3-6 hours. Afterwards the weight was removed and the pieces were tried to be separated by pulling using hand. A good adhesive is indicated by the surfaces sticking together not peeling away. The paste had excellent adhesive property in all the systems except in plywood and wood;ceramic. It was found to be most superior for ceramic:ceramic system and thus holds promise in the building sector. The drawback in the paper; wood and plywood; paper system was the hygroscopicity affected the stability and slowly there was decrease in the adhesiveness. For the ceramic:ceramic system, there was no letup in the stickiness even after prolonged storage even in humid conditions. This should serve as a good substitute for the synthetic pastes similar to fevicol. etc. In order to improve the resistance to moisture, hot water soluble PVA was used instead of coldwater soluble PVA and poly vinyl acetate was used in place of polyvinyl alcohol. Dioctyl phthalate was found to give good appearance and also good stability to the paste. It was also found that use of urea or borax in small quantities was helpful in increasing the pastiness of the samples.

   2. Carboxymethyl starch

      The conditions for the production of carboxymethyl starch were tsandardised at the laboratory level. The basic process consists of treating starch with alcoholic alkali to obtain a granular suspension to which the monochloroacetic acid or sodium salt of the acid is added and stirred for 4 hours. The product is precipitated in excess ethanol or methanol , filtered and dried to obtaina slight cream coloured product having high solubility and viscosity. The viscosity obtained was three times higher than that of starch at the same concentration. This product has high demand in textile and adhesive industries. The process was further modified to reduce the quantity of alcohol required for precipitation of the derivative.

      Three entrepreneurs are interested in the technology and it is expected that the know how will be transferred in the near future. Cold water miscible starch was prepared by solubilising the starch in dilute alkali and precipitaion in alcohol. The resulting granular product is transparent, miscible in water and possesses good viscosity. The product has potential as instant starch in textile applications.

   3. Starch phosphate

      Starch phosphate was prepared by reaction of starch with phosphates in presence of urea and borax. The required quantity of the chemicals was dissolved in minimum quantity of water and the starch was thoroughly mixed. After drying overnight, the powder was subjected to heat treatment for 4 hours at 145°C and then washed with water-alcohol mixture to remove dextrins. A final washing with excess methanol yields a product with high viscosity, stability and solubility.

   4. Fructose syrup

      In view of the increasing demand for sugar and harmful effects associated with the use of synthetic sweeteners, high fructose syrup in medicine and food. Acid and enzyme hydrolysis followed by treatment with the enzyme glucose isomerase and vacuum concentration gave a syrupy liquid . On passing through ion exchange column, a clear syrup having good sweetness was obtained. It was found to have a fructose content of 40%. Since fructose is three times sweeter than glucose, it can act as a sugar substitute and has excellent potential in the future.