Effect of processing on nutritional value

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Thermal processes can affect protein and starch properties (Table 46). The effect of boiling and parboiling was discussed in Chapters. Yellow rice from stack-burning of wet rough rice has a lower lysine content and NPU than normal rice (Eggum et al., 1984). Cystine and tryptophan are not affected.

TABLE 45 - Amylose type and other properties preferred for processed rice products

Product Amylose type Other properties
  Waxy Low Intermediate High  
Parboiled rice. + +  
Precooked and quick-cooking ricea + + + +  
Canned ricea + + + +  
Expanded rice products + + + + (Amylose content not a major factor)
Rice cereals and snacks + + +   Low fat; texture affected by amylose content
Extrusion-cooked rice foods + + +   Low fat
Rice-based infant formulations   + +   Low fat
Rice flour and rice starch + + + + Wet milling process, freshly milled
Rice puddings and breads   + + + Low GT
Rice cakes + + +   Low GT, aged (for fermented cakes), freshly milled
Flat rice noodles and rice paper (+)      
Extruded rice noodles       + Hard gel consistency
Rice wines + + Low protein and fat, higher ethanol yield for waxy and low amylose
Beer adjunct   +     Low GT and low fat
Fermnented rice foods (idli, dosai)     +   Parboiled
Rice frozen sauces, desserts,sweets + +     Slow retrogradation (syneresis)

a Preferred amylose type based on type of raw rice preferred.

Most preferred amylose type.

Extrusion cooking reduces lysine and cystine levels but not tryptophan, and reduces the NPU of milled-rice protein (Eggum et al., 1986). Hydrogen sulphide is observed during rice extrusion cooking. Other heat processes decompose lysine (IRRI, 1984a; Juliano, 1985a), except gun-puffing, which affects cysteine (Villareal and Juliano, 1987). The subsequent toasting step is probably where the lysine decomposition occurs (Khan and Eggum, 1979; Chopra and Hira, 1986). The tryptophan residues in model food proteins are more stable during processing and storage than methionine and lysine (Nielsen et al., 1985).

TABLE 46 - Effect of heat treatment and processing on the lysine and cystine content and net protein utilization of rice in growing rats

Processing method

Percent decrease in

  Lysine Cystine NPU  
Boiling, 20min 1-3 0 0 Eggum, Resurreccin & Juliano, 1977
Pressure parboiling        
(20-60 min, 120C, 35% H2O) 0 0 0 Eggum et al, 1984
Noodle extrusion (35% H2O, 55C) 0-3 - - Khandker et a/,1986
Pan baking (220-230C,7-10min) 0   0 Khan & Eggum, 1978
Accelerated aging (100C,3 hours, sealed) 3   - IRRI, 1984a
Pan toasting 5   - IRRI, 1984a
Stackburning (high H2O, <100C) 9-18   6-12 Eggum et al, 1984
Induced yellowing (60C, 4 days, sealed) 25-26% H2O 14-18   - IRRI, 1984a
14% H2O 9   - IRRI, 1984a
Popping, brown rice (207C, 45 see) 16-17   - IRRI, 1984a
Extrusion cooking, flour (15% H2O, 150C, 45 bars) 11 21 7-8 Eggum et al.,1986
Gun-puffing, milled rice (200-210C) 0 48 - Villareal & Juliano, 1987
Commercial steaming and puffing,milled rice 37 - - IRRI, 1984a
Manufacture of commercial rice krispies 53 - 41 Khan & Eggum, 1979
Roasting (220-280C, 2-2.5 min) 69 - 61 Chopra & Hira, 1986

Rice batter (dough) fermentation reduces the phytate content of 0.45 percent by 45 percent after one day, 74 percent after two days and 80 percent after three days (Marfo et al., 1990). Legume phytate hydrolysis is also reported during fermentation of idli.

Enrichment and fortification

The purpose of rice enrichment and fortification is to restore to milled rice the levels of B vitamins and minerals removed from the grain during milling. It is technically more difficult than enriching wheat flour since rice is consumed as a whole grain. Traditional methods include parboiling, acid parboiling with I percent acetic acid, thiamine enrichment, coating, production of artificial rice, dibenzoylthiamine enrichment and multinutrient enrichment by adding a nutrient-enriched premix (Mickus and Luh, 1980; Misaki and Yasumatsu, 1985). Premix is made by soaking milled rice in an acetic acid solution of the water-soluble vitamins thiamine, riboflavin, niacin, pantothenic acid and pyridoxine. Then it is steamed, dried and coated with separate layers of vitamin E, calcium and iron and then with a protective coating material and natural food colouring to prevent the loss of nutrients through washing. The nutrient levels are the same as those of brown rice. This multinutrient-enriched rice is blended with milled rice at a 1:200 ratio. Only 10 percent of any nutrient is lost through ordinary washing before cooking and another 10 percent on cooking.

The pioneering enrichment field studies in Bataan province, the Philippines in 1948-50 demonstrated that rice enrichment was practical, with striking reductions in the incidence of beriberi in the areas in which enrichment was introduced (Salcedo et al., 1950; Williams, 1956).

Obstacles to the successful introduction of rice enrichment by the premix method include the following (FAO, 1954):

- the cost of the imported premix,
- the difficulty of ensuring that the premix is added to milled rice in the correct proportion in the mill,
- the slightly greater cost of enriched rice as compared with that of ordinary rice, which affects its sale to lower income groups,
- losses of added vitamins which may occur when enriched rice is cooked in excess water that is subsequently discarded, according to current practice in some rice-eating countries,
- issues related to standards and analysis, particularly of imported rice,
- lack of knowledge about the loss of added nutrients during storage.

Undermilling has been employed to retain B vitamins in milled rice, but the shelf-life of undermilled rice is shorter than that of milled rice and the product is less white (FAO, 1954). Some consumers remill the undermilled rice to remove the rancid outer layer and to make the rice whiter, with an accompanying loss of B vitamins. Milled rice has also been used for enrichment programmes for vitamin A as well as vitamin B in Thailand and the Philippines (Gershoff et al., 1977). The results of a village-wide supplementation of lysine threonine, thiamine, riboflavin and vitamin A for Thai preschool children were not conclusive with respect to the Iysine-threonine fortification.

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