Dry Processes to Retard Quality Losses of Beans (Phaseolus vulgaris) during Storage

Can. lnst. Food Sci. Technol. J. Vol. 18, No. I, pp. 72-78, 1985

RESEARCH

Dry Processes to Retard Quality Losses of Beans (Phaseolus vulgaris) during Storage J.M. Aguilera and A. Steinsapir Department of Chemical Engineering Catholic University, P.O, Box 114-D Santiago CHILE

ing yet, nevertheless, may not soften sufficiently to be deemed of acceptable eating quality. In additon, protein quality decreases and energy expenditures increase with longer cooking times (Bressani et al., 1983). Several hypotheses have been proposed to explain the hardbean condition. Reduced water uptake (EI-Tabey and Youssef, 1979), incomplete breakdown of the middle lamella (Sefa-Dedeh et al., 1979) and, recently, reduced pectin solubility (Jones and Boulter, 1983) have all been implicated. Most current theories on the development of the hardbean phenomenon involve enzymatic steps. Steaming of dry beans followed by drying resulted in beans about 25% softer than untreated beans after 9 mo of storage at 25°C and 70% relative humidity (Molina et al., 1976). However, wet processing is energy intensive and the process changes the appearance of the beans. Recently, Aguilera et al. (1982) demonstrated the utility of dry heat processing using hot ceramic beads for large scale, continuous roasting of beans. The high temperatures (about 100°C) achieved in short times (1 to 2 min) were very effective in inactivating trypsin inhibitors. Another interesting dry process that induces physiological changes in dormant tissue is lowdose irradiation (Goldblith, 1967). The objective of this study was to investigate the effect of dry heat treatments and irradiation on physical and quality losses of Tortola beans, the preferred dry beans in Chile, during prolonged storage.

Abstract Six samples of beans (Phaseolus vulgaris cv. Tortola Diana) including a control and five dry-processed samples, were evaluated for hardness development after 2.5-10 mo of storage in sealed polyethylene bags at 22°C, Treatment consisted of irradiation (10, 50 or 100 krad), high temperature-short time roasting (HTST), and medium temperature-long time heating (MTLT). Most heat processed samples and approximately one-half of the irradiated samples were significantly softer than the control (P < 0.05) after autoclaving for 12 or 15 min. Scanning electron micrographs demonstrated that hard beans had a tougher middle lamella, showed no separation between cells when cooked, and contained ungelatinized starch granules. The processed samples showed no signs of insect infestation whereas insect losses in the control were in excess of 10070.

Resume Six echantillons d'haricots (Phaseolus vulgaris cv. Tortola Diane) dont un temoin et cinq echantillons traites iI sec furent evalues pour Ie developpement de durete apres 2.5-10 mo de stockage i122°C dans des sacs en polyethylene scelles, Les traitements consisterent d'irradiation (10, 50 ou 100 krad) de r6tissage iI haute temperature pour une courte duree (HTST), et de chauffage iI temperature moyenne pour une longue duree (MTLT). La plupart des echantillons traites ilia chaleur et environ la moitie des echantillons irradies furent significativement plus mous que Ie temoin (P < 0.05) apres la cuisson i11'autoclave pendant 12 ou 15 min. La microscopie electronique par balayage a revele que les haricots durs avaient une lamelle mediane plus resistante, etaient exempts de separation entre les cellules apres la cuisson, et contenaient des granules d'amidon non gelatines. Les echantillons traites furent exempts d'infestation par les insectes tandis que chez Ie temoin les pertes dues aux insectes furent plus de 10070.

Introduction Grain legumes or pulses are more difficult to store than cereals. Reported losses of legumes within the postharvest system in Latin America vary from 10 to 50% (NAS, 1978). Insects account for the greatest physical losses after harvest. For example, estimations of losses 'to beetles (mainly bruchids) in Nicaragua amounted to 5,240 tonnes, or about 110,10 of the bean supply of that country in 1975 (Giles, 1977). Losses in cooking quality due to development of the hard-to-cook or "hardbean" phenomenon during storage have not been quantified but are surmised to be significant. Hardbeans require more extensive cookCopyright

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Materials and Methods Mature, dry beans (Phaseolus vulgaris cv. T6rtola Diana) were harvested in February 1982 at the Platina Agricultural Experiment Station near Santiago. Clean seeds were processed within two weeks from the date of harvest. Processing, storage and soaking Beans were divided into 6 lots of 6 kg. Three lots

1985 Canadian Institute of Food Science and Technology

72

Table I. Hardness of irradiated and heat-processed beans as a function of storage and cooking (autoclaving) time Force l ,2 (g) Cooking time 100 krad (min) 50 krad Control 10 krad I53abc 140cd 193e 163a 12 113b 137a Illb 15 l13b 109a 106a 116a 108a 20 234bc 230bc 259a 221b 12 167b 168ab 190a 183ab 15 162a 164a 174a 20 178a 355ab 363ab 385a 364ab 12 310abc 314abc 236d 323a 15 241a 238a 241a 20 235a 443a 445a 448a 468a 12 348a 323b 360a 371a 15 261a 257a 280a 268a n 20 340b 400a 380ab 397a 10.0 15 243a 252a 269a 245a 22 10.0 IN = 20 for all samples 2Means in the same row followed by the same letter do not differ significantly (P <0.05) 3MTLT = Medium temperature-long time 4HTST = High temperature-short time Storage time (mo) 0.0 0.0 0.0 2.5 2.5 2.5 5.0 5.0 5.0 7.5 7.5

were placed in I-kg polyethylene bags, sealed, and irradiated by the Chilean Nuclear Commission at 10, 50 and 100 krad in a Cobalt 60 Gamma Cell 220 experimental reactor. Two lots were heat-processed, one using a high temperature-short time (HTST) treatment, and the other a medium temperature-long time (MTLT). HTST processing was accomplished in a rotating, 86 L metal drum externally heated by gas burners to produce an interior surface temperature of approximately 240°C. The sample was tumbled in the drum until the average temperature of the beans reached 105°C (approximately 3 min). After discharge, the beans were rapidly air-cooled to room temperature with the aid of a fan. MTLT processing consisted of heating layers of beans in a laboratory oven at 70°C for 1 h, followed by cooling as before. All processed samples, as well as an untreated control, were stored in I-kg lots in sealed polyethylene bags at a temperature of 22°C and a moisture content of 12010. Samples were retrieved from storage and evaluated after 2.5, 5, 7.5 and 10 months. Samples (50 g) of beans were soaked in distilled water (20°C, 4:1 w/w, water:bean ratio) for 16 h. After soaking, the hydrated beans, free of steep water, were placed in 211 x 400 cans and distilled water was added, leaving a 1 cm headspace. The sealed cans were autoclaved at 15 psig (121°C) for 12, 15,20 or 22 min and subsequently cooled by circulating cold water.

Assessment Water absorption capacity was determined by soaking beans, stored for 10 mo, in distilled water (20°C, 4:1 w/w, water: bean ratio) for 2, 4,6,8 or 18 h. After soaking, the beans were drained and blotted to remove surface water. The sample was weighed and water absorption capacity calculated as g of absorbed water per g bean dry weight. Moisture in all samples was determined according to AOAC (1960). Duplicate determinations were performed in both cases. The puncture test developed by Borne (1972) was used as an objective method to evaluate the hardness Con. Ins!. Food Sci. Technol. J. Vol. 18, No. I, 1985

MTLT3 212f 144a 143b 245ac 162b 163a 347ab 322ab 213b 416b 324b 270a 337b 252a

HTST4 133bd 119b Ilia 231bc 182ab 173a 338b 294c 216b 413b 327b 248b 345b 236b

of cooked beans. A cylindrical, flat-faced, steel punch 3 mm in diameter was attached to the crosshead bar of an Instrom Universal Testing Machine. A rotating platform held a cooked bean in place beneath the punch while another was inserted in the next test position, thus permitting 3 assays per minute. A 2-kg load cell and a crosshead speed of 20 cm/min were used. Twenty measurements per treatment were performed at ambient temperature for each storage time. A panel of 10 people, familiar with the texture properties of the bean variety, described the hardness of cooked beans using a hedonic scale ranging from 1 = very soft to 7 = very hard. Beans were heated to 45°C before tasting. Sensory evaluation was conducted following the recommendations of the 1FT (1981). After the 10 mo of storage, germination capacity was evaluated by placing 20 seeds in 9-cm Petri dishes lined with cotton moistened with 10 mL of distilled water. Dishes were placed in the dark for 60 h, after which the percentage of germinated seeds was calculated. Duplicate determinations were performed. After 2,5,5 and 10 mo of storage, bags were visually inspected for insect damage. Infestation was reported as the percentage of beans attacked by insects.

Scanning electron microscopy For scanning electron microscopy, raw, dry beans were fractured perpendicular to the main axis of the cotyledon with a razor blade, mounted on specimen stubs with silver conducting paint, and coated with gold. Undamaged portions of cooked beans from the puncture test were frozen in liquid nitrogen, freezedried and mounted as for the raw samples. Specimens were studied with a JEOL J8M -25 SII scanning electron microscope at an accelerating voltage of 12.5 kv. Comparison of sample means was accomplished using the SPSS-IO/KI program, release 7.02 A (14-Feb-1979) developed by the University of Pittsburgh and by a Multiple Range Test (Duncan, 1955). Analysis of germinations capacities was done using a Chi square test. Aguilera and Steinsapir / 73

Results and Discussion Physical effects of processing HTST-processed beans were slightly darker in color than other processed samples, although no cracking of the hulls was observed. Table 1 shows the textural changes that occured during storage of dry beans as a function of cooking (autoclaving) time. All samples became harder upon storage, corroborating the pervasive nature of the problem. The hardness of beans determined after autoclaving for 15 min, increased 2.5 times for samples HTST, MTLT and 50 krad, 2.8 times for 10 krad, and 2.9 times for control and 100 krad after 10 mo of storage, compared to that of untreated beans at harvest time. These values may have been higher had the beans been stored at a higher temperature or moisture content or both, since hardness development for a particular cultivar depends strongly on these two parameters (Gonzalez, 1982). All stored, heat processed beans, with the exception of HTST beans after 2.5 mo and MTLT beans after 5 mo, were significantly softer after 15 min of autoclaving than the corresponding controls (P < 0.05). The observed increases in hardness over time support the hypothesis that hardness development involves, at least partially, enzymatic reactions, e.g., polyphenol oxidase acting on catechol (Elias, 1982) or phytase hydrolyzing phytic acid (Varriano-Martson and Jackson, 1981). Some irradiated samples had lower hardness values than the corresponding controls (P < 0.05). However, there was not clear effect of radiation dosage upon hardness. Nene et al. (1975) reported that softening time could be reduced by 30070 after irradiation, but the dosages used were much higher (1 Mrad). The dosages used in our study were probably not high enough to cause enzyme inactivation but were of the order of magnitude that produces significant soften-

ing in fruits and vegetables by degradation of pectins and cellulose (Massey and Foust, 1969). Thus, pectin or cellulose degradation, or both, may be a minor contributor to the softening of cooked beans. Water uptake by beans stored for 10 mo is illustrated in Figure 1. More than 80% of the total water absorbed was picked up in the first 8 h of soaking. The control sample showed the slowest rate of water uptake, reaching a maximum of 0.76 gig after 18 h. Irradiated (50 and 100 Krad) and MTLT processed beans absorbed an amount of water almost equal to their dry weight, whereas the HTST and 10 Krad samples imbibed only 85% as much. The amount of water absorbed was not a good predictor of the puncture force values for the corresponding cooked beans, as was the case also for black beans (Silva et al., 1981).

Month 0 Cooking time 15min. Mean force 119 g.

(/)

Z


110

UJ

1LeO

170

200

CO

Month 2.5 Cooking time 12 min. Mean force 231 g.

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~ 3= 08 a

150

0.7

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200 250 300

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Month 10 Cooking time 15 min. Mean force 345 g.

0.5

lD 4 Q

03

l:I. Control • 10 krad • 50 krad • 100krad c MTLT o HTST

5 345 Fig. I. Effect of soaking time on hydration of beans after 10 months of storage.

74 / Aguilera and Steinsapir

Fig. 2. Histograms of puncture force values for individual cooked beans roasted under high temperature, short time (HTST) conditions. J. Inst. Can. Sci. Technol. Aliment. Vol. 18, No. I. 1985

of 0,5 and 10 mo. The hardening phenomenon sensed by the panelists followed the same trend as recorded using the Instron. The linear regression equation describing the relationship of objective and sensory tests was (P < 0.05; r2 = 0.93)

.......400

2 w

U 0:

o

lL

where:

xxxxxxxxx

e. Control

c MTLT o HTST Autociaving Time --15 min. ---20min. I

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2.5 Fig. 3. Effect of storage time on the hardness of heat-treated and control beans.

Measuring the puncture force of individual beans is tedious and time consuming, but it has an advantage over bulk testing in cells (Voisey and Larmond, 1971) in that it shows the dispersion of texture values among beans of the same class. Figure 2 illustrates this point. Texture values of individual cooked beans within a class seemed to conform to a normal distribution with a relatively large standard deviation. The sources of this variation could be genetic, differences in maturity at harvest, and position of the bean inside the can during autoclaving, among others. In a sample of 20 beans, as used here, it was sometimes possible to detect within a class "soft" beans (Figure 2a) and "hard" beans (Figure 2b), defined as those at either tail of the distribution curve. Figure 2c illustrates a case of small dispersion of data within a sample. Organoleptic testing of beans is important since it permits the relation of perceived hardness to the puncture force. Table 2 presents such data for storage times

SC SC FI

2.95 In (FI) - 11.44 score in sensory evaluation force measured in the Instron (g)

A numerical score of 5 (corresponding to a force of 263g) was found to be the minimum acceptable texture for consumption. Figure 3, based on data from Table 1, shows typical hardness vs storage time curves for the control and heat-processed samples. Both HTST curves originated at lower force values, as compared to the corresponding MTLT or control curves. This initial softening effect was also observed in previous work with navy, pinto and black beans (Aguilera and Lusas, 1982). It is possible that the moisture escaping during roasting, together with the resistence of temperature gradients inside the beans, softened the intercellular material and permitted easier separation of the cells. The texture of cooked beans apparently underwent an initial period of rapid deterioration until month 5, after which the rate of hardening began to decay. The HTST curve entered a plateau earlier and at a lower force value than did the control sample. Similar behaviour was observed by Molina et al. (1976) for untreated and steam-processed black beans. Figure 3 also shows that after 5 mo of storage samples cooked for 15 min were unacceptable. Cooking for 20 min was sufficient for HTST beans, but the control and MTLT samples remained unacceptably hard. Consequently, HTST beans not only required the least energy to attain the desired texture but, in addition, probably were highest in nutritional values since an inverse relationship exists between cooking time and protein quality (Bressani et al., 1983). In high altitude areas where water boils at a lower temperature, the rapid cooking characteristics of HTST beans may be even more marked.

Scanning electron microscopy Figure 4 presents the microstructural characteristics of selected bean samples after 10 mo of storage. Figure

Table 2. Sensory hardness of irradiated and heat processed beans as a function of storage and cooking (autoclaving) time1,2. Storage time (mo)

o o

o

5 5 5 10 10

Cooking time (min) 12 15 20 12 15 20 15 22

Score Control 4.0ab 2.6a 2.3ade 6.5a 5.6ab 4.5a 6.4a 5.0a

10 krad 3.3bc 3.la 2.3ac 6.3ab 5.7a 4.6a 6.0ad 4.6ade

50 krad

3.Oc 2.8a 2.3ab 6.0ab 4.9bc 4.8a 6.0ac 4.9abc

100 krad 3.0c 2.2b 2.Ibad 6.3ab 5.7a 4.6a 5.8bd 4.4bdf

MTLT3 4.2a 3.0a 3.0a 5.8ab 5.6a 4.la 5.2c 4.5af

3.lc 2.4a 2.Ibce 5.6b 4.8c 4.la 6.3ab 4.4cef

IN = 10 panelists 2Means in the same row followed by the same letter do not differ significantly (P <0.05) 3,4See Table I Con. Insl. Food Sci. Technol. J. Vol. 18. No. I. 1985

Aguilera and Steinsapir / 75

Fig. 4. Scanning electron micrographs showing structural properties of selected bean samples after 10 months of storage: A) Uncooked control bean. S = starch granule, PM = protein matrix, CW = cell wall; B) Uncooked HTST bean; C) Cooked "soft" bean. C = cell; D) Magnified view of the insert in C) showing intercellular material (lCM) and protein bodies (PB) underneath; E) Cross-section of a cooked "hard" bean illustrating intracellular components and absence of separation between cells; F) Cross section of a cooked "soft" bean showing gelatinized material (G). Arrows indicate places where extensive cell separation has occurred; G) Cooked "hard" bean with ungelatinized starch granules (S); H) Gelatinized starch (G) in a cooked "soft" bean. Markers = IOJLm.

Table 3. Germination capacity of irradiated and heat-processed beans I after 10 months of storage

Table 4. Insect infestation of untreated and dry-processed beans as a function of storage time

Germination Treatment

(%)

Control 10 krad 50 krad 100 krad MTLT 2 HTST 3

62.5 42.5

37.5 35.0 20.0 2.5

Storage time (mo) 2.5 5 10

infested beans (%) All other Control treatments I 0 5 0 10+ 0

IN = 40 2.3See Table I.

4A is a cross section of a raw bean showing several cotyledon cells with starch granules (S) embedded in the proteinaceous matrix (PM) and surrounded by cell wall material (CW). Microstructural components do not differ from those reported for other bean varieties (Rockland and Jones, 1974; Silva and Luh, 1978). After HTST processing, the cell boundaries became less distinguishable, although the intercellular matrix continued to adhere strongly to the starch granules (Figure 4B). The ultrastructure of irradiated beans (not shown) was similar to that of the control sample, with some starch granules detached from the matrix. Soaking and cooking of "normal" beans loosened the intercellular matrix of the middle lamella sufficiently to allow separation of individual cells without rupture of cell walls (Figures 4C and 4D). In contrast, "hard" beans separated intracellularly, presumably because the middle lamella was tougher and did not break apart (Figure 4E). This characteristic of "hard" beans was also observed in cowpeas (Sefa-Dedeh et al., 1978). Cell separation was very noticeable in beans classified as "soft" by the puncture test (Figure 4F) and complete gelatinization was achieved in this case (Figure 4H), whereas a few ungelatinized starch granules were detected in the "hard" beans examined (Figure 4G). Hardening of the cell wall material in "hard" beans may have impeded dilation of some starch granules that remained inmersed in gelatinized material. Variations in the gelatinization susceptibility of starch granules within a cell or in different cells were reported by Hahn et al., (1977) for large lima beans.

Germination capacity Table 3 presents the germination capacity of beans after 10 mo of storage. It is reasonable to assume that the viability of seeds is inversely proportional to the intensity of the treatment. Differences between the control, irradiated and heat-treated samples, considered as 3 different groups, were highly significant (P < 0.005) as determined by a Chi-square test. Hence, the irradiation processes were milder than the heat treatments. If it is assumed that the HTST beans had their enzymatic systems partially inactivated, the differences in hardness relative to the control might represent differences in the degree of deterioration due to enzymatic causes. Can. InSI. Food Sci. Technol. J. Vol. t8, No. t, 1985

Insect infestation of stored beans Treated beans exhibited no signs of infestation after 10 mo of storage in sealed polyethylene bags (Table 4). Control beans, however, showed signs of deterioration at 2,5 mo and at the end of 10 mo, more than 10% of the beans had been attacked by Acanthoscelides obtectus. Evidently, all treatments had a pasteurizing effect on beans, an effect that was subsequently preserved by the impermeability of the package. Due to the severity of losses because of insects, heat or irradiation treatments together with improved packaging may become economically attractive in the future.

Acknowlegement The authors thank Dr Jorge Garrido of the Institute of Biological Sciences for the scanning electron microscope work.

References Aguilera, J.M. and Lusas, E.W., 1982. Interim Report "Process Development for Preparing Ingredients and Products from Pinto, Black and Navy Beans". Research Agreement No. 59-2481-1-2-003-0 U.S. Department of Agriculture. 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. Food Sci. 47:996. AOAC, 1960. "Official Methods of Analysis" 9th ed. The Association of Official Agricultural Chemists. Washington, D.C. Bourne, M.C. 1972. Texture measurement of individual cooked dry beans of the puncture test. 1. Food Sci. 37:751. Bressani R., Braham, J .E. and Elias, L. 1983. Effects on nutritional quality of food legumes from chemical changes through processing and storage. In: "Chemistry and World Food Supplies: The New Frontiers". L.W. Shemilt, (Ed.). Pergamon Press. Duncan, D.B. 1955. Multiple range and multiple F tests. Biometrics II: I. Elias, L.G. 1982. Conocimientos actuales sobre el proceso de endurecimiento del frijo!. Arch. Latinoamer. Nutr. 32:233. EI-Tabey, A.M_ and Youseff, M.M. 1979. Relationship between physical properties and cooking quality of faba beans (Viciafaba). Fabis 1:33. Giles, P.H. 1977. Bean storage problems in Nicaragua. Tropical Stored Products Information No 33. Tropical Products Institute. London, England. Goldbith, S.A. 1967. Radiation processing of foods and drugs. In: "Fundamentals of Food Processing Operations". J.L. Heid and M.A. Joslyn, (Eds.). AVI Publishing Co., Westport, Conn.

Aguilera and Steinsapir / 77

Gonzalez, E. 1982. Efecto de diferentes condiciones de almacenamiento sobre el desarrollo de la dureza del frijo!. Arch. Latinoamer. Nutr. 32:258. Hahn, D.M., Jones, F.T., Akhavan, I. and Rockland, L.B. 1977. Light and scanning electron microscope studies on dry beans: intracellular gelatinization of starch in cotyledons of large lima beans (Phaseolus lunatus) J. Food. Sci. 42:1208. 1FT, 1981. Sensory evaluation guide for testing food and beverage products. Food Techno!. 35(11):50. Jones, P.M.B. and Boulter, D. 1983. The cause of reduced cooking rate in Phaseolus vulgaris following adverse storage conditions. J. Food Sci 48:623. Massey, L.M. and Foust, M. 1969. Irradiation-Effects on polysaccharides. In: "Carbohydrates and their Roles". H.W. Shultz, (Ed.). AVI Publishing Co. Westport, Conn. Molina, M.R., Baten, M.A., Gomez-Brenes, R.A., King, K.W. and Bressani, R. 1976. Heat treatment: A process to control the development of the hard-to-cook phenomenon in black beans (Phaseolus vulgaris). J. Food Sci 41;661. NAS, 1978. "Post-harvest Food Losses in Developing Countries". National Academy of Sciences, Washington, D.C. Nene, S.P., Yakil, U.K. and Sreenivasan, A. 1975. Improvement in the textural qualities of irradiated legumes. Acta Alimentaria 4(2):199.

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Rockland, L.B. and Jones, F.T. 1974. Scanning electron microscope studies on dry beans. J. Food Sci 39:342. Sefa-Dedeh, S., Stanley, D.W. and Voisey, P.W. 1978. Effects of cooking time and cooking conditions on texture and microstructure of cowpeas (Vigna unguiculata) J. Food Sci. 43:1832. Sefa-Dedeh, S., Stanley, D.W. and Voisey, P.W. 1979. Effect of storage time and conditions on the hard-to-cook defect in cowpeas (Vigna unguiculata). J. Food Sci. 44:790. Silva, H.C. and Luh, B.S. 1978. Scanning electron microscopy studies on starch granules of red kidney beans and bean sprouts. J. Food Sci. 43:1405. Silva, C.A.B., Bates, R.P. and Deng, J.C. 1981. Influence of soaking and cooking upon the softening and eating quality of black beans (Phaseolus vulgaris). 1. Food Sci. 46:1716. Varriano-Martson, E. and Jackson, G.M. 1981. Hard-to-cook phenomenon in beans: Structural changes during storage and imbibition. J. Food Sci. 46: 1379. Voisey, P.W. and Larmond, E. 1971. Texture of baked beansA comparison of several methods of measurements. J. Text. Stud. 2:96.

Accepted July 18, 1984

J. Ins!. Can. Sci. Technol. Aliment. Vol. 18, No. I, 1985