Czech J. Food Sci. Vol. 23, No. 1: 1–11 REVIEW Optimum Conditions of Rice Puffing KAREL HOKE, JIŘINA HOUŠOVÁ and MILAN HOUŠKA Food Research Institute Prague, Prague, Czech Republic Abstract HOKE K., HOUŠOVÁ J., HOUŠKA M. (2005): Optimum conditions of rice puffing – Review. Czech J. Food Sci., 23: 1–11. The presented paper reviews the existing literature dealing with rice puffing and optimum conditions of this process. The rice pre-treatment procedures and puffing conditions (dry heat, microwave, gun-puffing) are considered. The optimum composition of the raw material is also mentioned. Keywords: rice; puffing Puffed rice is very popular in many countries as The goal of this study was to review in detail the a cereal breakfast component or as a light food. influence of the main quality parameters of the It is prepared from treated (hydro-heated) milled rice sorts, the conditions of hydro-heat treatment rice by heating in high temperature air, oil, sand, of rice and further treatment procedures on the or also from raw milled rice in microwave field quality of the puffed product. The quality of puff- or by the puffing gun. ing is given by the expansion ratio (ratio of the During puffing, rice kernels increase their vol- expanded rice volume and volume of the original ume several times and a fully heat-treated crisp, prepared rice) – the higher the expansion ratio, the porous, ready-to-eat product is created. Regardless higher the quality of the final product. Optimum of the puffing process two important parameters selected sort, optimum treatment of rice kernels should be taken into account: the selection of an and optimum conditions during puffing gun or appropriate sort of rice, and the use of a proper microwave heating combined with vacuum can hydro-heat treatment of raw rice. assure the optimum conditions for puffing. Raw rice (paddy) is the rice in the husk. Raw rice wetted in water, heated in steam and dried CONDITIONS OF PARBOILING, is treated rice (special steamed rice). Wetted rice MILLING AND PUFFING heated in sand is rice treated with dry heat. Milled rice is partially or fully free of the hull. The cor- CHINNASWAMY and BHATTACHARYA (1983a) responding milled rice is regarded as treated by studied optimum conditions of parboiling, mill- heat in steam or by dry heat. ing and puffing for the preparation of expanded Supported by the Ministry of Agriculture of the Czech Republic, Project No. QF3291. 1 Vol. 23, No. 1: 1–11 Czech J. Food Sci. rice on a small laboratory scale. The experiments sand heated to the temperature of 250°C was used were carried out with Intan variety of paddy. It was for all subsequent experiments. obtained from the market in bulk, cleaned, dried To study the effect of salt on the expansion vol- [12–13% moisture (wet basis, w.b.)] and stored ume, 0.5 ml of various salt solutions was sprinkled in metal containers in the laboratory at room on 20 g of milled parboiled rice, the sample was temperature. All experiments were conducted mixed thoroughly with a glass rod for 5 min and 5 to 10 months after the harvest of the lot. For the rice was expanded. parboiling, 0.5–2 kg of paddy was added to 2–3 l The expansion ratio was determined by meas- of tap water previously heated to 80°C, the vessel uring the bulk volume of 20 g of the original was covered with jute sacks and left overnight parboiled rice and that of the resulting expanded (BHATTACHARYA & INDUDHARA SWAMY 1967). The rice in 50 ml and 200 ml graduated measuring water was drained off and the paddy was cooked cylinders. either by steaming (steam-parboiled paddy) or by heating with sand as done in industry [dry-heat- Heat expansion condition parboiled paddy (DH-parboiled paddy)]. The initial studies (CHINNASWAMY & BHATTA- Soaked paddy was steamed on a wire mesh tray CHARYA 1983a) were devoted to determining the for 10 min in an autoclave. Various steam pres- optimum conditions of heat expansion of milled sures (0–3 bar gauge) were tested initially, and parboiled rice with the laboratory roaster. For this the pressure of 1.5 bar gauge was selected for the purpose, dry-heat-parboiled (DH-parboiled) rice subsequent studies. For parboiling by sand heat- prepared for heat expansion after salting was ob- ing, the method was essentially that of ALI and tained and used. These results showed that for the BHATTACHARYA (1976b) for the production of laboratory roasting conditions, heating 20 g of rice flaked rice. Briefly, about 250 g of fine sand was with 300 g of sand preheated to 250°C for about heated to the desired temperature (200–300°C) 10 s gave the maximum expansion ratio 7.6. in a hand-operated, rotating, cylindrical, electri- However, sand temperature, sand quantity, and cally heated laboratory coffee roaster of about rice quantity are interrelated and the same effect 1 kg capacity. About 130 g of soaked paddy was can be received by different combinations of condi- poured quickly into the roaster and the latter was tions. At too high temperatures or too high sand rotated at a rate of 80–100 rpm for the desired to rice ratio, the rice was slightly to moderately time (0.5–4.5 min) or until a few grains started charred which may explain the decrease of the to pop. The sand was sieved out and the paddy expansion ratio. The rpm of the roaster (10–100) was cooled and tempered for 30 min. The steam- did not seem to have a significant effect on the parboiling method was adopted for all studies of expansion ratio; for routine experiments it was other process variables, as it was simpler than the maintained at about 90 (about 15 rotations for a dry-heat-parboiling method. batch lasting 10–11 s). Too coarse sand gave low Paddy parboiled by the above methods was air- expansion, perhaps due to a poor heat transmis- dried in the laboratory for 1–2 days (to reach sion, but otherwise the mesh size of sand did not 10.5–11% moisture, w.b., in the milled rice) and appear to have any significant effect. then milled to 8–10% degree of milling for rou- In comparison, ROBERTS et al. (1951) found that tine studies, or to the degree of milling desired. the optimum temperature was 200–220°C and 250 Any broken rice was removed with an indented to 300°C for heating in oil and air, respectively. plate. To test the effect of the moisture content of Effect of parboiling condition milled parboiled rice on its expansion volume, the parboiled paddy was collected at different times In the industry, water is first brought to boil in during air-drying, milled and used. a drum, heating is discontinued and the paddy is Milled parboiled rice (20 g adjusted to 10.5–11% put into water for overnight soaking (Figure 1) moisture) was heated with 10–20 times its weight (CHINNASWAMY & BHATTACHARYA 1983a). of preheated sand (200–300°C) for 10–11 s, i.e. This appeared, from experience, to be an un- until the crackling sound just ceased, in the same necessarily high temperature for optimal soaking manner and in the same roaster as described above. (BHATTACHARYA & SUBBA RAO 1966; BHATTA- Based on the preliminary experiments, 300 g of CHARYA & INDUDHARA SWAMY 1967). Therefore, 2 Czech J. Food Sci. Vol. 23, No. 1: 1–11 By our opinion, the substantial feature of any 50 kg paddy (14% M) treatment is the increase of humidity inside the | Soak overnight in just-boiled water in covered kernel centre and the formation of a dry solid drum (3% M) layer on the kernel surface. This creates the mini- | ature pressure vessel that can work as a puffing Drying gun (inside the kernel is overheated water and | during the rupture of the surface the expansion Roast 1.5 kg with fine sand (1:2) in a shallow occurs). iron pan over a strong fire at 240–250°C, When soaked paddy was steamed under differ- for 22 s (100°C, 15% M) | ent pressures, the expansion ratio of the resulting Sieve parboiled rice was minimal with mildly parboiled | rice (nearly equal to that of raw rice) but increased Parboiled paddy up to about 1.5 bar gauge pressure and then levelled | off. These results were quite similar to those of Temper 1 h, shade dry 1 hour (13% M) | ROBERTS et al. (1954). Mill in Engelberg huller → husk-bran for fuel However, the maximum expansion ratio obtained | by the above procedure (just over 5) was much less Milled parboiled rice than that obtained in the industry, as was the case | in the laboratory with the salted DH-parboiled Preheat 10 kg without sand in an iron pan rice produced industrially with the same variety over a low fire for 5 min (80°C, 10.5% M) | of paddy (about 7.5). Therefore, soaked paddy Temper 30 min was parboiled by sand roasting under various | conditions. The DH-parboiled rice yielded a much Mix 40 ml saturated salt solution to 800 g rice higher expansion ratio than did steam-parboiled (11% M) rice, the optimum conditions being heating soaked | paddy at 250°C for 2.5 min. Roast with sand (1:10) in an iron pan The essential difference between dry-heat-par- at 240–250°C for 11–13 s | boiled and steam-parboiled rice is a lower starch Sieve re-association, or retrogradation in the former | (ALI & BHATTACHARYA 1976a, b).