Quality Assessment of Dried‐Bean with Biochemical Parameters

Abstract

This work reports the quality assessment study of beans preserved through batch dehydration introducing tempering cycles and a control without them. The parameters considered were viability, germination, lipoxygenase activity, and digestibility percentages, as well as changes in the molecular weights of the albumin and globulin protein fractions. Since a similar final humidity content was reached, it can be considered that from the bean dehydration treatments, those with batch tempering cycles always resulted better compared to those without the application of tempering periods. The viability, germination, and lipoxygenase activity tests showed lower values compared to a control sample in all cases; however, these indexes were even lower when the sample was dried without the application of tempering cycles. The effect of the drying process with and without tempering cycles on digestibility showed a slight increase in this parameter compared to the control sample. The effect of the different drying treatments on protein molecular weights showed that, when the process temperature was higher, molecular aggregates were apparently present with weights similar to those in the control sample. From the treatments with tempering cycles, the best scheme was at 70°C and 5 drying time × 30 tempering time periods.

Keywords:

• Bean drying
• Tempering cycles
• Viability
• Germination
• Bean lipoxygenase
• protein molecular weights

Introduction

In some cases, beans represent the only source of proteins for a wide portion of the population in Latin America, so that it becomes necessary to preserve this food at a lower cost while maintaining its quality. In Mexico, bean cultivation has profound ancient roots. Today this leguminous food still plays a fundamental role, because it represents an important source of labor and income for agricultural workers, as well as a guarantee of feeding insurance through self‐consumption, while it represents the main and only protein source in the diet for an ample portion of the population.

The drying process of low‐humidity products such as grains and seeds has been described in the literature1–6 as a diffuse drying. Here the modification in the structure is not significant; however, under prolonged drying periods, longer residence times of the material in the equipment are required, which may cause thermal damage that in most cases becomes irreversible.[7]

One of the drying methods that may be recommendable for food and other biomaterials that are essentially dried during the decreasing rate period, is drying with tempering cycles. The tempering process can be carried out by removing the solid load from the drier and introducing it either in containers with controlled temperature or in a container at room temperature. In the latter, thermal efforts are generated during bean cooling and reheating. This process, therefore, favors the occurrence of drying conditions resembling the initial ones at the beginning of a new cycle, but at progressively lower average humidity content each time, as the number of cycles increase.[8]

Regardless of a conventional or intermittent drying, there is always the risk of a quality decline, since food products are multicomponent systems consisting of biomolecules such as proteins, carbohydrates, and lipids.[9] Nutrients losses as a function of temperature, humidity content, process duration, and presence of catalysts lead to its decomposition. In the case of grains, the most usual method is visual inspection, reporting damage levels as a percentage by weight of broken grains during dehydration, chiefly due to the tension caused not only by temperature gradients within grains, but also because of the tension caused by the humidity gradient or the combination of both.[7]

This work aims to preserve beans using fluidized bed dehydration as a work methodology, preserving the dehydrated product quality. The determination of dehydrated product quality was carried out in terms of biochemical (changes in the molecular weight of proteins), enzymatic (lipoxygenase activity), nutritional (in vitro digestibility), and biological parameters (percentage of viable seeds and germination).

Materials and Methods

The raw material was the bean (Phaseolus vulgaris), “bayo mecentral” variety, provided by the National Institute of Forestry, Agriculture, and Livestock Research (INIFAP), Mexico's Valley Experimental Field, autumn–winter 2000 harvest. Grain baseline humidity content was 12% (Brainweigh MB‐300 moisture balance), so that a conditioning phase was needed to achieve the required humidity levels. This conditioning was carried out placing water instead of the desiccant in the drier, and leaving beans to hydrate, using a maximum of 500 g of sample, that is, a thickness not exceeding 3E‐2 m per container of 3E‐1 m in diameter. Samples were taken at 48 hour intervals for five weeks approximately, aimed at determining the time when the sample reached the required hydration level. These humidity content determinations were done at different conditioning times for two to four weeks.

The fluidized bed dryer consists of a squared‐section stainless steel tunnel measuring 0.2 m per side, through which air is circulated. At one end, a funnel is attached allowing to place a detachable acrylic tube of 0.1‐m inner diameter, with 80 mesh‐size, used as drying area. Air is extracted from the environment using a 1‐HP three‐phase fan capable of handling 0.01963 m³ of air per second. The motor is connected to a Siemens voltage transformer, Micromaster Vector model, that allows the choosing of different air speeds. Air was heated using LP gas. Air speed was 1.3 times the minimum fluidizing speed in a bed height with a packed bed height/dryer diameter ratio (L/D ratio) of 0.5. The incoming air temperature was conditioned to 60, 70, and 80°C measured with a digital recorder including a J‐type constantano‐copper thermocouple (Cole Parmer). The drying process was carried out in two ways: batch with tempering periods and, as control, simply batch. For the first one, the experimental design is shown in Table 1. Humidity losses in samples is measured through weight loss, recorded at 1‐min intervals in a digital balance (Mettler Toledo PB302) recorded as percentage. During tempering periods (tr) the drier load was placed in an air‐tight container kept inside of a chamber at 28°C (Hotpack Incubator controlled‐temperature storage chamber) for the required period of time. To obtain the steady‐state humidity (express as dry basis), desorption isotherms were determined using Wink's method,[10] which is based in the discontinuous recording of material weight (g), caused by the water adsorption/desorption that the material undergo when it is in an atmosphere of controlled relative humidity. The packed L/D ratio and the drying air speed were determined according to the drier dimensions. Based on preliminary testing, the test temperatures chosen were those which did not affect bean quality when drying with tempering cycles was conducted. The response variables (quality parameters) considered were: Germination: briefly, portions of 100 beans were soaked in water in a cotton cloth and incubated at 28°C along 48 h. After this time, the germinated seeds were counted. Viability: the sprouts of 25 bean seeds were treated with 2,3,5‐triphenyltetrazolium chloride (SIGMA Chemical Co.) along 10 min. After this time were counted those which were stained and percentages calculated.[11] Lipoxygenase activity,[12] measured by a spectrophotometric assay, were the oxidative enzymatic activity upon the linoleic acid was recorded as A _245 nm/min/mg of protein changes in a Shimadzu UV‐1201 spectrophotometer. Digestibility[13] performed using an enzymatic mixture, composed by bovine pancreatic quimotrypsin type II, porcine pancreatic trypsine type IX, and porcine intestinal peptidase grade II (all of them provided by SIGMA Chemical Co.). After mixing the sample with the enzymatic mixture, the pH decrease was measured during 5 min in a potentiometer (Conductronic Model pH15) in a controlled temperature bath (HAAKE RV2) and the results expressed as digestibility percentages. Finally, we assessed changes in molecular weight (expressed in kD) of bean globulin fractions (extracted with 5% NaCl), through the Laemmli–PAGE's method,[14] performed in an electrophoresis apparatus (BRL Mini‐V8.10 Life Technologies Inc.), set at 100 V, 1 h, in a polyacrylamide gel (10%). Then the gel was stained with Coomassie Blue G250 (SIGMA Chemical Co.). The proteins bands length migrations were measured and interpolated in a curve constructed with protein markers of known molecular weight ran at same time (Bio‐Rad protein ladder from 21 to 110 kDa).

Table 1.  Tempering cycles at various temperatures for bean dehydration in the fluidized bed equipment

Temperature (°C)	Cycle: ts × tr (min × min)
60	5 × 30	15 × 90	15 × 120	30 × 180
70	5 × 30	15 × 90	15 × 120	30 × 180
80	5 × 30	15 × 90	15 × 120	NA

Note: NA, not available; ts, drying time; tr, tempering time.

The statistic typical deviation analysis of Table 2, was made accordingly with Spiegel.[15] For the data on the other tables the statistical analysis was made accordingly to Massart et al.,[16] applying the arcsine transformation.

Table 2.  Final humidity content of dehydrated bayo mecentral bean in a fluidized bed equipment

Cycle: ts × tr (min × min)	60°C			70°C			80°C
	Residence duration (min)		Final humidity (d.b.)	Residence duration (min)		Final humidity (d.b.)	Residence duration (min)		Final humidity (d.b.)
	Total	Actual		Total	Actual		Total	Actual
5 × 30	355	55	0.1314b	390	60	0.0998a	320	50	0.1069a
15 × 90	435	75	0.1344a	330	60	0.1267b	435	75	0.1259b
15 × 120	555	75	0.1399a	555	75	0.107a	555	75	0.1077a
30 × 180	660	120	0.1437b	450	90	0.1079a		NA	NA
Batch	365		0.1344a	405		0.0972a	535		0.0487b
Mean			0.13676			0.10762			0.0923
Standard deviation			0.0044			0.01033			0.0336
Variance			0.0049			0.01158			0.0011

Note: N.A, not available; ts, drying time; tr, tempering time; d.b., dry basis (g water/g dry solid).

a: Within the variance interval.

b: Outside the variance interval.

Results and Discussion

Final Grain Humidity Content

Table 2 shows total process duration (drying time × tempering time) and actual beans residence times in the equipment, as well as the humidity content at the end of each trial. The reduction in total drying time compared to residence time, is in some cases, of up to 10 times compared to the total process duration, reaching a humidity content similar to the one obtained without tempering cycles. This behavior may be translated into energy savings, since the material remains less time in the dryer, which also results in the possibility of obtaining a dehydrated product of a better quality. With respect to final humidity content, in the case of 60°C values, those of 15 × 90 and 15 × 120 as well as that of batch treatment without tempering cycles (bottom of Table 2), while that of 5 × 30 and 30 × 180 treatments were outside of variance range. For the 5 × 30 treatment, the final humidity content was below the lower level, and for 30 × 180 it was above the upper level. This indicates that under these conditions, dehydration under the 5 × 30 scheme is the most convenient one, since the lowest humidity content is achieved. When drying is carried out at 70°C, treatments that fall outside the variance range correspond to batches without tempering cycles (below) and those of the 15 × 90 treatment (above), suggesting that at this temperature any of the other treatments have the same final humidity content, and further testing would be required to define the best scheme. For plots obtained at 80°C, the 5 × 30 and 15 × 120 treatments do not fall outside the variance range. It is worth mentioning that, except for 60°C, the final moisture contents are acceptable for grain storage. Thus, it may be considered that, from all treatments used for bean dehydration, those by batch with tempering cycles always turned out to be better than those without the application of tempering periods. From the treatments with tempering cycles, the best scheme was the one at 70°C and 5 × 30 (drying × tempering); where it was attained the lowest final humidity content.

Biochemical Parameters

Table 3 shows product quality parameters germination, viability, lipoxygenase activity, and digestibility, expressed as percentages. The control sample was considered as the one which did not undergo any kind of treatment, and is considered as having 100% values. Treatments are compared to samples batch dried without tempering cycles.

Table 3.  Biochemical parameters for quality assessment of beans subjected to dehydration by batch tempering cycles, compared to those dehydrated without tempering

Cycle: ts × tr (min × min)	Viability (%)			Germination (%)			Lipoxygenase activity (%)			Digestibility (%)
	60°C	70°C	80°C	60°C	70°C	80°C	60°C	70°C	80°C	60°C	70°C	80°C
5 × 30	74.0	53.0	0.0	88.0	61.0	5.6	22.3	11.4	31.0	79.7	75.3	77.0
15 × 90	87.0	76.0	0.0	86.5	57.0	0.0	13.1	22.9	24.6	81.3	77.2	76.8
15 × 120	78.5	58.0	0.0	96.0	69.5	0.0	20.0	26.3	14.8	79.9	77.0	78.8
30 × 180	58.5*	51.2	NA	92.0	36.0*	NA	32.6	14.3	NA	77.6	76.8	NA
Batch	56.0*	0.0	0.0	49.5	23.5 *	NA	18.4	21.0	29.9	76.4	75.5	75.2
Control		100			100			100			72.66

Note: All the values are average of three determinations. The controls were grains without any drying treatment. NA, not available; ts, drying time; tr, tempering time.

* No significative differences (p < 0.05).

Viability

A drop in viability is observed in all cases with tempering cycles, when the process temperature increased from 60 to 70°C; it completely disappears when a batch is dried without tempering cycles at 70°C. For the highest temperature, loss of viability is observed in all cases. For this test, the best results are obtained at 60°C with tempering periods in the 15 × 90 scheme. It is worth pointing out that the viability of seeds batch dried at 70°C totally disappears, but this capacity is maintained when tempering cycles are introduced. Brown,[17] have also reported that with exception of viability, the quality of corn dried with dryeration was improved over that corresponding batch drying. This behavior can be explained considering that in the cases without tempering, there is not a period which allows the water redistribution inside the grains, hence affecting profoundly to the sprouts. While the case of tempering such water redistribution causes the sprout not drying completely allowing partially its survival.

Germination

Dehydrated beans using tempering cycles show a better germination capacity than those batch dehydrated. For 60°C and 70°C, the 15 × 120 drying process was the one showing the best results. To note, the 5 × 30 cycle shows germination even at the highest drying temperature. It is worth pointing out that ca. 50% of the cases (underlined values) the germination percentages are somehow higher than the corresponding figures of viability. It could be due to the difference on the determinations basis. That of viability is judged only by the penetration of a dye, while that of germination resides in the total metabolism integrity of the grain. It is quite possible that crust formed impedes the penetration of the dye. Also it is noticeable that in the case of batch at 70°C without tempering there is a germination percentage with no significative difference with respect to the case of same temperature with 30 × 180 tempering cycle. It could be explained invoking that at this temperature, the humidity lost is so quick that it might be formed a crust, that impede a total loss of humidity which in turn allows to sprouts its survival.

Lipoxygenase Activity

In general, a similar activity was observed in the 30 × 180 treatment at 60°C compared to the one at 80°C by lot and to the 5 × 30 treatment. To this respect, drying at a higher temperature occurs more rapidly, which may cause the formation of a crust and/or spots of different humidity content, restraining the migration of water into the seed, which in turn, favors enzymatic activity. It likely could be due to the protector effect that the structure modification (crust formation) of the grain undergo when the drying is faster at higher temperatures and longer drying times.

Digestibility

A higher digestibility was observed when the seed is treated at 60°C compared to the other two temperatures. This is likely due to protein association and dissociation phenomena at 70 and 80°C. Comparing the batch experiments with and without tempering cycles, the introduction of these cycles increases digestibility regardless of the process temperature. Badweel[18] reported a lower digestibility value than those that were found in this work.

Change of Molecular Weight

Tables 4–6 show molecular weights of the protein fractions corresponding to bean albumins and globulins. Table 4 shows that when tempering cycles are included in the dehydration process at 60°C, except for the 5 × 30 cycle, in all other treatments molecular species appear with a lower molecular weight compared to the control, including the batch process without tempering cycles, most likely as a result of molecular dissociation. In the case of the 30 × 180 treatment, the species with the highest molecular weight corresponds to the phaseolin monomer.

Table 4.  Molecular weights (kDa) of proteins from beans dehydrated by batch with tempering cycles at 60°C

Batch	Control	Tempering cycles (min × min)
		5 × 30	15 × 90	15 × 120	30 × 180
Molecular weight (kDa)
ND	119	119	ND	ND	ND
ND	98	101	ND	98	ND
88	87	82	87	87	ND
61	59	52	59	59	59
44	41	ND	43	43	41
39	31	ND	37	37	31
34	31	ND	33	33	31

Note: All values are average of two determinations. ND, not detected.

Table 5.  Molecular weights (kDa) of proteins from beans dehydrated by batch with tempering cycles at 70°C

Batch	Control	Tempering cycles (min × min)
		5 × 30	15 × 90	15 × 120	30 × 180
Molecular weight (kDa)
124	ND	ND	ND	ND	124
116	119	ND	108	109	116
96	98	100	100	98	96
84	87	88	93	81	84
ND	59	61	82	71	ND
ND	41	44	72	ND	ND
ND	31	39	ND	ND	ND
ND	31	34	ND	ND	ND

Note: All values are average of two determinations. ND, not detected.

Table 6.  Molecular weights of proteins from beans dehydrated by batch with tempering cycles at 80°C

Batch	Control	Tempering cycles (min × min)
		5 × 30	15 × 90	15 × 120
Molecular weight (kDa)
160	ND	ND	ND	ND
127	119	119	119	119
97	98	98	98	98
67	87	87	87	87
60	59	59	59	59
54	41	41	41	41
ND	31	31	31	31
ND	31	31	31	31

Note: All values are average of two determinations. ND, not detected.

Table 5 shows that when tempering cycles are applied to the dehydration process at 70°C, molecular species with a higher molecular weight appear when seeds are left in the drier less than 15 min, which suggests the occurrence of molecular aggregates, even when compared to the control, and equivalent to those that occur in the batch process.

Table 6 indicates that when tempering cycles are included in the dehydration process at 80°C, molecular species identical to those in the control appear, while there is a difference in molecular weight in the treatment without tempering cycles, where species with a higher molecular weight are formed. That is, dissociation phenomena occur as a result of the tempering cycles.[19]

Conclusions

Since a similar final humidity content was achieved, it may be considered that from all the bean dehydration treatments, those with batch tempering cycles always turned out to be better than those lacking the application of these tempering periods. This might result in energy savings, since the material remains less time in the drier, which in turn leads to the possibility of achieving a dehydrated product of a better quality. The viability, germination, and lipoxygenase activity tests showed lower values compared to the untreated sample in all cases; however, these indexes were even lower when the sample was dried without the application of tempering cycles. The effect of the drying process with and without tempering cycles on digestibility showed a slight increase in this parameter compared to the control sample. The effect of the different drying treatments on protein molecular weights shown that when the process temperature is higher, apparently molecular aggregates with weights similar to those of the control sample appear. From the treatments with tempering cycles, the best scheme was at 70°C and 5 × 30 (drying × tempering).

Acknowledgments

The authors acknowledge the financial support from Instituto Politécnico Nacional of Mexico, Grant CGPI. We also acknowledge to Dr. María Valdés by critically reading the manuscript.