Apr. 29, 2024
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FAQs
Q: What are the key challenges facing the puffed rice manufacturing industry in India?
Ans:
The key challenges facing the puffed rice manufacturing industry in India are the excessive cost of production and the competition from other countries. The high cost of production is making it difficult for manufacturers to compete in the export market, while the competition from other countries is making it difficult to grow the domestic market.
Q: What are the key growth drivers for the puffed rice manufacturing industry in India?
Ans:
The key growth drivers for the puffed rice manufacturing industry in India are the growing population and increasing disposable incomes. The growing population is leading to an increase in the demand for puffed rice, while the increasing disposable incomes are resulting in more people being able to afford the product.
Q: What are the key markets for the puffed rice manufacturing industry in India?
Ans:
The key markets for the puffed rice manufacturing industry in India are the domestic market and the export market. The domestic market is the largest, with puffed rice consumed by many people across the country. The export market is also significant, with India exporting puffed rice to several countries around the world.
Q: What are the key products/services of the puffed rice manufacturing industry in India?
Ans:
The key products of the puffed rice manufacturing industry in India are puffed rice, rice bran, and rice flour. Puffed rice is the most popular product, with manufacturers producing a variety of diverse types of puffed rice for both the domestic and export markets.
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Popping/puffing have been traditionally practiced for enhancing storage life, improving organoleptic properties and ease of incorporation in ready-to-eat-foods. Currently, batch type sand and electric popping/puffing machines involving conduction mode of heat transfer are employed. The major drawbacks of these methods are high-energy consumption, scorching of grains, non-uniform product quality, contamination (by sand/ash) and problems in scale-up. Since fluidization is known to increase heat and mass transfer, a continuous fluidized popping/puffing machine (capacity 10–20 kg/h) involving convective mode of heat transfer is designed/developed. Hot-flue gas generating from burning of LPG was used as the eco-friendly fuel. Process parameters such as expansion ratio, fluidization velocity, terminal velocity, carry over velocity, bulk density and voidage were estimated for un-popped and popped/puffed rice, maize, jowar (sorghum) and paddy. Fluidization and carry over velocities for these grains were in the range of 4.18–5.78 m/s and 2.15–6.18 m/s, respectively. Based on the terminal velocity of the grains and volumetric air flow rate of the blower, fluidization chamber diameter was arrived. Chamber diameter of 0.15 m was found to be sufficient to generate required air velocity of 6.89 m/s which met the fluidization and carry over velocities of popped/puffed grains. The designed fluidization chamber was analyzed for heat and mass transfer during popping/puffing. Convective heat and mass transfer coefficients were estimated to be in the range of 103–187 W/m 2 °C and 0.124–0.162 m/s, respectively. Theoretical values for total heat and mass transfer were similar to the experimental values.
Accordingly, the objective of the present study is to design and develop a continuous popping/puffing machine (10–20 kg/h of raw material) using flue gas/hot air (generated by burning the LPG) as the fluidizing medium. This paper presents also the heat and mass transfer in popping and puffing of different grains namely rice, maize, jowar and paddy at conditions standardized for achieving maximum efficiency for grain expansion.
Although the basic kitchen methodology (know-how/do-how) for the production of traditional snack foods such as puffed rice, puffed beaten rice, puffed paddy is known, considerable R&D studies are necessary to adapt them into technologies for the large-scale production. For this, major inputs from food engineers and technologists are required. In order to facilitate the production of Indian traditional food and ready-to-eat snack food, our research group has designed and developed a few equipment (Hrishikesh et al. 2014 ; Venkateshmurthy et al. 2003 ; Venkateshmurthy et al. 2004 ). Analysis of heat and mass transfer immensely help in the design and development of such food processing equipment (Saxena et al. 1995 ; Venkateshmurthy and Raghavarao 2015 ).
Conventional conduction based machines (static bed) result in particle–particle contact. On the other hand, during fluidization, the stream of fluid flowing upward through contact with the solid particles transforms them into fluid like state (Daizo and Levenspiel 1991 ; Ennis et al. 1994 ; Nedderman 1992 ). The mean surface area of the dispersed particles increases during fluidization of the grains which in turn increases the degree of heat and mass transfer. The major advantages of fluidization are low maintenance cost, ease of scale-up simple design, uniform particle exposure without mechanical agitation and intimate particle to gas contact (Heywood 1978 ). Reports on application of fluidization for popping/puffing in food processing industry are available (Chandrasekhar and Chattopadhyay 1988 ; Inoue et al. 2009 ; Iyota et al. 2005 ; Markowski et al. 2006 ; Nath et al. 2007 ; Shimoni et al. 2002 ). Studies on puffing and popping are already reported for grains such as maize (Park and Maga 2006 ; Sweley et al. 2012 ; Vázquez-Carrillo et al. 2019 ), amaranth (Castro-Giráldez et al. 2012 ; Zapotoczny et al. 2006 ), sorgum (Aruna et al. 2020 ), rice (Mir et al. 2016 ) and chickpeas (Mukhopadhyay et al. 2015 ). In our earlier work, fluidization was successfully used for roasting of grains (Murthy et al. 2008 ).
Popping/puffing process results in aerated, porous, texture and an increase in volume, with the added benefits of dehydration. The main parameters affecting puffing are starch content and initial moisture content of the raw materials, puffing duration and temperature (Arya 1992 ), whereas the main factor affecting popping is the initial moisture content of the raw material (Stewart 1923 ). Currently these operations are performed in sand and electric popping/puffing machines involving conduction mode of heat transfer. The major disadvantages of the above mentioned methods are non-uniform product quality, scorching of grains, high-energy consumption (Nath et al. 2007 ), problems in scale-up besides being grain specific. The popped material obtained from these conventional techniques contain contaminants like sand, sawdust ash etc. and the popping operation is carried out in non-hygienic conditions. Different types of popping machines are needed for different raw materials and each has its own drawbacks. Reports on design and development of popping/puffing machine which is flexible enough to handle a wide variety of raw material (maize, paddy, rice, sorghum etc.) hygienically in a continuous mode depending on the requirement of the small scale/cottage industry are scarce. Accordingly, attempts are made in the present work to address these issues by the application of fluidization.
The demand for the ready-to-eat food is increasing day by day (Nath et al. 2007 ) and some of the snack foods such as popcorn, puffed paddy (Aralu), puffed beaten rice (Avalakki puri) and puffed rice (Murmuri) are gaining popularity in different parts of the world. Popcorn which is produced by heat expansion of maize (popcorn variety), gained popularity as a favorite snack/breakfast cereal not only in India but also in many other countries. Powder salt and other spices are added, mixed and cooled before packaging.
In the present application, the second type of mass transfer is most appropriate than the other two. This is because the mass transfer involved in popping or puffing is with respect to moisture loss from the product during expansion, as the moisture present in the grain absorbs the heat from the fluidizing hot air and gets evaporated. The corresponding correlation (Beek 1971 ) is given by
where Q A is total heat absorbed (kJ), W m mass of moisture in the grain (kg), C pw specific heat of water (kJ/kg °C), W p weight of popped grain (kg), C pg specific heat of grain (kJ/kg °C), T v temperature of vaporization (°C), T R room temperature (°C). W d weight of the unexpanded grain (kg), T d surface temperature of the unexpanded grain (°C), L weight of moisture evaporated (kg) and λ v is latent heat of evaporation (kJ/kg).
where the Nusslet number N Nu = hd p /K; Reynolds number N Re = ρd p v/µ; Prandtl number N Pr = C p µ/K and h is convective heat transfer coefficient (W/m 2 °C), d p diameter of the grain (m), C P specific heat of hot air (kJ/kg °C), K thermal conductivity (W/m ° C), µ viscosity (Ns/m 2 ), v velocity (m/s) and ρ is density (kg/m 3 ). The bulk fluid properties of the hot air are calculated at average temperature from the following equation (Geankoplis 1993 ).
where h F is heat transfer coefficient between fluidized particles and surrounding heating medium (W/m 2 °C), A P area of heat transfer surface (m 2 ), T I inlet hot air temperature and T P is expanded (popped/puffed) grain temperature (°C). In the present application, the fluid (hot air) is passes through the grains while fluidizing them (flow past immersed objects) and heat transfer depends on the different variables such as shape, moisture content and quantity of the particles, flow rate and bulk fluid characteristics of hot air.
Though supply of required amount of heat is an important aspect for achieving desired organoleptic properties (color, crispiness and flavor) of food, mode of heat transfer to the food product from heating medium is known to have more effect (Rattanadecho et al. 2007 ; Saxena et al. 1995 ; Standing 1974 ). Hence, it was thought desirable to carry out theoretical analysis of heat and mass transfer during popping/puffing of different grains and compare them with the experimental values.
The performance evaluation of popping/puffing machine over the designed feed rate of 10–20 kg/h was carried out as described below. The grain to be popped/puffed was fed into the popping chamber mounted on to the combustion chamber. The grain was fed on to the hot air distributor (wired mesh) at a standardized rate, air velocity and air temperature (each different for different grains). The popped/puffed grain was discharged through machine’s outlet chute and collected. Temperature and final moisture content (dry basis) of the popped/puffed material was measured. The product was analysed for the texture and uniformity in color. The popping/puffing efficiency, expansion ratio of the grains and thermal efficiency of the machine were analyzed.
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Hot air velocity was measured with an anemometer (Prova Instruments Inc., AVM 01) having the least count of 0.1 m/s. Temperature of the product as well as the hot air were measured employing digital temperature indicator (Model-TFF200, make-EBRO, Germany, PT-100, Range: − 50 to 400 °C). The temperature indicator had a least count of 0.1 °C.
A known amount of popped maize was taken and placed on hot air distributor/wired mesh of fluidization chamber of the popping/puffing machine, forming a bed of 0.02 m height. The velocity of air from centrifugal blower was gradually increased and flow rate of air was noted. The air velocity at which the entrained particles leave the fluidized bed column entirely is called the carry over velocity (Smith 2007 ). The experiment was repeated for different batches of the same grain and average carry over velocity for that particular grain was calculated. Similarly, experiments were carried out for puffed rice and other popped grains (jowar and paddy) and corresponding carry over velocities were obtained.
A known quantity of maize is taken and the average diameter of the grains was measured. These grains were placed on hot air distributor/wired mesh of fluidization chamber of the popping/puffing machine, forming a bed of 0.02 m height. The velocity of the air generated by a centrifugal blower, provided at the bottom of machine was gradually increased. The air velocity at which the bed just becomes fluidized was noted as minimum fluidization velocity. The experiment was repeated for different batches of the same grain and average minimum fluidization velocity for that particular grain was calculated. Similarly, experiments were carried out for other grains (rice, jowar and paddy) and corresponding fluidization velocities were obtained.
When a fluid flows upward through a packed bed of particles at low velocities, the particles remain stationary. As the fluid velocity is increased, the pressure drop increases according to the Ergun equation. Upon further increase in velocity, conditions finally occur where the force of the pressure drop times the cross-sectional area just equals the gravitational force on the mass of the particles. Then the particles just begin to move, and this is the onset of fluidization or minimum fluidization. The fluid velocity at which fluidization begins is the minimum fluidization velocity (Geankoplis 1993 ).
The terminal velocity of the unexpanded/expanded (popped and puffed) grains was obtained by dropping the unexpanded/expanded grains through a tube of 0.05 m diameter and 5.16 m length, and the time required for the vertical travel of the grains was recorded at room temperature (26 ± 1 °C) using a stop-watch.
In gravitational setting, acceleration due to gravity remains constant. Also, the drag always increases with velocity. The acceleration decreases with time and approaches zero. The particle quickly reaches a constant velocity, which is the maximum attainable under the circumstances. This velocity is called Terminal velocity (McCabe et al. 1993 ).
Since the grains are to be fluidized (for popping/puffing) using hot air, revised volumetric flow rate ( Q bulk o m 3 /s) has been calculated using the properties of hot air at bulk fluid temperature using following equation (McCabe et al. 1993 ).
Velocity of air generated by the blower and employed for fluidization was measured at 0.25 m from the outlet of blower using anemometer (Prova Instruments Inc., AVM 01). Based on the measured air velocity, the volumetric air flow rate of blower ( Q blower o m 3 /s) was calculated using following equation (Smith 2007 ).
Similar experiments were carried out for popped/puffed grains as well. Predetermined quantity of popped/puffed grain was taken in a measuring cylinder and after measuring the volume, toluene was added slowly till the lower meniscus reaches the upper level of the grain bed in the measuring cylinder. The ratio of the volume of popped grain to the initial volume (before popping/puffing) was considered as expansion ratio for that particular grain. The ratio of weight of popped/puffed grain to the initial weight (db, by subtracting the initial and final moisture content of un-popped/un-puffed and popped/puffed grains, respectively from total weight) of grain is considered as popping/puffing efficiency.
Predetermined quantity of grain was taken in a measuring cylinder and after measuring the volume, toluene was added slowly till the lower meniscus reaches the upper level of the grain bed in the measuring cylinder. The ratio of the volume of toluene added and the total volume of grains was recorded as voidage. Toluene was used instead of water because of its low surface tension and dissolution (Kashaninejad and Tabil 2009 ). Difference between Volume of grain in the measuring cylinder and Volume of toluene added gives the actual volume of grain taken.
Based on the average diameter of each grain, outer surface area and volume of each grain were calculated. These values were used to determine the number of grain particles and the total surface area available for heat and mass transfer. The bulk density of the grains was determined for both expanded and unexpanded grains by calculating the ratio of the weight and volume of the given grain. The values thus obtained were used for heat and mass transfer calculations.
The popping/puffing variety of different grains were (rice, maize, jowar and paddy) procured from the local market, and the moisture content was analyzed by using moisture analyzer (M/s Wensar, HMB 100, German). Vernier caliper was used to measure the dimensions (length, width and thickness) of the grains by taking the average of 10 randomly selected grains and average diameter for each grain was noted.
In case of popping of grain, no treatment is required. A grain of desired moisture content only can result in popping. The popping variety of maize, jowar and paddy were procured from the local market, and the moisture content was analyzed and the dry grains (without addition of extra moisture) was used for popping.
Certain pretreatments are essential for the preparation of grains for puffing. For puffing, 500 gm of rice of moisture contents (10%) were subjected to hydrothermal treatment (the grain was soaked in hot water at a temperature of 60 to 70 °C and kept under pressure) prior to puffing. After preheating the rice to a temperature of 150 °C, 10% (w/v) water containing rock salt 15% (w/v) was added and rested for 10 minutes. The samples were puffed at 260 °C and expansion ratio of final product was calculated.
The prototype of fluidized bed popping/puffing machine was assembled and the orthogonal (front) view/engineering drawing of the same is as shown in Fig. a. It consists of a combustion chamber (1) mounted on the centrifugal blower (2). The flue used at 750–800 °C was mixed with air of ambient temperature in desired proportion to bring down the temperature to 250–300 °C in the combustion chamber. The raw material is fed to the popping machine by a screw feeder (3). The feed rate of the raw material is controlled by the rotation of the screw feeder using a timer. A hot air distributor/wired mesh (4) is provided at the end of combustion chamber which holds the grains and also allows hot air to pass through it, facilitating fluidization and in turn popping/puffing of grains. A thermocouple of the temperature indicator (5) is provided inside the popping chamber for measuring the hot air temperature and temperature of popping is varied to suit the given raw material. The popped material is conveyed out of the machine pneumatically through the outlet chute (6). The hot air is generated in the combustion chamber by burning liquid petroleum gas. The centrifugal blower (2) generates a stream of air to be mixed with the flue and is blown through the hot air distributor. Regulating the supply of LPG varies the temperature of the hot air. The machine is provided with a set of castor wheels (7) for easy mobility. Main cover/electrical panel (8) houses the timer (9) and main switch (10).
The physical properties of the grains such as voidage, bulk density were estimated, and the results are presented in Table a. Fluidization velocity, carry over velocity, terminal velocity were determined, and the results are presented in Table b.
The fluidization velocity for different grains was found to be in the range 4.18 to 5.78 m/s as shown in the Table b. Accordingly the blower was selected for fluidization that can generate an air velocity of 6.4 m/s (at 0.25 m from outlet) which is sufficient to fluidize the grains. The terminal velocity of different grains was found to be in the range 2.66 to 4.20 m/s as can be seen from the Table b. The volumetric air flow rate (Qblowero) of the blower was calculated to be 0.122 m3/s using the outlet cross sectional area and air velocity generated by the blower. Subsequently, considering the terminal velocity of the grains (over the range 2.66–4.20 m/s) and the volumetric air flow rate generated by the blower (0.122 m3/s), the theoretical diameter of the fluidization chamber was calculated and is in the range of 0.20–0.23 m for different grains.
Since the grains are to be fluidized (for popping/puffing) using hot air, volumetric flow rate (Qbulko) of the hot air has been calculated using the properties of hot air at bulk fluid temperature. Accordingly, revised volumetric flow rate of hot air was estimated to be in the range of 0.17–0.18 m3/s (as can be seen is supplementary material) and an equivalent diameter of chamber for fluidization employing hot air was calculated to be in the range of 0.24–0.27 m (as can be seen is supplementary material).
The maximum value of the carry over velocity was observed to be 6.18 m/s (for popped jowar). Accordingly the fluidization chamber diameter has been selected as 0.15 m, which generates air at a velocity of 6.89 m/s for a volumetric flow rate of 0.12 m3/s generated by the blower. This is sufficient to meet the carry over velocity ranging from 2.15 to 6.18 m/s, fluidization velocity ranging from 4.18 to 5.78 m/s and terminal velocity ranging from 2.66 to 4.20 m/s. The details of the calculations for the determination of diameter of fluidization chamber are shown in Supplementary material. The fabricated Fluidized popping/puffing machine is shown in Fig. b.
The average moisture content of 12–14% (weight basis) is reported to be the most suitable for the preparation of popcorn (Metzger et al. 1989). Even in our preliminary experiments, similar range of moisture content was observed to be suitable. When corn is heated, the natural moisture present inside the kernel turns into steam and the corn casing acts like a pressure vessel which is holding the steam (Metzger et al. 1989). When the pressure exceeds beyond a point, the kernel breaks releasing the pressure (Hoseney et al. 1983), resulting in popping of grain.
Grains such as rice, maize, jowar and paddy were fluidized by hot air (180–250 °C) at a velocity ranging from 2 to 7 m/s. In the hot air popping/puffing machine, the startup (heat up) and shutdown are instantaneous and practically there is no loss of heat energy. The air velocity was kept uniform during the experimentation. Bulk density of the puffed and popped grains was found to decrease (Table a), due to decrease in weight associated with moisture loss and also increase in volume of the grains on popping/puffing. For instance, in case of maize, the bulk density of unexpanded grain was 0.77 × 103 which was observed to decrease to 0.03 × 103 kg/m3 after popping. Similarly, the decrease in bulk density from un-popped grain to popped grain was from 0.62 × 103 to 0.15 × 103 kg/m3, from 0.70 × 103 to 0.15 × 103 kg/m3 and from 0.50 × 103 to 0.08 × 103 kg/m3 in case of rice, jowar and paddy, respectively. The air velocity that aided the fluidization helped also to carry the popped/puffed material away from fluidization zone. The expansion ratio of different popped/puffed grains were calculated as explained in “Physical properties of grains and determination of expansion ratio and popping/puffing efficiency” section. The values of expansion ratio of different grains along with the corresponding feed rate are presented in Table b. The fluidized popping/puffing machine has a capacity of 10–20 kg/h depending on the grain. During experimentation, channeling of gas and unstable fluidization was observed which may be attributed to the fact that all the four grains fall into Geldart’s group ‘D’ category (Geldart 1973).
The heat transfer Eqs. (1) to (4) were used for the popping and puffing of grains in the continuous fluidized bed. Temperature distribution in the bed will be uniform in case of fluidization of the particles with hot gas. This is because of the rapid particle mixing occurring because of the temperature drop taking place between the bed and the inlet hot air over very short distance (a few particle diameters), immediately above the hot gas distributor (wire mesh). It is also possible to use high inlet temperature without causing much thermal damage to temperature sensitive food particles.
Convective mode of heat transfer was found to be predominant in the present case, and heat transfer coefficients were estimated in the temperature range of 230–296 °C using Eq. (2) and found to be in the range of 103–186.81 W/m2 °C as presented in Table . The gas-particle convective heat transfer coefficients reported in the literature are in the range 170–230 W/m2 °C for the particle of size 600 µm and 60–101 W/m2 °C for the particles of size 9000 µm (Smith 2007). It can be observed that the values obtained in the present work are for the particles of 2000 µm and hence falling in between these ranges. At the same time, these heat transfer coefficients can be of even lower magnitude (about 20 W/m2 °C) according to Botterill (1975). Total heat supplied for the popping/puffing was calculated by considering calorific value of LPG as 48,651.92 kJ/kg and arrived to be in the range of 60,815–72,978 kJ for different grains and the experimental value for total heat absorbed during popping/puffing of grains was found to be in the range of 4597–5463 kJ. Theoretical values for total heat absorbed was calculated and arrived to be in the range of 573–4386 kJ. The major results obtained in Tables and are compared with the experimental results in Table . The theoretical values for total heat absorbed during popping/puffing are found to be similar to that of experimental values except for maize as can be seen from Table .
QAQT×100
(%)PuffingRice1275.3 (27.12%)†3429.12 (72.88%)†4704.7260,814.907.73PoppingMaize1672.47 (30.61%)†3790.68 (69.38%)†5463.1572,977.887.40Jowar1672.47 (36.81%)†3317.29 (72.16%)†4596.7760,814.907.55Paddy1319.65 (25.82%)†3791.18 (74.17%)†5110.8372,977.887.00Open in a separate windowEstimation of mass transfer coefficient is rather difficult due to the scarcity of diffusion coefficient data for grains. In this study, it was considered as 2 × 10−9 m2/sec, based on our earlier work (Rastogi and Raghavarao 1994).
Mass transfer coefficient and mass transfer factor have been estimated using Eqs. (5) and (6), respectively and the results are given in Table . The mass transfer coefficients were in 0.124–0.154 m/s range and mass transfer factor were in 0.58 × 10−3–2.13 × 10−3 Ns/kg range (Table ). The gas-particle mass transfer coefficients reported in the literature are in the range 0.188–0.253 m/s for the particle of size 600 µm and 0.063–0.103 m/s for the particles of size 9000 µm (Smith 2007). It can be observed that the values obtained in the present work are for the particles of 2000 µm and hence falling in between these ranges.
The transfer of mass taking place from all the particles in the bed is represented by the whole bed mass transfer coefficients. These coefficients are not equal to the coefficients of individual particles unless the gas flow through the bed is plug flow type. In case of beds with large particles and at high Reynolds numbers, the gas flow follows plug flow pattern approximating the whole bed coefficient and single particle coefficient. In case of fine particles, the degree of particle gas contact is less as majority of the gas passes through the bed in bubble phase. As a consequence, the mass transfer coefficient estimated was lower than single particle coefficient predicted.
Experimental value of Total mass transfer for the popping/puffing was calculated to be in the range of 3.38 × 10−3–4.95 × 10−3 kg/s for different grains and the theoretical values for total mass transfer during popping/puffing was found to be in the range of 0.61–2.43 kg/s. The theoretical and experimental values for total mass transfer are found to be of comparable order of magnitude except for maize as can be seen in Table . It can be attributed to the very low volume of material used for experimentation (about 0.87 kg).
The performance evaluation of popping/puffing machine was carried out at the rated capacity as described below with respect to different grains. The maize to be popped was fed into the popping chamber mounted on to the combustion chamber. The maize was fed on to the hot air distributor (wired mesh) at a rate 18 kg/h. The air velocity was set at 1.3–1.4 m/s over an air temperature range from 225 to 235 °C. The popped maize was discharged through machine’s outlet chute and collected. Temperature of the popped material was around 55–60 °C with the final moisture of around 4–5% (dry basis), wherein the product is of uniform color and is crisp.
Similarly, the paddy to be popped was fed into the popping chamber at a feed rate of 11.25 kg/h. The air velocity has been set at 1.5–1.8 m/s, over an air temperature range from 295 to 300 °C. The popped paddy was discharged through the machine’s outlet chute and collected. The temperature of the popped material was around 65–70 °C with the final moisture of around 3–5% (dry basis), wherein the product is of uniform color and is crisp. Similar trials were carried out for rice and jowar as well.
Trials for performance evaluation were carried out and was observed to perform the popping/puffing satisfactorily over the designed feed rate of 10–20 kg/h. The highest popping/puffing was observed in case of maize and jowar with 88.23% popping efficiency and the lowest puffing in case of rice with 73.05% efficiency. The lowest un-popped material was observed in case of maize and jowar (11.76%) and the highest in case of paddy (23.51%). The expansion ratio of the final product was satisfactory for all the grains. Jowar exhibited highest expansion ratio (12.5) whereas the lowest was observed in case of rice (6.37). The machine was observed to handle different grains satisfactorily.
The product obtained after popping is uniform in color, texture and even unsorted raw materials could be used for popping. The startup/heatup time and shutdown time of the popping are rather instantaneous unlike other popping machines which need at least 25–30 min. The present continuous popping machine using flue gas/hot air was found suitable for popping of the wide variety of raw material like maize, paddy, rice, and sorghum, which is not possible in other types of popping machines.
The total heat (sum of sensible heat and latent heat) absorbed during popping and puffing grains was calculated using the Eq. (4). The contribution of sensible and latent heat varied from 27–37 to 69–74% as shown in Table for the grains employed in the present study. The thermal efficiency of the fluidized popping/puffing machine was calculated considering the total heat absorbed during popping/puffing and total heat supplied by LPG and the values are given in Table . Recirculation of hot air will definitely increase the thermal efficiency of a given machine (Murthy et al. 2008) and this possibility can be explored for the continuous popping and puffing machine.
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