منبع پایان نامه درباره determination

same screen aperture (35 to 40 mesh or 355 to 425 μm) as in the first pass, until minimal starch content of the pulp is achieved. Moisture content of the pulp is then reduced by a screw press and the pulp finally discharged out of the process. The moisture content of pressed pulp is about 60 to 70% and starch content about 45 to 55% (dry basis). High starch content of the pulp reflects both low efficiency of the rasping stage and composition of the cell membrane and cell walls. Composition of polysaccharides, pectic substances and non-starch polysaccharides are in turn influenced by variety and environmental factors. Fig 2.19 shows starch granules trapped in a ruptured cell, present in discharged pulp.
Figure 2. 17: Mass balance of cassava starch manufacturing process in a starch factory with a decanter (Sriroth 2000).
Figure 2. 18: Mass balance of cassava starch manufacturing process in a starch factory without a decanter (Sriroth, 2000).
Figure 2. 19: Starch granules trapped in discharged pulp of cassava starch process (Sriroth, 2000).
Starch slurry exiting the coarse extractor contains a large amount of fine fiber which must be removed in the fine extractor. The extractors are equipped with a filter cloth and screen with an aperture of 100 (150 μm) to 120 (125 μm( mesh. Fine extraction is repeated with a finer screen aperture (140 to 200 mesh). The extraction is normally completed by a series of coarse and fine extractors. Extraction is a continuous process; fresh pulp slurry is fed into the top of the first extractor together with water and SO2 water (for fine extractors). During transport to each extraction stage starch slurry is passed through a hydrocyclone to ensure complete removal of sand. Extra filtration equipment, such as rotary brush strainer, is installed to protect from the passage of a starch clump (Sriroth, Piyachomkwan et al. 2000).
– Starch separation and dewatering
Starch slurry received from fine extraction (with or without vibrating screener) has a concentration of 10 to 17 °Bé. Water is then separated from the starch slurry, increasing the concentration to 18 to 20 °Bé, using a separator. The common separators are of the two-phase nozzle type such as TX 310 or TX 612 from Alfa Laval (Sweden) or DA30 or SDA 70 from Westfalia Separator Industry GmbH (Germany). Separators from Chinese and Japanese manufacturers are also installed in some factories. To increase the concentration from 10 to 17 °Bé up to 18 to 20 oBé, either two stages of separation or re-circulation are required. Three-phase separators such as SDA 60 and SDA 130 of Westfalia Separator Industry GmbH (Germany) have been introduced into some factories. Three-phase separators help to reduce the number of processing stages and water consumption. Only two factories in Thailand are using a hydrocyclone series for starch separation (Sriroth, Piyachomkwan et al. 2000).
– Dewatering
Normally dewatering is done by a horizontal centrifuge, with baskets of 1.20 m diameter and 0.65 m length, operated at 900 rpm. The capacity of a typical centrifuge is 1.5 t dry solid per hour. The filter cloth has a specific weight of 2,440 g/m2 and total filter area of about 2.45m2. One cycle (about 10 min.) of dewatering discharges about 240 kg cake. Moisture content in the starch cake is about 35 to 40%. A typical factory with capacity of 200 t starch per day needs about 8 to 10 dewatering centrifuges. The performance of a typical dewatering centrifuge is shown in Tab. 4. In a conventional system, the dry solid content in the filtrate is high (Table 2.2). Thus, by using the filtrate as circulating liquid, the probability of acid formation due to microbial activity is increased. An alternative process of dewatering, namely high-pressure filtration has been studied. The results are promising. The high-pressure filtration (Larox Oy, Finland) is now applied in some modified starch factories.
Table 2. 2: Performance of starch slurry dewatering by a conventional centrifuge from a typical cassava starch factory.
)a) Values are the mean ± S.D. of four determinations.
(b) Values are the ratio of dry solid in the circulating filtrate to total dry solid of starch slurry having entered the dewatering centrifuge.
(c) Values are the ratio of dry solid in starch cake to total dry solid of starch slurry having entered the dewatering centrifuge (Sriroth, Piyachomkwan et al. 2000).
– Drying
A pneumatic conveying dryer, known as flash dryer, is commonly used in Thai starch factories. There are two groups of operation depending on the source of thermal energy, namely simple hot air or thermo-oil. Using the hot air as a heat exchanger is causing problems in terms of temperature fluctuation, therefore many factories are switching to use thermo-oil as a heat exchanger. The installed thermo-oil boilers are from Scherrer (Germany), Konus Keisel (Germany), Fr. K. Bay GmbH (Germany( and S.W. Multitech Starch Co., Ltd. (Thailand). Normally, about 100 t/h of hot air (170 to 200 °C) is blown into the drying tube of the flash dryer. Starch cake with a moisture content of about 38% is blown with hot air and dried to 12% moisture within 6 s. The throughput of one normal dryer is 8 to 10 t dried starch per hour. Temperature fluctuation remains a problem, often due to variation in the moisture content and feeding rate of cake entering the dryer. At this range of moisture and temperature, heatmoisture treatment can be a problem. Starch quality ultimately is affected. Loss through the two cyclones is also problematic. Venturi scrubbers to trap this loss have been installed in some factories, 37.5% of the starch loss, or 2.7% of total starch produced, can be recovered (Sriroth, Piyachomkwan et al. 2000).
– Storage
Starch discharged out of the dryer is of low moisture content, about 9 to 12%. It is also dusty. Cassava starch is packed into 30 or 50 kg double layer polyethylene bags or linen cloth jumbo size (600 kg). The 50-kg starch bags are laid on each other in the storehouse. A pile of bags up to 4 to 5m (20 to 30 bags) causes no problem for storage in short time. Effects of storage of cassava starch for three weeks under different conditions of temperature (20 and 30 °C) and relative humidity (22, 52 and 93%) has been studied. Relative humidity seems to negatively influence the flow properties of the starch. Total water uptake and crystallinity are apparently not affected. Storage of cassava starch for four weeks at 22, 50, 52, 80 and 92% relative humidity and 30 ± 2 °C has been investigated. Paste viscosity values of all samples were the same. However, prolonged storage (0 up to 7 months) under ambient condition (30 °C and 53% relative humidity) of cassava starch that contains sulfur dioxide (0, 153 and 190 mg sulfur dioxide/kg starch) led to a decrease in peak viscosity, which varied depending on sulfur dioxide content in starch. Changes in peak viscosity as a function ofstorage time for starches are different, influence of sulfur dioxide content is well described by a non-linear regression model (R2 ≥ 0.95). Starch with extremely high level of sulfur dioxide (190 mg/kg), after seven month of storage, presented a peak viscosity reduction from 417 to 360 RVU (3 g starch at 14% moisture content in 25mL water) (Sriroth, Piyachomkwan, Wanlapatit & Oates, 2000).
2.2.2 Carrageenan
The carrageenans are polysaccharides extracted from red algae of the Rhodophyceae family. Their name derives from the lichen Carageen they are initially extracted. They are obtained by heat treatment under alkaline, filtered, precipitated in alcohol before being recovered in powder form. Carrageenan is used mainly in the food industry as texture agents and for their ability to form reversible gels in aqueous media. They are also found in some cosmetics and pharmaceutical applications to stabilize emulsions or dispersions.
2.2.2.1 Chemical Structure
Carrageenan is composed of linear sulfated chains whose repeating unit is a disaccharide consisting of two galactose residues, β-D-galactose and anhydrogalactose related to β (1, 3) and α (1, 4)(Rees, 1969) (Figure 2.17). There are three main types of carrageenan used by industry according to their degree of sulfation and the position of S03- residue on the carbon cycle: the kappa (κ), iota (ι) and lambda (λ) carrageenan. These different types are present in algae in varying proportions depending on the species, location and season of harvest. The average molecular weight (Mw) ranges from 3 to 10 ×105 g/mol. The κ-and ι-carrageenan gelled with the formation of helices. The most sulfated fraction, the λ-carrageenan has no residue anhydrogalactose and do not form a gel(Rees, 1963).
Figure 2. 20: Ideal repeating units of λ-carrageenan (a) (R = H or SO3-), and (b) for ι- carrageenan (R1 = R2 = SO3-) and κ- carrageenan (R1 = H ; R2 = SO3-).
2.2.2.2 Conformation of κ-carrageenan
2.2.2.2.1 Conformation in the solid state
The properties of κ-carrageenan and ι-carrageenan were the subject of numerous studies (Morris, 1998; Piculell, 2006; Vreeman, Snoeren & Payens, 1980). They depend on both temperature and ionic environment (presence of salts, concentration). In solution (hot), carrageenans behave as random coil whose degree of expansion depends on the degree of sulfation and the ionic environment due to its polyelectrolyte character. On cooling, they adopt a helical conformation. The temperature of helix-coil transition which depends on the nature of

مطلب مرتبط :   پایان نامه رایگان با موضوعسیاست خارجی، صدام حسین، عراق جدید، سازمان ملل

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