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s-linking, has been demonstrated to provide stability against acid, thermal and mechanical degradation of starch and to delay retrogradation during storage. Dual modified starches are used widely in salad dressings, canned foods, frozen foods and puddings (Jane, 1997; Wurzburg, 1986a, 1986b). The control of reaction conditions is important during preparation of dual-modified cross-linked/hydroxypropylated starches using different cross-linking reagents with different starch bases such as maize, tapioca, wheat, waxy maize, waxy barley, rice and sago (Tessler, 1975; Wattanchant et al., 2003; Wu & Seib, 1990; Yeh & Yeh, 1993; Yook et al., 1993). The quantity of cross-linking reagent required to prepare a dual modified starch (with desirable properties) vary with the source of starch, the type of cross-linking reagent, the efficiency of the crosslinking reaction, the degree of substitution required and the specified range of final modified-starch properties (Wattanchant et al., 2003). The effects of different reaction conditions such as starch base concentration, temperature, pH and the concentration of the catalyst salt plays an important role during preparation of duly modified hydroxypropylated crosslinked starch (Lim & Seib, 1993; Smolka & Alexander, 1985; Tessler, 1975; Wu & Seib, 1990; Yeh & Yeh, 1993). The effects of chemical modifications on thermal, morphological and pasting/rheological behaviour of starches may be quantified using instrumentation such as differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Viscoamylograph/ Rapid Visco Analyser (RVA) and dynamic rheometer, respectively (Singh, Kaur et al. 2007; Kaur, Singh, & Singh, 2004; Kaur et al., 2005a; Kim, Hermansson, & Eriksson, 1992; Liu et al., 1999a, 1999b; Shi & BeMiller, 2000; Singh & Singh, 2003; Singh, Kaur, & Singh, 2004; Singh, Chawla, & Singh, 2004; Yeh & Yeh, 1993).
2.2.1.6 Cassava
Cassava or tapioca (Manihot esculenta Crantz) is the third most important crop in Thailand after rice and sugar cane. Cassava was introduced to South Thailand from Malaysia during the period 1786 to 1840, after this time it was gradually distributed throughout the country. This crop is now mainly cultivated in the Northeast, primarily in Nakhon Ratchasima (57% of total root production), followed by the Central plains (31% of total root production) (Sriroth, Piyachomkwan, Wanlapatit & Oates, 2000).
It is considered a foodstuff of high nutritional value and one of the most economical sources of energy; since the carbohydrate yield is 40% greater than in rice and 20% greater than in corn (Tonukari, 2004). For this reason, processing of cassava root in order to obtain cassava starch has increased in the last decades (Perdomo, Cova, Sandoval, García, Laredo & Müller, 2009).
Tapioca is a pancake-like delicacy made from gelatinisation of wet cassava starch by roasting it over a hot plate. It is dipped in a solution of condensed or coconut milk in plain milk, and immediately consumed. Tapioca is largely consumed and appreciated in Brazil’s northeast. Cassava flour fortification with iron and bioproteins was considered as a promising technology by Metri et al. (2003) and Tuma et al. (2003). Thus, tapioca-based products could be a good vehicle for nutritional improvement (de Brito, dos Santos Garruti & Silva, 2007). Cassava starch, as all other starches, mainly contains two polymers that contribute to its molecular structure: amylose in a percentage varying from 13 to 24% (Hoover, 2001; Rickard, Asaoka, & Blanshard, 1991), essentially a linear molecule, and a non-linear and highly branched molecule, amylopectin, both consisting of glucose repeating units. Both molecules form semicrystalline superstructures (with crystalline and amorphous layers arranged in an onion like structure), where most of the crystalline regions are formed by amylopectin although parts of the amylose molecules are also present in them (Perdomo, Cova, Sandoval, García, Laredo & Müller, 2009).
One of the advantages cassava has over other starchy crops is the variety of uses to which the roots can be put. Apart from being a staple food for humans (especially in Africa), it additionally has an excellent potential as livestock feed, and in textile, plywood, paper, brewing, chemical and pharmaceutical industries. Thus, a lot of research has been done on cassava. Originally, the research focussed on improved yields, cultivation practices and crop protection. Cassava research has, since 1985, also focussed on processing, quality control and new product development (Dufour et al., 2002). A major constraint is that cassava deteriorates rapidly. Cassava has a shelf life that is generally accepted to be of the order of 24–48 h after harvest (Wenham, 1995). Hence, fresh cassava roots must be processed into a more shelf-stable form within 2 to 3 days from harvest. One such cassava product is tapioca grit. Tapioca grit is a partly gelatinised dried cassava starch, which appears as flakes or irregularly shaped granules. It is consumed in many parts of West Africa, and widely accepted as a convenient diet (Adebowale, Sanni et al. 2007; Hollesman & Ates, 1956).
2.2.1.6.1 Manufacturing Process
– Root preparation
Despite the introduction of mechanization, roots are mainly manually harvested – a legacy of the small farm sizes. Roots are transported to factories by small trucks or trolleys. At the factory gate 5 kg-samples are weighed in water using a Rieman balance, a method adopted from the potato industry for estimating root starch content. A Rieman balance, in fact, measures root apparent density from root weight in air and in water. The relationship between apparent density and starch content in cassava root is highly correlated, this has been verified using roots produced under different growth conditions; Colombia and Thailand. Note there is still disparity between the different methods for measuring starch content. For trading purpose, root price is based on root starch content. Root purchasing using this system is well accepted, as the farmers are rewarded for high root starch content. Starch content above 25% is rewarded; each 1% increase leads to a 5% higher payment. Unfortunately, for the farmer, for each 1% drop in starch content a 5% price reduction is imposed. In general, root starch content varies in the range of 20 to 30%, as estimated by the balance. On arrival at the factory, roots are deposited onto a cement floor for short-term storage. Generally, factories will only purchase sufficient roots to fulfill a daily crushing capacity of 600 to 800 t.
Roots are presented for processing by transferring to a root hopper (5m3). The roots are moved onto chain conveyors and transported with a velocity of 20 to 40 m/min.
Each conveyor set has a carrying capacity of about 15 to 20 t of roots per hour. Soil and sand are removed as the roots (15 to 20 t/h) pass through a cylindrical root sieve, dimensions (W × L × H) of 1.20 × 1.50 × 40 m, rotating at 10 to 15 rpm.
Soil, sand, pieces of broken peels and impurities pass through the sieve. Roots after sieving are transported into a water chamber where they are washed and moved by a paddle blade rotating at 10 rpm. Water used for washing is re-circulated from later processing stages. Capacity of washing is 15 to 20 t of roots per hour. Consequently, for a standard sized factory with a starch capacity of 200 t per day (requiring 700 to 800 t of roots), two sets of root hopper, root sieve and washers must be installed (Sriroth, Piyachomkwan et al. 2000).
– Rasping
Washed roots are chopped with a cutting blade running at 250 rpm, driven by a 10HP electric motor. The small root pieces are gravimetrically fed to the raspers. Popular are simple local made saw-tooth raspers, which consist of a drum (diameter 77.5 cm) with 144 blades on its surface; 210 teeth are distributed along the 30 cm length of the blade. Attempts have been made to optimize the drum speed, but usually 1,000 rpm is adopted by most factories in Thailand. The domestically produced rasper operates at a capacity of 5 to 6 t of chopped roots per hour. About 6 to 8 raspers are installed to support each chopper. Neither oxidation nor discoloration takes place during rasping. Liquid recycle from the process is fed along with chopped root into the rasper. As factories prepare to move up the quality ladder, some are planning to install higher speed and capacity raspers such as HOVEX (HOVEX Engineering, Veendam, the Netherlands). Some are trying with the local modified higher speed raspers (Sriroth, Piyachomkwan et al. 2000).
– Extraction
Early in the modernization phase, cassava starch processors commonly incorporated a single stage decanter for fruit water separation; this was adopted from the potato starch processing industry. However, many processors (about 50%) believe that the levels of protein and other impurities are too low to warrant the use of a decanter. The fruit water is used for root washing or is directly discharged as wastewater. The material balance and flow diagrams of a starch factory working with and without a decanter are presented in Figs 2.17 and 2.18 Fresh rasped root slurry from the rasper (if no decanter) is pumped through a series of extractors, from coarse to fine. The extractors are continuous centrifugal perforated baskets, dimensions (W × L × H) of 1.14 × 1.36 × 2.10 m, operating at 800 rpm. The capacity of this stage is 0.75 to 1.50 t dry solid per hour. Pulp from the coarse extractor is repeatedly re-extracted using the

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