The effect of solar drying and heating on the hardness of Phaseolus beans during storage

J. stored Prod. Res. Vol. 22. No. 4, pp. 243 247. 1986 Printed in Great Britain. All rights reserved

THE HEATING

EFFECT

0022-474X 86 83.00 ~ 0.00 (opyright ~: 1986 Pergamon Journals Ltd

OF SOLAR

ON THE BEANS

DRYING

HARDNESS DURING

AND

OF PHASEOLUS

STORAGE

J. M. AGUILERA,M. I. HAU and W. VILLABLANCA Department of Chemical Engineering, Catholic University, P.O. Box 6177. Santiago, Chile

(Received 16 June 1986) A b s t r a c ~ S m a l l white beans (Phaseolus vulgaris) were dried and heat processed betbre storage. Water removal in a solar drier was four times faster than conventional field drying in the pods. Heat treatments included particle-to-particle roasting using hot sand, and microwave heating. All samples stored for 212 days at low moisture contents (4~5%) showed a slow rate of hardening at a low temperature of 18C and a high temperature of 34°C. However, heat treated and control beans became much harder when stored at a humidity of 9% and a temperature of 34'~C. Heating and solar drying are effective treatments prior to bean storage, as they greatly reduce moisture to safer levels.

INTRODUCTION

Legumes are used as food in nearly all the temperate and tropical areas of the world. They are important sources of protein, and provide as much lysine as the total cereal crop which is twice as large (Rockland and Radke, 1981). Beans become increasingly hard during storage and they require a longer cooking time. This phenomenon, known as "hard-to-cook" results in a reduction in the rate of cell separation during cooking caused by structural changes probably induced by enzymes (Jones and Boulter, 1983). Two reviews on this subject, one on the biochemical and structural aspects (Stanley and Aguilera, 1985) and another on the effect of processing and storage (Aguilera and Stanley, 1985) have been published. Insect attack and hardening are the two major causes for legume losses in the developing world. Moisture content and temperature during storage are critical parameters of the "hard-to-cook" phenomenon (Burr et al., 1968). Heat treatments and drying reduce the moisture content of the beans and may have beneficial effects in partially inactivating enzymatic systems. A novel dry heating process using hot solid particles as a heat transfer medium was used to slightly roast beans (Aguilera et al., 1982). The process was also effective in reducing the rate of hardening and in controlling insect infestation (Aguilera and Steinsapir, 1985). The present research had two objectives: (1) To study solar drying of fresh mature beans as an alternative to field drying; (2) to assess the effect of different drying treatments on the cooking quality of stored small white beans. MATERIALS AND METHODS Beans Fully developed small white beans (Phaseolus vulgaris) obtained from mature pods harvested in the Agricultural Experiment Station, University of Chile, are referred to by the code MB. Dried beans picked from the same field after natural drying in the pods are referred to as field-dried beans, code FB. Drying Mature beans (96% moisture) were hand removed from the pods. Solar drying was performed in a box-type solar drier similar to the one described by Lumley et al. (1978). Beans were loaded into shallow perforated trays previously dried to a constant weight. The following variables were monitored at regular intervals: weight, temperature of beans and dryer interior, and wet and dry bulb temperature of the air. Later, another sample of mature beans (78% moisture) were dried in sPR 22,~ v

243

244

J.M. AGUILERAet al.

the open and used as controls. Beans left in the field were dried in the pods and sampled every other day for moisture determination. Bean treatments

Dry (FB) and mature (MB) beans were subjected to different heat treatments representing various degrees and forms of thermal processing: FB-Control corresponded to beans dried in the pods while in the field in the manner commonly practiced by small farmers. FB-HTST were field-dried beans (same as FB control) roasted for 2 min to 61°C using hot sand as a heat transfer medium as described by Aguilera et al. (1982). Product temperature was measured by holding roasted beans immediately after discharge in an insulated box with a thermocouple along the main axis. MB-HTST were prepared from mature beans by roasting as described for FB-HTST, followed by solar drying. MB-MWave is the code for mature beans heated in a Philips Cooktronic 7915 microwave oven at a power setting of 6 for 2 min. The beans reached an internal temperature of 60°C as measured by a thin thermocouple inserted into a bean. MB-MWave beans were later dried in the solar drier. MB-Solar were beans from the solar drying experiment. MB-Solar-HTST were MB-Solar beans roasted as previously reported. Analysis

Moisture content was determined in duplicate on ground beans after oven drying at 105°C to a constant weight. Water absorption (WA) was determined by soaking beans in distilled water (1:4, w/w ratio) for 16 h and calculating the weight gain after blotting with a paper towel. Germination capacity was assayed on 20 seeds placed on wet cotton (water plus 0.3% chlorine to prevent microbial growth) and stored in a dark place. Nitrogen solutility index (NSI) was determined as described by the AACC (1976). Texture was measured by organoleptic and mechanical methods. Organoleptic testing was done by a panel of 10 members familiar with the eating quality of white beans. A 9-point hedonic scale was used to rate texture (1 = extremely soft; 9 = extremely hard) flavor and acceptability (1 = extremely poor; 9 = extremely good). Bean preparation for texture analysis consisted of 14 h soaking at room temperature (approx. 18-20°C) in a 1:4 bean-to-water ratio followed by autoclaving for 10 min at 118°C. Hardness was measured objectively by two methods: the puncture test using a 3 mm plunger attached to an Instron Universal Testing Machine for piercing 20 individual beans, as described by Aguilera and Steinsapir (1985) and after 100 days of storage by using an extrusion cell press. This method was similar in principle to the Kramer shearpress method and was used to assay 40 g of cooked beans at a time. Storage

Bean samples were stored in sealed polyethylene bags for 7 months at two temperature ranges: low (LT), 18 + 3°C and high (HT), 34 ___3°C. RESULTS AND DISCUSSION Drying experiments were performed at the peak of the summer season and no abnormal variations in temperatures or insolation rate (i.e. rainy days) were observed during the period. Figure 1 compares the drying behaviour of beans that had been removed from the pods and solar dried and those left in the pods for conventional field drying. The drying rate in this latter case was slowed by the moisture transfer resistance provided by the pods enclosing the beans. Drying time in the field from the time mature beans were fully developed (96% moisture, dry basis) until dry (9.1% moisture, dry basis) was 12-14 days. The shape of the drying curves for beans dried in the solar drier and in the open (sun drying) was almost similar for the first 9-h period. The average and maximum temperatures of the beans during drying were different (53.6 and 73.3 vs 32.7 and 39.3°C, respectively, solar vs sun drying), reflecting the higher energy collected and conserved in the solar drier. The only explanation for the similar initial drying rates is that the relative humidity of the outside air was of the order of 45-50% while the lack of forced convection inside the solar drier may have led to higher relative

Solar drying and heating

100 9C

100

\ \

80

70 \ r~

So,or

\

X ~"

Z

drying

\

Field drying

\

~ 40

\

\

:~

c:i 80 * 60

Xm

60

~50 2

245

40

\

0

\% k

20 10 0

Sun

i

i

i

4

6

8

Night

",,,

3o

L

2

Time

t

t

-

10 12

(days)

Night fl'.

~

"-.,...

drying I I I l I I I I I/f

2

4

6

8

I I I I I I I I I~

24

26 28 30 Time

Ifj

I I

I I I I I I I l I

48 50 52 54 56

2

(h)

Fig. 1. Kinetics of solar and field drying of white beans.

humidities. Final moistures at the end of the first day, when the constant rate period was over, were 22.3 and 15.1%, which is equivalent to 5-6 days of field drying. Drying to a safe moisture content was smoother inside the solar dryer since no moisture pick-up from the cool wet air occurred during the night. This phenomenon apparently also affected the appearance of the sun dried beans which had about 37% shrinked coats as opposed to only 16% shrinked coats for solar dried beans. The final moisture contents after 57 h of drying were 8.1 and 4.7%, respectively. Table 1 shows the moisture content of beans before and after treatment and at the beginning and end of the storage period. Variations in the initial moisture of MB beans was due to desiccation during processing. Both microwave and H T S T treatments, involved heating and drying effects that reduced the moisture content. Since all MB samples were ultimately dried in the solar dryer their moisture content at beginning of the storage test were similar (4.1-4.7%) and lower than those of field-dried beans (8.5 and 9.4%). Beans stored at low temperature showed small variations in moisture content during storage while those stored at high temperatures lost significant amounts of water by diffusion through the package (Table 1). The extent of the heating effect was analyzed through indicators such as NSI, germination capacity and water absorption and the results are presented in Table 2. Dry roasting (HTST) induced minor changes in NSI when the beans were dry (NSI = 28.2) but significantly reduced the NSI when mature beans were used (NSI = 15.9). This is in agreement with the higher protein denaturation rates found when heating is performed in the presence of moisture (Smith and Circle, 1972). The microwave processing induced only light heating effects (NSI = 34.3) which was confirmed by the other indicators. As expected, the FB control had the highest percentage of viable seeds and, based on this indicator, the most severe processes were those involving HTST roasting. Water absorption was similar for all samples (between 102.0 and 105.6 g water/g dry bean) except Table 1. Effect of treatments on the moisture content of beans (percent dry basis)* After storagef Sample FB-Control FB-HTST MB-Solar MB-Wave MB-HTST MB-Solar-HTST

Before treatment

After treatment

Before storage

LT

HT

9.4 9.4 65.6 43.8 26.7 4.7

9.4 8.5 4.7 33.8 16.5 4.1

9.4 8.5 4.7 4.6 4.4 4.1

9.9 9.0 5.3 4.4 4.4 4.1

5.8 4.2 3.0 3.2 2.7 3.1

*Average of duplicates. # L T = s t o r e d at low temperature (18 + 3 C ) ; H T = stored at high temperature (34 ± 3 C ) .

246

J.M.

AGUILERA et al.

Table 2. Effect of drying and/or heating on physicochemical properties of beans* NSI

Germination

Water absorption

Sample

(%)

(%)

(%)

FB-Control FB-HTST M B-Solar MB-Wave MB-HTST MB-Solar-HTST

31.5 28.2 38.4 34.3 15.9 35.6

43.3 0 15.0 11.6 1.7 0

105.6 102.6 102.0 102.9 89.0 104.3

*Average of duplicates.

for MB-HTST. This characteristic of the MB-HTST sample in combination with the low NSI and germination capacity, may signify that particle roasting caused biochemical as well as structural (case hardening) changes. The force required to puncture individual grains is a well accepted indicator of texture quality (Bourne, 1972). At the beginning of the study hardness development was followed by this method and results for time 0 and 100 days are presented in Table 3. Bean hardness at the beginning of the storage test varied between 209 and 326 kg-f. After 100 days, all beans except those FB stored at high moisture contents and high temperatures (FB-Control-HT and FB-HTST-HT) showed practically no signs of hardening. In these two exceptions about double the puncture force required for penetration. However, when the dispersion of force values for individual beans is wide, the puncture test applied on small samples (e.g. 20 beans) becomes highly irreproducible. This has been demonstrated by Aguilera and Steinsapir (1985) who found that within a sample of that size, the presence of an "extremely hard" or an "extremely soft" bean could alter the results completely. Moreover, testing is tedious and slow as each bean has to be prepared and tested individually, preventing the use of larger samples. The extrusion cell used 40 g of cooked beans. The coefficient of variation of the test in the force range involved was 1-2%. Results of hardness measured by the extrusion cell test for 176 and 212 days are also shown in Table 3, although results from both tests are not directly comparable. Initial moisture contents of below 10% did not appear to have a significant effect on the rate of hardening when storage temperatures were low (18 _ 3°C). Heating or additional drying did not further affect hardening. However, hardening proceeded at a slow pace even under these conditions. The beans stored at a higher initial moisture content (8-10%) hardened more than those stored at a low moisture content (4-5%) after 212 days at 34 __+3°C. Thus, particle-to-particle heating and solar drying are effective in preventing hardening during storage. Bean hardening in tropical areas could be slowed significantly if low cost methods of drying and moisture proof packaging systems were available. This slow hardening would be independent of storage temperatures.

Table 3. Hardness of samples during storage (in kg-f) Puncture test*

Hardness determined by: Storage time (days):

0

Extrusion cell testt 100

176

212

Sample Low temperature storage FB-Control FB-HTST MB-Solar

MB-Wave MB-HTST MB-Solar-HTST

0.326 d 0.291 ca 0.209 a 0.296 cd 0.244 ab 0.270 ~

0.249 b~ 0.314 c 0.267 bc 0.217 ab 0.256 ~ 0.246 ~

47.5 39.0 39.5 39.5 43.5 34.5

57.0 48.0 41.0 53.5 48.5 38.5

0.326 ~ 0.291 cd 0.209 a 0.296 ~d 0.244 ab 0.270 b¢

0.659 ~ 0.525 d 0.219 ab 0.216 ab 0.281 c 0.176 ~

177.5 160.0 33.5 33.5 41.5 40.0

217.0 164.0 52.5 42.5 46.0 69.5

High temperature storage FB-Control FB-HTST MB-Solar MB-Wave MB-HTST MB-Solar-HTST

*N = 20. Means followed by the same letter are not significantly different (P < 0.05)

according to Duncan's test. tAverage of duplicates.

Solar drying and heating

247

Table 4. Organoleptic analyses of beans after storage Sample

Acceptability

Texture

Flavor

5.0 5.6 6.0 4.6 4.2 5.0

4.2 4.0 3.5 3.8 3.8 3.5

5.0 5.8 6.0 5.2 6.8 6.8

2.0 1.3 5.3 5.5 5.0 5.4

9.0 9.0 3.1 3.2 3.0 3.2

4.2 5.2 5.2 4.5 4.7 4.3

Low temperature storage FB-Control FB-HTST MB-Solar MB-MWave MB-HTST MB-Solar-HTST

High temperature storage FB-Control FB-HTST MB-Solar MB-MWave MB-HTST MB-Solar-HTST

*Scores for texture ranged from 1 = extremely soft to 9 = extremely hard; and for flavor and acceptability from 1 = extremely poor to 9 = extremely good.

These results are in agreement with those of Burr et al. (1978) who found that the temperature effect on hardening could be offset by low moisture contents during storage. Differences in hardness were similarly perceived by organoleptic testing, as shown in Table 4. The correlation coefficient between hardness after 212 days of storage and organoleptic texture was 0.978. Acceptability scores paralleled those of texture. Field beans stored at higher moisture contents and temperatures were rated very poorly. Flavor scores of beans stored at lower temperatures were higher although no differences were detected among beans held at 34 _+ 3°C. Hence, texture is a critical parameter in determining acceptability by native consumers and greater efforts should be devoted to preserve it. Current work is aimed at developing kinetic data for hardening as a function of moisture and temperature. Acknowledgements--Financial support from the International Development Research Centre (Canada) and the Office of Research (DIUC) Catholic University of Chile is acknowledged with thanks.

REFERENCES A A C C (1976) Approved Methods, 7th edn (revised). American Association of Cereal Chemists, St Paul, Minn. Aguilera J. M., Lusas E. W., Uebersax M. A. and Zabik M. E. (1982) Roasting of navy beans (Phaseolus vulgaris) by particle-to-particle heat transfer. J. f d Sci. 47, 1000-1005. Aguilera J. M. and Stanley D. W. (1985) A review of textural defects in cooked reconstituted legumes: The influence of processing and storage. J. Fd Proc. Preserv. 9, 145-169. Aguilera J. M. and Steinsapir A. (1985) Dry processes to retard quality losses of beans (Phaseolus vulgaris) during storage. Can J. f d Sci. Technol. J. 18, 72 78. Bourne M. C. (1972) Texture measurement of individual cooked dry peas by puncture test. J. j d Sci. 37, 751 754. Burr H. K., K o n S. and Morris H. J. (1968) Cooking rates of dry beans as influenced by moisture content and temperature and time of storage. Fd Technol. 22, 336-338. Jones P. M. B. and Boulter D. (1983) The cause of reduced cooking rate in Phaseolus i~ulgaris following adverse storage conditions. J. f d Sci. 48, 623-626, 649. Lumley F., Thimoty G., C h u a n g Y. and Elvin T. (1978) Technical and economical characteristics of two solar kiln designs. Forest Prod. J. 29, 4 3 4 8 . Rockland L. B. and Radke T. M. (1981) Legume protein quality. Fd Technol. 35, 79 82. Smith A. K. and Circle S. J. (1972) Soybeans: Chemistry and Technology. Vol. 1. Proteins. AVI Publishing, Westport. Conn. Stanley D. W. and Aguilera J. M. (1985) A review of textural defects in cooked reconstituted legumes: The influence of structure and composition. J. f d Biochem. 9, 277- 323.