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Development and performance evaluation of a continuous rice cooker
Journal of Food Engineeting 27 (1996) 371-387 Cowrinht 0 1996 Elsevier Science Limited Pr&ed-in Great Britain. All rights reserved 0260.8774196$KOO+O.OO 0260-8774(95)00017-8
Development and Performance Evaluation of a Continuous Rice Cooker M. N. Ramesh” Food Engineering
(Received
& P. N. Srinivasa Rao
Department, Central Food Technological Mysore 570013, India
Research
Institute,
28 June 1994: revised version received 14 March 1995; accepted 27 March 1995)
ABSTRACT A continuous food cooker was developed and the peformance evaluation of the unit was carried out. The parameters were standardized for 30 kg of rice per haul: The final product was evaluated in terms of moisture pick up, volume expansion factor with reference to the milled rice, total percentage yield, stickiness of the product using an Instron tester and microbial analysis for the commercial acceptance of the product. The percentage loss of solids in the overjlowing water at the outlet of the cooker was also determined. These product characteristics were compared with the laboratory scale experimental results available in the literature. A preliminary cost estimate of cooking rice at pilot scale is found to be $0.17 per kg. However; with necessary sealing at the outlet of the cooker; the cost can be reduced to $0.1 per kg.
NOTATION
2 m 9 T
Specific heat (kJ/kg “C) Enthalpy (kJ/kg) Mass flow rate (kg/h) Heat transfer rate (kJ/s) Temperature (“C)
Subscripts C Liquid/condensate *To whom correspondence
should be addressed. 377
M. N. Ramesh, P N. Srinivasa Rao
378 i 0
s V
Initial Outer Steam Saturated
vapour
INTRODUCTION Catering is developing rapidly with the spread of lunch services in large firms and industries, central kitchens for chain restaurants and the lunch supply scheme for schools. Accordingly, the present age requires large food cooking plants. Presently, batch type or semicontinuous cooking units using jacketed kettles heated by steam are adopted in institutional cooking. Since the texture varies between batches a uniform product as desired by a consumer cannot be consistently prepared. These units are labour oriented and are not economical when large scale applications are envisaged. Due to batch operations the hygeine is also poor (Glew, 1983). Furthermore, the large scale cooking process is usually controlled by a chef, not by a food technologist and the process of cooking is regarded as an art not a science. Hence, the chefs control over the cooking conditions depends on the degree of development of his skills and not on the use of scientific instruments (Glew, 1983). The present study reports the development and performance data of a continuous food cooker. So far, the food cooker has been standardized for cooking rice.
MATERIALS
AND METHODS
Description of the cooker A schematic diagram of the rice cooker is shown in Fig. 1 and the two end views are shown in Fig. 2. The equipment is a continuous conveyor with a facility for open steaming into the chamber. A water inlet is provided through a flow meter to add a measured quantity of water during processing. The chamber is steam jacketed for additional heating. A variable speed drive is used to vary the residence time of cooking. A rotary valve is fixed at the inlet end to control the material feed rate. The conveyor speed and the rotary valve speed are matched with sprocket and chain drive. A stationary water draining device having a stainless steel trough, stainless steel and nylon sieving screens is installed at the discharge end of the machine, which largely helps quick separation of water from cooked grains (patent accepted). Operation of the cooker (for rice processing) Milled rice, of Sona masuri, a high amylose variety, was gently washed to remove dust and adhering bran but not excessively to conserved vitamins and minerals. The washed rice was soaked for 30 min to equilibrate to a suitable moisture content of preferably 30% w.b. and the excess water drained. From preliminary studies on soaking 10 g sample at room
Development and performance evaluation of a continuous rice cooker
Rice cooker cap: SOkglhour
Fig. 1.
I
\
Water li (n-
“‘-A Steam
line
View A
View B
Fig. 2.
/
379
M. N. Ramesh, R N. Srinivasa Rao
380
temperature for various periods ranging from 5 to 35 min it was determined that, beyond 30 min of soaking, there is no considerable change in the moisture pick up. Water was fed into the cooker at a measured rate for the entire process (flow rate depending upon the output and variety) and steam was let into the system by opening the steam valves at controlled pressures. Steam supply was maintained at a constant rate for both the spreader and the jacket. Careful control of steam pressure is necessary to obtain a very good product. Excessive steam pressures will lead to scorching and lower pressures to uncooked products. The system was allowed to stabilise at the operating conditions such as temperature of outlet water at 95-96°C constant discharge rate of condensate. The water at the outlet of the cooker was collected to determine the steam flow rate. The condensate at the jacket outlet was also collected. The sum of the two gave the total steam flow rate. During cooking the come-up time of rice was about 6-8 min depending on the variety, whether waxy or non-waxy. The residence time was set through the variable speed drive according to the rice variety. The soaked rice was continuously fed from the hopper through the rotary valve at the required rate. At the end of the set residence time uniformly cooked rice was discharged through the outlet chute. The product was collected from the mesh and was ready for consumption. Methodology of product evaluation (a) Moisture analysis
The mositure content increase after 10 min of discharge of the material was determined by drawing samples every 10 min; the range of moisture ratio determined is reported. The samples were weighed using a Mettler digital balance and kept in an oven at 110°C for 8 h. The weight after drying was determined and the moisture content was evaluated:
MC&,=
weight before drying -weight
after drying
weight before drying
x 100
(b) Volume expansion factor
The degree of swelling is a significant quality parameter and is closely related to the moisture content. The amount of swelling can be expressed in terms of the volume expansion factor (VU’) which is the ratio of the swollen kernel to its non-swollen volume (Mercer et al., 1985) I/EF=
]( mi l1e d rice density) x (1.0 - (% M&,/100))] [(cooked rice density) x (1.0 - (% M&,/100))]
Percentage moisture in the numerator is of milled rice, percentage moisture in the denominator is of cooked rice. Moisture content was evaluated as described in (a) above. The density was determined as described below. A portion (10 g) of the sample (milled/cooked rice) was weighed in a
Development and performance evaluation of a continuous rice cooker
381
Mettler digital balance and added to a graduated cylinder with kerosene of known initial volume. The final volume of kerosene and the sample was noted: density=
final volume Ynitial
volume gicm”
(c) Percentage yield
The initial weight of the milled rice before soaking was determined. total cooked rice obtained at the outlet of the cooker was measured: % yield=
(d) Stickiness
with Instron
final weight initial weight
The
x 100
tester
An objective method of measuring the surface stickiness of cooked rice with an Instron tester (Model 4301) was used. The Instron tester consists of a stationary bottom plate and upper movable cross head. The movable cross head is connected to a load cell to record the force exerted on the sample. A portion (10 g) of the cooked sample was placed in a stainless steel cylinder of dimensions 50 mm dia. and 75 mm height. A plunger was attached to the upper movable cross head. The plunger moves in close tolerance within the sample cylinder. The sample cylinder was placed on the stationary plate and cooked rice was compressed to 50% of its original height, at a rate of 50 mm/min. The on-line deformation graph was obtained through a computer interfaced with the instrument. After compressing, the plunger was retracted and the cycle was repeated. The type of graph obtained is as shown in Fig. 3. The area of the negative part of the graph below the x-axis indicates the Instron stickiness value (Mossman et al., 1983).
4
I
-1 0
10
I 20
I 30
I 40
I 50
I 60
70
Time (s)
Fig. 3. Ins&on profile. Cooking conditions: material feed rate=05 kgimin; water feed rate=4 litres/min; steam feed rate= 120 kg/h. Product characteristics: moisture content=75% w.b.; volume expansion factor=4.5.
M. N, Ramesh, I! N. Srinivasa Rao
382
(e) Microbial analysis The total number of counts of colis and mesophilic
spores were determined from three samples. One at the beginning of the material discharge at the outlet, one after 15 min of feeding the soaked rice into the cooker and one at the end of the experiment. This evaluation was carried out to ensure the safety of the product produced from the cooker and to determine whether there is any contamination due to direct steam injection.
(f) Percentage
solid loss
The drained water from the cooker was collected at different intervals. The initial weight of water with the solids (lost) was determined using a Mettler digital balance. The water was evaporated by keeping the sample in an oven at 110°C for 10 h. The weight of the dry solids after drying was determined: % solid loss=
initial weight -final
RESULTS
weight
initial weight
x 100
AND DISCUSSION
Steam inlet pressure and water inlet ratio were varied for different trials to determine the optimum rice water feed ratio required for complete and uniform cooking. Table 1 gives the performance data. Experiments were also conducted with the following variations: (a) (b) (c) (d) (e)
without soaking rice, without allowing steam into the jacket of the conveyor chamber, with a varying residence time range of 15-20 min, with a reduced water feed ratio of less than 3.5 litres/min, without adding water during cooling.
But the cooking was incomplete and nonuniform. Hence, these results are not reported. From Table 1 it can be observed that the water feed ratio is 8 times the material feed rate. There are two reasons for this. One is that excess water is required to keep the cooked rice floating so as to be easily conveyed. Otherwise, the cooked rice will mash. Second is that complete gelatinisation needs excess water. On cooking, rice takes up about 2.5 times water and attains a moisture content of 75% w.b. (Battacharya & Sowbhagya, 1971). Also, by cooking in excess water, the cooked rice becomes fluffy, non-sticky and soft which is an Indian consumer preference (Desikachar, 1980). It can be observed from Table 1 that the average steam consumption is 120 kg/h, which is very high. This is due to the escape of steam from unsealed outlets which was observed to be considerable. A similar observation has been reported by Elaine P. Scott et al. (1983) on the study of energy consumption in steam blanchers. It has been reported that the losses range from 80% for blanchers with no end seal and was reduced to 60% with water curtains. Attempts are being made to reduce the steam loss by providing a suitable steam locking system at the outlet, without impairing the product characteristics and discharge.
3.7
0.5
;::
05
1
:
4
(kglmin)
2
(kglmin)
05
0.5 0.5 0.5
1
2 3 4
170 180 180
160
(kglh)
Steam feed rate
120
100 120
Steam feed rate (kglh)
74-76
1.00
68-70
73-75 74-76 74-76
2.5-3.0 3.0-3.5 3.0-3.5
450
3.99 4.32
4.14
Volume ex~~$r
275 300 300
250
Total Te&l 0
4.3 4.5 4.5
3.7
Nil Nil Nil Nil
Znstron stickck;s
Rice Cooker”
Nil
Nil Nil
Instron stickck;s
Rice Cooker0
Volume e~~;~n&on
TABLE 2 Data on Continuous
MC of cooked n’ce (70 w.b.)
2.0-2.5
Steam pressure (kglm ‘)
Performance
2.50 275 300
300
72-74
Total $ly 0
68-76 73-75
MC of cooked n’ce (70 w.b.)
TABLE 1 Data on Continuous
0.85 11.00 .oo
Steam pressure (kg/m ‘)
’ Rice variety: Bangara Tega rice and Basmati rice.
3 4 4
Water feed rate
Material feed rate
SL no.
u Rice variety: Sona masuri.
4.0
4.3 5.0
Water feed rate (kglmin)
Material feed rate (kglmin)
SL
IZO.
Performance
Not cooked No uniform cooking Cooking complete Uniform cooking
Observation
No uniform cooking Water ratio not is higher Cooking complete Uniform cooking
Observation
Not acceptable Not acceptable Acceptable Acceptable
Remarks
Not acceptable Not acceptable acceptable Acceptable
Remarks
384
IV. N. Ramesh, P N. Srinivasa Rao
The moisture increase is about 74-77% w.b. and the total yield is about 300%. This is in accordance with the observations made by Battacharya and Sowbhagya (1971) and Sowbhagya et al. (1994). They have used a sample size of 10 g and quoted that any rice (with a moisture content of about 13% w.b.) when optimally cooked, absorbs about 25 times its weight of water and attains a moisture content of 75% w.b. The volume expansion ratio was found to be about 4.5. This is in accordance with the observation made by Mercer et al. (1985) that at 75% w.b. moisture content, the swelling is about 4.40. The volume expansion is in two stages, viz. after soaking and after cooking. The volume expansion ratio due to soaking is 1.6-1.8 and due to cooking is 2.6-2.8. Hence, the volume expansion ratio due to processing is 2.6-2.8. The stickiness measured using an Instron tester was much lower as evident from the graph shown in Fig. 3. The solid content in excess water at the outlet of the cooker was negligiable, i.e. it was less than 1%. However, recirculating the excess water will further reduce the loss as there is no loss of water from the system. Microbial analysis indicated the commercial acceptance of the product. However, a deviation from the literature value and the present study was observed in the time of cooking. According to Lund (1984) and Suzuki et al. (1976) the time required for complete cooking at 75°C and above is only 3 min. But, the time required in the present study for complete cooking at 95-96°C was about 18-20 min. However, Suvendu Battacharya (1993) has also reported that cooking of 800 g of raw rice in a pressure cooker requires about 20 min. One probable reason may be that the sample size in those studies were about 2-5 g using differential scanning calorimetry and a parallel plate plastometer. Both are analytical instruments which can be applied for the basic studies. To cross check this fact, 1 kg of rice was cooked in open steam at 95-96°C in a steel vessel and the time required for cooking was determined. This showed similar results to those of the continuous cooker. Overall residence time of cooking was maintained within a range of 18-20 min. A residence time of less than 18 min resulted in undercooking and more than 20 min caused overcooking. It can be observed from Table 1 that for the same conditions with only the water feed ratio decreasing, the moisture content had a different trend. This is because excess water feed ratio moves the rice before it is completely cooked and hence leads to lower moisture pick up. At the same time with a lower water feed ratio there is not enough moisture for complete gelatinisation. Hence the most suitable water feed ratio is 1: 8. Table 2 shows similar results for the cooking of Basmati rice and Bangara Tega rice. Basmati is a waxy variety and Bangara Tega is a non-waxy variety.
CONCLUSIONS A continuous rice cooker was developed and its performance was evaluated. The final product was analysed and the results were compared with the experimental values of the literature. The standardised parameters were
Development and performance
evaluation of a continuous rice cooker
385
checked by repeated trials. The performance of the cooker gave consistent results. The cooker developed can be used in lunch services in large firms and industries, central kitchens for chain restaurants and the lunch supply services. The unit is simple to operate and does not need any skill to get consistently acceptable products. The art of cooking can be shifted from the chef on to the cooker. The unit is versatile as different varieties of rice can be accommodated by suitable adjustment of operating parameters. The process does not need any pretreatment except soaking which is very economical.
ACKNOWLEDGEMENTS The authors wish to thank Shri A. Ramesh, Head, Process Engineering & Plant Design for his encouragement and Smt Nirmaladevi of the Microbiology department for microbial analysis. We thank Smt Prapulla and her colleagues of the Bioengineering Laboratory and Mr Kumar of Central Instrumentation for providing facilities for product evaluation. Special thanks are due to Mr V. N. Subbarao of the Pilot Plant for technical assistance
during
experimental
studies.
REFERENCES Battacharya, K. R. & Sowbhagya, C. M. (1971). Water uptake by rice during cooking. Cereal Sci. Today, 16 (12), 420-4. Battacharya, S. (1993). Energy economy in food processing - cooking. In Food Processing, ed. V. H. Potty. Oxford & IBH Publishing Co., New Delhi. Desikachar, H. S. R. (1980). Three decades of research on the processing and utilization of food grains. J. Food Sci. Technol., 17 (1, 2) 24-32. Glew, G. (1983). Industrial cooking - Introduction - Preparation, warm holding and reheating of foods in catering. In Thermal Processing and Quality of Foods, ed. Zeuthen et al. EASP, New York, pp. 368-70. Lund, D. (1984). Influence of time, temperature, moisture, ingredients and processing conditions on starch gelatinization. CRC Crit. Rev. Food Sci. Nutrit., 20 (4) 249-73.
Mercer, D. G., Sirett, R. R. & Maurice, T. J. (1985). Mathematical modelling of rice kernel expansion as a function of hydration. In Food Engineering and Process Application, Vol. 1, Transport Phenomena, ed. Le Maguer. EASP, New York, paper No. 32, pp. 347-53. I. Mossman, A. P., Fellers, D. A. & Suzuki, H. (1983). Rice stickiness. Determination of rice stickiness with an Instron tester. Cereal Chem., 60 (4), 286-92.
Scott, E. P., Ccarroad, P. A., (1983). Energy consumption Singh, R. & Heldman, R. D. Food Engineering, 2nd edn, New York, pp. 104-5. Sowbhagya, C. M., Ramesh, solubility behaviour of rice
Rumsey, T. R., Horn, J., Buhlert, J. & Rose, W. W. in steam blanchers. J. Food Proc. Engng, 5, 77-88. (1993). Energy for food processing. In Introduction to Chapter 3. Academic Press, Harcourt Brace & Co., B. S. & Ali, S. Z. (1994). Hydration, swelling and in relation to other physicochemical properties. J. Sci.
M. N. Ramesh, ff N, Srinivasa Rao
386
Food Agric., 64, l-7. Suzuki, K., Kubota, K., Omichi, M. & Hosaka, H. (1976). Kinetic studies on cooking of rice. J. Food Sci., 41, 1180-3.
Togeby, M., Hansen, N. & Mosekiide, E. (1986). Modelling energy consumption, loss of firmness and enzyme inactivation in an industrial blanching process. 1 Food Engng, 5, 251-67.
APPENDIX
A
Actual steam requirement for cooking (Singh & Heldman, 1993) Product flow rate Specific heat of rice (based on potato at 80% w.b. moisture content) Initial product temperature Final product temperature Steam quality Steam temperature (selected to assure a minimum temperature gradient of 5°C between steam and product) For a steam temperature
30 kg/h 3-68 kJ/kg
25°C 95°C 90% 100°C
of lOO”C, pressure will be 101.35 kPa, and
Enthalpy H,=419*04 kJ/kg,
H,=2676*1 kJ/kg
The thermal energy requirements for the product the mass flow rate of steam required.
will be used to establish
(1) Thermal energy requirement q=mc,(To-Ti)=30(3*68)(100-25)x82
800 kJ/h
For 85% heat exchange efficiency. q=82 800/O-85=97 411.765 kJ/h (2) For steam quality of 90% H=(0*1)419.04 (3) The thermal will be
+ (O-9)2676*1 =2450.394 W/kg
energy content
of condensate
leaving the heat exchanger
H,=specific heat of water x steam temp. =4.211(100)=421.1 kJ/kg (4) Since the thermal energy provided by steam will be qs=m2(h -K) this must match the steam requirements,
then
m,=97 411*765/(2450*394-421.1)=48*003 with 25% radiation and conduction
kg/h
losses, losses=48(0*25)=12
kg/h.
Development and performance
evaluation of a continuous rice cooker
387
Total actual steam requirement=48+ 12=60 kg/h. Hence, with respect to Table 2, it can be cited that the steam requirement can be reduced by 50%.
APPENDIX Specific energy consumption
B
of continuous cooker
120 kg/h 30 kg/h
Steam consumption Material feed rate
Steam consumption per kg of material= 120/30=4 kg/kg Assuming that steam loss in the worst case is 80% (Scott et al., 1982). Actual steam requirement=4/1*8=2*22 kg/kg With 80% heat transfer efficiency steam requirement
=2.22(0*8) = 1.776 kg/kg
latent heat of steam=504 Therefore
kJ/kg
specific energy consumption = l-776(504) =895-104 kJ/kg
=885*104 x 10” J/kg=O*9 x 10’ J/kg=0.9 x 10’ J/t This is in accordance with the value reported by Togeby et al. (1986). They have reported that a steam blancher without end seals was found to have a specific energy consumption of 2-l GJ/t of product. With water curtains to reduce the leakage of live steam through the blancher ends, the specific energy consumption was reduced to 1.6 GJ/t, while the specific energy consumption for a steam blancher with hydrostatic water seals was found to be O-95 GJ/t. Thus, the continuous cooker developed is comparable to that with hydrostatic end seals.
Development and Performance Evaluation of a Continuous Rice Cooker M. N. Ramesh” Food Engineering
(Received
& P. N. Srinivasa Rao
Department, Central Food Technological Mysore 570013, India
Research
Institute,
28 June 1994: revised version received 14 March 1995; accepted 27 March 1995)
ABSTRACT A continuous food cooker was developed and the peformance evaluation of the unit was carried out. The parameters were standardized for 30 kg of rice per haul: The final product was evaluated in terms of moisture pick up, volume expansion factor with reference to the milled rice, total percentage yield, stickiness of the product using an Instron tester and microbial analysis for the commercial acceptance of the product. The percentage loss of solids in the overjlowing water at the outlet of the cooker was also determined. These product characteristics were compared with the laboratory scale experimental results available in the literature. A preliminary cost estimate of cooking rice at pilot scale is found to be $0.17 per kg. However; with necessary sealing at the outlet of the cooker; the cost can be reduced to $0.1 per kg.
NOTATION
2 m 9 T
Specific heat (kJ/kg “C) Enthalpy (kJ/kg) Mass flow rate (kg/h) Heat transfer rate (kJ/s) Temperature (“C)
Subscripts C Liquid/condensate *To whom correspondence
should be addressed. 377
M. N. Ramesh, P N. Srinivasa Rao
378 i 0
s V
Initial Outer Steam Saturated
vapour
INTRODUCTION Catering is developing rapidly with the spread of lunch services in large firms and industries, central kitchens for chain restaurants and the lunch supply scheme for schools. Accordingly, the present age requires large food cooking plants. Presently, batch type or semicontinuous cooking units using jacketed kettles heated by steam are adopted in institutional cooking. Since the texture varies between batches a uniform product as desired by a consumer cannot be consistently prepared. These units are labour oriented and are not economical when large scale applications are envisaged. Due to batch operations the hygeine is also poor (Glew, 1983). Furthermore, the large scale cooking process is usually controlled by a chef, not by a food technologist and the process of cooking is regarded as an art not a science. Hence, the chefs control over the cooking conditions depends on the degree of development of his skills and not on the use of scientific instruments (Glew, 1983). The present study reports the development and performance data of a continuous food cooker. So far, the food cooker has been standardized for cooking rice.
MATERIALS
AND METHODS
Description of the cooker A schematic diagram of the rice cooker is shown in Fig. 1 and the two end views are shown in Fig. 2. The equipment is a continuous conveyor with a facility for open steaming into the chamber. A water inlet is provided through a flow meter to add a measured quantity of water during processing. The chamber is steam jacketed for additional heating. A variable speed drive is used to vary the residence time of cooking. A rotary valve is fixed at the inlet end to control the material feed rate. The conveyor speed and the rotary valve speed are matched with sprocket and chain drive. A stationary water draining device having a stainless steel trough, stainless steel and nylon sieving screens is installed at the discharge end of the machine, which largely helps quick separation of water from cooked grains (patent accepted). Operation of the cooker (for rice processing) Milled rice, of Sona masuri, a high amylose variety, was gently washed to remove dust and adhering bran but not excessively to conserved vitamins and minerals. The washed rice was soaked for 30 min to equilibrate to a suitable moisture content of preferably 30% w.b. and the excess water drained. From preliminary studies on soaking 10 g sample at room
Development and performance evaluation of a continuous rice cooker
Rice cooker cap: SOkglhour
Fig. 1.
I
\
Water li (n-
“‘-A Steam
line
View A
View B
Fig. 2.
/
379
M. N. Ramesh, R N. Srinivasa Rao
380
temperature for various periods ranging from 5 to 35 min it was determined that, beyond 30 min of soaking, there is no considerable change in the moisture pick up. Water was fed into the cooker at a measured rate for the entire process (flow rate depending upon the output and variety) and steam was let into the system by opening the steam valves at controlled pressures. Steam supply was maintained at a constant rate for both the spreader and the jacket. Careful control of steam pressure is necessary to obtain a very good product. Excessive steam pressures will lead to scorching and lower pressures to uncooked products. The system was allowed to stabilise at the operating conditions such as temperature of outlet water at 95-96°C constant discharge rate of condensate. The water at the outlet of the cooker was collected to determine the steam flow rate. The condensate at the jacket outlet was also collected. The sum of the two gave the total steam flow rate. During cooking the come-up time of rice was about 6-8 min depending on the variety, whether waxy or non-waxy. The residence time was set through the variable speed drive according to the rice variety. The soaked rice was continuously fed from the hopper through the rotary valve at the required rate. At the end of the set residence time uniformly cooked rice was discharged through the outlet chute. The product was collected from the mesh and was ready for consumption. Methodology of product evaluation (a) Moisture analysis
The mositure content increase after 10 min of discharge of the material was determined by drawing samples every 10 min; the range of moisture ratio determined is reported. The samples were weighed using a Mettler digital balance and kept in an oven at 110°C for 8 h. The weight after drying was determined and the moisture content was evaluated:
MC&,=
weight before drying -weight
after drying
weight before drying
x 100
(b) Volume expansion factor
The degree of swelling is a significant quality parameter and is closely related to the moisture content. The amount of swelling can be expressed in terms of the volume expansion factor (VU’) which is the ratio of the swollen kernel to its non-swollen volume (Mercer et al., 1985) I/EF=
]( mi l1e d rice density) x (1.0 - (% M&,/100))] [(cooked rice density) x (1.0 - (% M&,/100))]
Percentage moisture in the numerator is of milled rice, percentage moisture in the denominator is of cooked rice. Moisture content was evaluated as described in (a) above. The density was determined as described below. A portion (10 g) of the sample (milled/cooked rice) was weighed in a
Development and performance evaluation of a continuous rice cooker
381
Mettler digital balance and added to a graduated cylinder with kerosene of known initial volume. The final volume of kerosene and the sample was noted: density=
final volume Ynitial
volume gicm”
(c) Percentage yield
The initial weight of the milled rice before soaking was determined. total cooked rice obtained at the outlet of the cooker was measured: % yield=
(d) Stickiness
with Instron
final weight initial weight
The
x 100
tester
An objective method of measuring the surface stickiness of cooked rice with an Instron tester (Model 4301) was used. The Instron tester consists of a stationary bottom plate and upper movable cross head. The movable cross head is connected to a load cell to record the force exerted on the sample. A portion (10 g) of the cooked sample was placed in a stainless steel cylinder of dimensions 50 mm dia. and 75 mm height. A plunger was attached to the upper movable cross head. The plunger moves in close tolerance within the sample cylinder. The sample cylinder was placed on the stationary plate and cooked rice was compressed to 50% of its original height, at a rate of 50 mm/min. The on-line deformation graph was obtained through a computer interfaced with the instrument. After compressing, the plunger was retracted and the cycle was repeated. The type of graph obtained is as shown in Fig. 3. The area of the negative part of the graph below the x-axis indicates the Instron stickiness value (Mossman et al., 1983).
4
I
-1 0
10
I 20
I 30
I 40
I 50
I 60
70
Time (s)
Fig. 3. Ins&on profile. Cooking conditions: material feed rate=05 kgimin; water feed rate=4 litres/min; steam feed rate= 120 kg/h. Product characteristics: moisture content=75% w.b.; volume expansion factor=4.5.
M. N, Ramesh, I! N. Srinivasa Rao
382
(e) Microbial analysis The total number of counts of colis and mesophilic
spores were determined from three samples. One at the beginning of the material discharge at the outlet, one after 15 min of feeding the soaked rice into the cooker and one at the end of the experiment. This evaluation was carried out to ensure the safety of the product produced from the cooker and to determine whether there is any contamination due to direct steam injection.
(f) Percentage
solid loss
The drained water from the cooker was collected at different intervals. The initial weight of water with the solids (lost) was determined using a Mettler digital balance. The water was evaporated by keeping the sample in an oven at 110°C for 10 h. The weight of the dry solids after drying was determined: % solid loss=
initial weight -final
RESULTS
weight
initial weight
x 100
AND DISCUSSION
Steam inlet pressure and water inlet ratio were varied for different trials to determine the optimum rice water feed ratio required for complete and uniform cooking. Table 1 gives the performance data. Experiments were also conducted with the following variations: (a) (b) (c) (d) (e)
without soaking rice, without allowing steam into the jacket of the conveyor chamber, with a varying residence time range of 15-20 min, with a reduced water feed ratio of less than 3.5 litres/min, without adding water during cooling.
But the cooking was incomplete and nonuniform. Hence, these results are not reported. From Table 1 it can be observed that the water feed ratio is 8 times the material feed rate. There are two reasons for this. One is that excess water is required to keep the cooked rice floating so as to be easily conveyed. Otherwise, the cooked rice will mash. Second is that complete gelatinisation needs excess water. On cooking, rice takes up about 2.5 times water and attains a moisture content of 75% w.b. (Battacharya & Sowbhagya, 1971). Also, by cooking in excess water, the cooked rice becomes fluffy, non-sticky and soft which is an Indian consumer preference (Desikachar, 1980). It can be observed from Table 1 that the average steam consumption is 120 kg/h, which is very high. This is due to the escape of steam from unsealed outlets which was observed to be considerable. A similar observation has been reported by Elaine P. Scott et al. (1983) on the study of energy consumption in steam blanchers. It has been reported that the losses range from 80% for blanchers with no end seal and was reduced to 60% with water curtains. Attempts are being made to reduce the steam loss by providing a suitable steam locking system at the outlet, without impairing the product characteristics and discharge.
3.7
0.5
;::
05
1
:
4
(kglmin)
2
(kglmin)
05
0.5 0.5 0.5
1
2 3 4
170 180 180
160
(kglh)
Steam feed rate
120
100 120
Steam feed rate (kglh)
74-76
1.00
68-70
73-75 74-76 74-76
2.5-3.0 3.0-3.5 3.0-3.5
450
3.99 4.32
4.14
Volume ex~~$r
275 300 300
250
Total Te&l 0
4.3 4.5 4.5
3.7
Nil Nil Nil Nil
Znstron stickck;s
Rice Cooker”
Nil
Nil Nil
Instron stickck;s
Rice Cooker0
Volume e~~;~n&on
TABLE 2 Data on Continuous
MC of cooked n’ce (70 w.b.)
2.0-2.5
Steam pressure (kglm ‘)
Performance
2.50 275 300
300
72-74
Total $ly 0
68-76 73-75
MC of cooked n’ce (70 w.b.)
TABLE 1 Data on Continuous
0.85 11.00 .oo
Steam pressure (kg/m ‘)
’ Rice variety: Bangara Tega rice and Basmati rice.
3 4 4
Water feed rate
Material feed rate
SL no.
u Rice variety: Sona masuri.
4.0
4.3 5.0
Water feed rate (kglmin)
Material feed rate (kglmin)
SL
IZO.
Performance
Not cooked No uniform cooking Cooking complete Uniform cooking
Observation
No uniform cooking Water ratio not is higher Cooking complete Uniform cooking
Observation
Not acceptable Not acceptable Acceptable Acceptable
Remarks
Not acceptable Not acceptable acceptable Acceptable
Remarks
384
IV. N. Ramesh, P N. Srinivasa Rao
The moisture increase is about 74-77% w.b. and the total yield is about 300%. This is in accordance with the observations made by Battacharya and Sowbhagya (1971) and Sowbhagya et al. (1994). They have used a sample size of 10 g and quoted that any rice (with a moisture content of about 13% w.b.) when optimally cooked, absorbs about 25 times its weight of water and attains a moisture content of 75% w.b. The volume expansion ratio was found to be about 4.5. This is in accordance with the observation made by Mercer et al. (1985) that at 75% w.b. moisture content, the swelling is about 4.40. The volume expansion is in two stages, viz. after soaking and after cooking. The volume expansion ratio due to soaking is 1.6-1.8 and due to cooking is 2.6-2.8. Hence, the volume expansion ratio due to processing is 2.6-2.8. The stickiness measured using an Instron tester was much lower as evident from the graph shown in Fig. 3. The solid content in excess water at the outlet of the cooker was negligiable, i.e. it was less than 1%. However, recirculating the excess water will further reduce the loss as there is no loss of water from the system. Microbial analysis indicated the commercial acceptance of the product. However, a deviation from the literature value and the present study was observed in the time of cooking. According to Lund (1984) and Suzuki et al. (1976) the time required for complete cooking at 75°C and above is only 3 min. But, the time required in the present study for complete cooking at 95-96°C was about 18-20 min. However, Suvendu Battacharya (1993) has also reported that cooking of 800 g of raw rice in a pressure cooker requires about 20 min. One probable reason may be that the sample size in those studies were about 2-5 g using differential scanning calorimetry and a parallel plate plastometer. Both are analytical instruments which can be applied for the basic studies. To cross check this fact, 1 kg of rice was cooked in open steam at 95-96°C in a steel vessel and the time required for cooking was determined. This showed similar results to those of the continuous cooker. Overall residence time of cooking was maintained within a range of 18-20 min. A residence time of less than 18 min resulted in undercooking and more than 20 min caused overcooking. It can be observed from Table 1 that for the same conditions with only the water feed ratio decreasing, the moisture content had a different trend. This is because excess water feed ratio moves the rice before it is completely cooked and hence leads to lower moisture pick up. At the same time with a lower water feed ratio there is not enough moisture for complete gelatinisation. Hence the most suitable water feed ratio is 1: 8. Table 2 shows similar results for the cooking of Basmati rice and Bangara Tega rice. Basmati is a waxy variety and Bangara Tega is a non-waxy variety.
CONCLUSIONS A continuous rice cooker was developed and its performance was evaluated. The final product was analysed and the results were compared with the experimental values of the literature. The standardised parameters were
Development and performance
evaluation of a continuous rice cooker
385
checked by repeated trials. The performance of the cooker gave consistent results. The cooker developed can be used in lunch services in large firms and industries, central kitchens for chain restaurants and the lunch supply services. The unit is simple to operate and does not need any skill to get consistently acceptable products. The art of cooking can be shifted from the chef on to the cooker. The unit is versatile as different varieties of rice can be accommodated by suitable adjustment of operating parameters. The process does not need any pretreatment except soaking which is very economical.
ACKNOWLEDGEMENTS The authors wish to thank Shri A. Ramesh, Head, Process Engineering & Plant Design for his encouragement and Smt Nirmaladevi of the Microbiology department for microbial analysis. We thank Smt Prapulla and her colleagues of the Bioengineering Laboratory and Mr Kumar of Central Instrumentation for providing facilities for product evaluation. Special thanks are due to Mr V. N. Subbarao of the Pilot Plant for technical assistance
during
experimental
studies.
REFERENCES Battacharya, K. R. & Sowbhagya, C. M. (1971). Water uptake by rice during cooking. Cereal Sci. Today, 16 (12), 420-4. Battacharya, S. (1993). Energy economy in food processing - cooking. In Food Processing, ed. V. H. Potty. Oxford & IBH Publishing Co., New Delhi. Desikachar, H. S. R. (1980). Three decades of research on the processing and utilization of food grains. J. Food Sci. Technol., 17 (1, 2) 24-32. Glew, G. (1983). Industrial cooking - Introduction - Preparation, warm holding and reheating of foods in catering. In Thermal Processing and Quality of Foods, ed. Zeuthen et al. EASP, New York, pp. 368-70. Lund, D. (1984). Influence of time, temperature, moisture, ingredients and processing conditions on starch gelatinization. CRC Crit. Rev. Food Sci. Nutrit., 20 (4) 249-73.
Mercer, D. G., Sirett, R. R. & Maurice, T. J. (1985). Mathematical modelling of rice kernel expansion as a function of hydration. In Food Engineering and Process Application, Vol. 1, Transport Phenomena, ed. Le Maguer. EASP, New York, paper No. 32, pp. 347-53. I. Mossman, A. P., Fellers, D. A. & Suzuki, H. (1983). Rice stickiness. Determination of rice stickiness with an Instron tester. Cereal Chem., 60 (4), 286-92.
Scott, E. P., Ccarroad, P. A., (1983). Energy consumption Singh, R. & Heldman, R. D. Food Engineering, 2nd edn, New York, pp. 104-5. Sowbhagya, C. M., Ramesh, solubility behaviour of rice
Rumsey, T. R., Horn, J., Buhlert, J. & Rose, W. W. in steam blanchers. J. Food Proc. Engng, 5, 77-88. (1993). Energy for food processing. In Introduction to Chapter 3. Academic Press, Harcourt Brace & Co., B. S. & Ali, S. Z. (1994). Hydration, swelling and in relation to other physicochemical properties. J. Sci.
M. N. Ramesh, ff N, Srinivasa Rao
386
Food Agric., 64, l-7. Suzuki, K., Kubota, K., Omichi, M. & Hosaka, H. (1976). Kinetic studies on cooking of rice. J. Food Sci., 41, 1180-3.
Togeby, M., Hansen, N. & Mosekiide, E. (1986). Modelling energy consumption, loss of firmness and enzyme inactivation in an industrial blanching process. 1 Food Engng, 5, 251-67.
APPENDIX
A
Actual steam requirement for cooking (Singh & Heldman, 1993) Product flow rate Specific heat of rice (based on potato at 80% w.b. moisture content) Initial product temperature Final product temperature Steam quality Steam temperature (selected to assure a minimum temperature gradient of 5°C between steam and product) For a steam temperature
30 kg/h 3-68 kJ/kg
25°C 95°C 90% 100°C
of lOO”C, pressure will be 101.35 kPa, and
Enthalpy H,=419*04 kJ/kg,
H,=2676*1 kJ/kg
The thermal energy requirements for the product the mass flow rate of steam required.
will be used to establish
(1) Thermal energy requirement q=mc,(To-Ti)=30(3*68)(100-25)x82
800 kJ/h
For 85% heat exchange efficiency. q=82 800/O-85=97 411.765 kJ/h (2) For steam quality of 90% H=(0*1)419.04 (3) The thermal will be
+ (O-9)2676*1 =2450.394 W/kg
energy content
of condensate
leaving the heat exchanger
H,=specific heat of water x steam temp. =4.211(100)=421.1 kJ/kg (4) Since the thermal energy provided by steam will be qs=m2(h -K) this must match the steam requirements,
then
m,=97 411*765/(2450*394-421.1)=48*003 with 25% radiation and conduction
kg/h
losses, losses=48(0*25)=12
kg/h.
Development and performance
evaluation of a continuous rice cooker
387
Total actual steam requirement=48+ 12=60 kg/h. Hence, with respect to Table 2, it can be cited that the steam requirement can be reduced by 50%.
APPENDIX Specific energy consumption
B
of continuous cooker
120 kg/h 30 kg/h
Steam consumption Material feed rate
Steam consumption per kg of material= 120/30=4 kg/kg Assuming that steam loss in the worst case is 80% (Scott et al., 1982). Actual steam requirement=4/1*8=2*22 kg/kg With 80% heat transfer efficiency steam requirement
=2.22(0*8) = 1.776 kg/kg
latent heat of steam=504 Therefore
kJ/kg
specific energy consumption = l-776(504) =895-104 kJ/kg
=885*104 x 10” J/kg=O*9 x 10’ J/kg=0.9 x 10’ J/t This is in accordance with the value reported by Togeby et al. (1986). They have reported that a steam blancher without end seals was found to have a specific energy consumption of 2-l GJ/t of product. With water curtains to reduce the leakage of live steam through the blancher ends, the specific energy consumption was reduced to 1.6 GJ/t, while the specific energy consumption for a steam blancher with hydrostatic water seals was found to be O-95 GJ/t. Thus, the continuous cooker developed is comparable to that with hydrostatic end seals.