Design and development of a machine for continuous popping and puffing of grains

Abstract

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/m² °C and 0.124–0.162 m/s, respectively. Theoretical values for total heat and mass transfer were similar to the experimental values.

Electronic supplementary material

The online version of this article (10.1007/s13197-020-04680-3) contains supplementary material, which is available to authorized users.

Introduction

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.

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.

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).

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).

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.

Theoretical aspects: heat and mass transfer

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.

In fluidized bed popping machine, the flue gas/hot air fluidizes the grains and convection is the major mode of heat transfer. The equation for total heat transfer (McCabe et al. 1993) to the grains (Q_T) can be written as,

$${\text{Q}}_{\text{T}}={\text{h}}_{\text{F}}{\text{A}}_{\text{P}}({\text{T}}_{\text{I}}-{\text{T}}_{\text{P}})$$ 1

where h_F is heat transfer coefficient between fluidized particles and surrounding heating medium (W/m² °C), A_P area of heat transfer surface (m²), 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.

For heat transfer between a flowing fluid and the surface of a single sphere (for 1 < N_Re < 70,000 and 0.6 < N_Pr < 400), the Nusslet number can be written as (Singh and Heldman 2001)

$${\text{N}}_{\text{Nu}}=2+0.60{\text{N}}_{\text{Re}}^{0.5}{\text{N}}_{Pr}^{0.33}$$ 2

where the Nusslet number N_Nu = hd_p/K; Reynolds number N_Re = ρd_pv/µ; Prandtl number N_Pr = C_pµ/K and h is convective heat transfer coefficient (W/m² °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/m²), v velocity (m/s) and ρ is density (kg/m³). The bulk fluid properties of the hot air are calculated at average temperature from the following equation (Geankoplis 1993).

$${\text{T}}_{\text{f}}=\frac{{\text{T}}_{\text{R}}+{\text{T}}_{\text{M}}}{2}$$ 3

where T_f is average bulk fluid temperature, T_R room temperature (°C) and T_M is temperature of medium (°C).

The total heat absorbed during the popping/puffing operations can be calculated from the following equation (Singh and Heldman 2001)

$${\text{Q}}_{\text{A}}=[\{{\text{W}}_{\text{m}}{\text{C}}_{\text{pw}}({\text{T}}_{\text{v}}-{\text{T}}_{\text{R}})\}+\{{\text{W}}_{\text{p}}{\text{C}}_{\text{pg}}({\text{T}}_{\text{p}}-{\text{T}}_{\text{R}})\}+\{{\text{W}}_{\text{d}}{\text{C}}_{\text{pg}}({\text{T}}_{\text{d}}-{\text{T}}_{\text{R}})\}+\{\text{L}\times {\mathrm{\lambda }}_{\text{v}}\}$$ 4

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).

Three different types of mass transfer are common in gas fluidized bed operations (Beek 1971). The mass transfer occurs between

1. Fluidized bed and a wall or an object in the bed.

2. Bed particles and the fluidizing medium.

3. Bed particles and the gas bubbles in a bubbling bed.

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

$$\frac{{\text{k}}_{\text{P}}}{\text{U}}\times \mathrm{\epsilon }{\left[\frac{\mathrm{\mu }}{\mathrm{\rho }\text{D}}\right]}^{2/3}={(1-\mathrm{\epsilon })}^{0.5}{\left[\frac{{\text{Ud}}_{\text{P}}}{\text{V}}\right]}^{-0.5}$$ 5

where U is fluidization velocity (m/s), d_p diameter of the particle (m), ν kinematic viscosity of the hot gas (Ns/m²), ε voidage, D diffusion coefficient of moisture in grains (m²/s) and k_p is mass transfer coefficient (m/s)

For the mass transfer factor (J_D), the correlation suggested by Chilton and Colburn (1934) can be written as (Chilton and Colburn 1934)

$${\text{J}}_{\text{D}}=\frac{{\text{k}}_{\text{p}}}{\text{U}}{\left[\frac{\mathrm{\mu }}{\mathrm{\rho }\text{D}}\right]}^{2/3}$$ 6

Food materials being porous, impose intra-particle (independent of fluidizing conditions) resistance to heat and mass transfer. As a result, the intra-particle heat and mass transfer resistances dominates the external heat and mass transfer resistance.

Materials and methods

Design and operation

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. 1a. 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).

Fig. 1.

a Fluidized popping and puffing machine (Orthogonal View). b Fluidized popping and puffing machine

Raw materials, pretreatment and capacity of processing

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.

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.

Physical properties of grains and determination of expansion ratio and popping/puffing efficiency

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.

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.

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.

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.

Design of fluidization chamber

In order to design suitable fluidization chamber it is desirable to arrive at the appropriate fluidization chamber diameter to meet the fluidization requirements (Fluidization velocity, Terminal velocity) of different grains during popping/puffing

The fluidization chamber diameter was calculated using the equation of continuity as described below.

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 ($\underset{\text{blower}}{\overset{\text{o}}{\text{Q}}}$ m³/s) was calculated using following equation (Smith 2007).

$$\underset{\text{blower}}{\overset{\text{o}}{\text{Q}}}={\text{A}}_{\text{blower}}\times {\text{V}}_{\text{blower}}$$ 7

where A_blower is cross sectional area of blower outlet (m²), v_blower is air velocity generated by blower (m/s)

Subsequently, the required diameter of the fluidization chamber (D_req) for different grains has been calculated by using the following equation (McCabe et al. 1993).

$${\text{D}}_{\text{req}}=\sqrt{\frac{4\underset{\text{blower}}{\overset{\text{o}}{\text{Q}}}}{\mathrm{\pi }{\text{v}}_{\text{T}}}}$$ 8

where v_T is the Terminal velocity of a given grain (m/s).

Since the grains are to be fluidized (for popping/puffing) using hot air, revised volumetric flow rate ($\underset{\text{bulk}}{\overset{\text{o}}{\text{Q}}}$ m³/s) has been calculated using the properties of hot air at bulk fluid temperature using following equation (McCabe et al. 1993).

$$\underset{\mathrm{bulk}}{\overset{\text{o}}{\text{Q}}}=\frac{{\mathrm{\rho }}_{\mathrm{RT}}\times {\stackrel{\mathrm{o}}{\text{Q}}}_{\text{blower}}}{{\mathrm{\rho }}_{\mathrm{bulk}}}$$ 9

where ρ_RT and ρ_bulk are density of air (blower) at room temperature (kg/m³) and at bulk temperature (kg/m³), respectively.

Accordingly, diameter of the fluidization chamber (D_fc) for popping/puffing with hot air was calculated as

$${\text{D}}_{\text{fc}}=\sqrt{\frac{4\underset{\text{bulk}}{\overset{\text{o}}{\text{Q}}}}{\mathrm{\pi }{\text{v}}_{\text{T}}}}$$ 10

Terminal velocity

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).

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.

Minimum fluidization velocity

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).

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.

Carry over velocity

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.

Hot air velocity and temperature

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.

Heat balance and thermal efficiency (η) of machine

The sensible heat (Q₁) and the latent heat (Q₂) of different grains of given quantity (rice, maize, jowar and paddy) were estimated and total heat absorbed (Q_A) was obtained by adding Q₁ and Q₂.

Total heat supplied (Q_T) was calculated using the equation

$${\text{Q}}_{\text{T}}=\left(\text{m}\times \text{Calorific}\text{value}\text{of}\text{LPG}\right)$$ 11

where m is the weight of LPG (kg) consumed for the given quantity of grain and Calorific value of LPG used was 48,651 kJ/kg

Thermal efficiency (η) of machine was calculated using the equation

$$\mathrm{\eta }=\frac{{\text{Q}}_{\text{A}}}{{\text{Q}}_{\text{T}}}\times 100$$ 12

Performance evaluation and energy efficiency of the machine

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.

Results and discussion

Design of fluidization chamber

The physical properties of the grains such as voidage, bulk density were estimated, and the results are presented in Table 1a. Fluidization velocity, carry over velocity, terminal velocity were determined, and the results are presented in Table 1b.

Table 1.

(a) Physical properties (b). Fluidization and Terminal Settling Velocities, of different grains

(a)
Unexpanded grains											Expanded grains
Material	Feed rate (kg/h)	Moisture content (%)	Diameter of grain particle (m)	Weight (kg)	Volume of grain in cylindrical flask (m3)	Volume of toluene (m3)	Voidage ε	Actual volume of grains taken (m3)	Overall surface area of grains (m2)	Bulk density ρ (g/cc)	Weight (kg)	Volume (m3)	Bulk density ρ (kg/m3)
Rice	10.50	10	2.00 × 10−03	0.05	8.00 × 10−05	4.40 × 10−05	0.55	3.60 × 10−05	10.8 × 10−02	620	103 × 10−3	660 × 10−6	0.15 × 103
Maize	18	13	6.30 × 10−03	0.04	5.20 × 10−05	2.10 × 10−05	0.40	3.10 × 10−05	2.95 × 10−02	770	30 × 10−3	1000 × 10−6	0.03 × 103
Jowar	9.8	12	3.50 × 10−03	0.05	7.20 × 10−05	3.50 × 10−05	0.47	3.70 × 10−05	6.34 × 10−02	700	85 × 10−3	550 × 10−6	0.15 × 103
Paddy	11.25	10	2.50 × 10−03	0.05	10.00 × 10−05	5.60 × 10−05	0.58	4.40 × 10−05	10.6 × 10−02	500	80 × 10−3	1000 × 10−6	0.08 × 103

(b)
Material	Fluidization velocity (m/s)	Carryover velocity (expanded Grain) (m/s)	Terminal velocity (m/s)
			Unexpanded	Expanded
Rice	4.18	2.15	5.47	3.30
Maize	5.78	5.78	5.33	2.66
Jowar	5.57	6.18	4.80	4.20
Paddy	4.31	2.19	5.60	3.60

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 1b. 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 1b. The volumetric air flow rate ($\underset{\text{blower}}{\overset{\text{o}}{\text{Q}}}$) 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 ($\underset{\text{bulk}}{\overset{\text{o}}{\text{Q}}}$) 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. 1b.

Popping and puffing

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 1a), 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 1b. 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).

Heat transfer

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 2. 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 3 and 4 are compared with the experimental results in Table 5. 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 5.

Table 2.

Convective heat transfer coefficient

Material	Feed rate (kg/h)	Feed material (kg) Feed Unexpanded Expanded		Hot air temp. (°C) Inlet Outlet		Temp. of expanded material (°C)	Moisture loss (kg)	Expansion RATIO	Bed height (m)	LPG consumed (kg/h)	Convective heat transfer coefficient (W/m2 °C)
Puffing
Rice	10.50	1.83	6.56	250	190	72	1.52	6.37	0.020	1.25	186.81
Popping
Maize	18	1.92	14.4	230	160	60	1.68	10.42	0.020	1.50	103.84
Jowar	9.8	0.98	7.35	235	194	85	1.47	12.50	0.020	1.25	154.86
Paddy	11.25	2.25	7.31	296	240	70.5	1.68	10.25	0.020	1.50	166.83

Table 3.

Heat balance and thermal efficiency

Material	Total Sensible heat absorbed Q1 (kJ)	Total latent heat absorbed Q2 (kJ)	Total heat absorbed QA = Q1 + Q2 (kJ)	Total heat supplied QT = (m × Calorific value of LPG*) (kJ)	Thermal Efficiency of fluidized popping and puffing machine $\frac{Q_A}{Q_T}\times 100$ (%)
Puffing
Rice	1275.3 (27.12%)†	3429.12 (72.88%)†	4704.72	60,814.90	7.73
Popping
Maize	1672.47 (30.61%)†	3790.68 (69.38%)†	5463.15	72,977.88	7.40
Jowar	1672.47 (36.81%)†	3317.29 (72.16%)†	4596.77	60,814.90	7.55
Paddy	1319.65 (25.82%)†	3791.18 (74.17%)†	5110.83	72,977.88	7.00

m Mass of LPG (kg)

^†Relative percentage, *Calorific value of LPG is 48,651.92 kJ/kg

Table 4.

Mass transfer coefficient and mass transfer factor

Material	Diameter of the particle dp (m)	Mass transfer coefficient Kp (m/s)	Mass transfer factor JD (Ns/kg)
Rice	2.0 × 10−03	0.152	2.13 × 10−03
Maize	6.3 × 10−03	0.124	0.585 × 10−03
Jowar	3.5 × 10−03	0.154	1.11 × 10−03
Paddy	2.5 × 10−03	0.154	2.00 × 10−03

Table 5.

Comparison of theoretical and experimental values for heat and mass transfer

Grains	Weight of raw material (kg)	Heat transfer			Mass transfer
		ΔT TI-[(TR + TP)/2]	Theoretical Q (kJ)	Experimental Q (kJ)	Cg	Ca	ΔC (Cg-Ca)	Theoretical mass transfer N	Experimental mass transfer (kg/s) N
Rice	10.5	200.5	4055.27	4704.72	1.39 × 102	0.0304	1.39 × 102	2.43	4.23 × 10−03
Maize	18	186.5	572.83	5463.15	1.68 × 102	0.0304	1.68 × 102	0.614	3.38 × 10−03
Jowar	9.8	179	1763.14	4596.77	1.62 × 102	0.0304	1.62 × 102	1.58	4.95 × 10−03
Paddy	11.25	247.5	4385.62	5110.83	1.14 × 102	0.0304	1.14 × 102	1.85	3.52 × 10−03

Mass transfer

Estimation 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 4. 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 4). 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 5. It can be attributed to the very low volume of material used for experimentation (about 0.87 kg).

Performance evaluation and energy efficiency of the machine

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 3 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 3. 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.

Conclusion

A continuous popping and puffing machine (rated capacity of 10–20 kg/h) was designed and developed employing fluidization by hot air, flexible enough to handle a wide variety of raw material (maize, paddy, rice, sorghum etc.) hygienically in a continuous mode. The fluidization velocities, terminal velocities and carry over velocities of these grains before and after popping/puffing were measured and used to determine appropriate fluidization chamber diameter (to meet the fluidization requirements of different grains during popping/puffing) and arrived to be 0.15 m. The experimental values of the calculated heat (103–187 W/m² °C) and mass transfer coefficients (0.124–0.162 m/s) were compared with the theoretical values and were found to be of comparable order of magnitude except for maize. The thermal efficiency of the popping/puffing machine was found to be 7–8% for both popping and puffing. The machine was evaluated for its performance over the designed feed rate and was found to be satisfactory with respect to the popping/puffing efficiency (73–88%) besides offering product of good expansion ratio (6.37–12.50) with minimum un-popped material left (11–23%). The machine was found to be able to handle different grains satisfactorily.

Electronic supplementary material

Below is the link to the electronic supplementary material.

List of symbol

λ_v

Latent heat of evaporation (kJ/kg)

µ

Viscosity of the flue gas (Ns/m²)

A_blower

Cross sectional area of Blower outlet (m²)

A_P

Area of heat transfer surface (m²)

C_p

Specific heat of flue gas (kJ/kg °C)

C_Pw

Specific heat of water (kJ/kg °C)

C_Pg

Specific heat of grain (kJ/kg °C)

D

Diffusion coefficient of moisture in grains (m²/s)

D_fc

Diameter of the fluidization chamber (m)

d_p

Diameter of the particle (m)

D_req

Diameter required at ambient temperature (m)

h

Heat transfer coefficient (W/m² °C)

h_F

Heat transfer coefficient between fluidized particles and surrounding heating medium (W/m² °C)

J_D

Mass transfer factor

K

Thermal conductivity of the hot air (W/m °C)

k_p

Mass transfer coefficient (m/s)

L

Weight of the moisture loss (kg)

m

Weight of LPG (kg)

η

Thermal efficiency of machine

Nu

Nusselt number

Pr

Prondtl number

Q_A

Total heat absorbed (kJ)

$\underset{\text{Blower}}{\overset{\text{o}}{\text{Q}}}$

Volumetric flow rate of air at blower outlet (m³/s)

$\underset{\text{Bulk}}{\overset{\text{o}}{\text{Q}}}$

Volumetric flow rate of air at bulk temperature (m³/s)

Q_T

Total heat transfer to grains (kJ)

Re

Reynolds number

T_d

Temperature of unexpanded grain (°C)

T_P

Temperature of expanded grain (°C)

T_f

Average bulk fluid temperature (°C)

T_I

Inlet hot air temperature (°C)

T_M

Temperature of medium (°C)

T_R

Room temperature (°C)

T_v

Temperature of vaporization (°C)

U

Minimum fluidization velocity (m/s)

Ν

Kinematic viscosity of the flue gas (Ns/m²)

v

Velocity of the inlet air (m/s)

v_blower

Air velocity generated by blower (m/s)

V_T

Terminal velocity (m/s)

W_m

Moisture content of grain (kg)

W_d

Weight of the unexpanded grain (kg)

W_p

Weight of the expanded grain (kg)

ε

Voidage

ρ

Density of the flue gas (kg/m³)

ρ_Bulk

Density of air at bulk temperature (kg/m³)

ρ_RT

Density of air at room temperature (kg/m³)