Effects of storage temperature and relative humidity on cooking time and moisture uptake of selected common bean (Phaseolus vulgaris L.) varieties grown in Ethiopia

Abstract Common beans (Phaseolus vulgaris L) are important legumes rich in nutrients. However, the defect to cook or hard-to-cook (HTC) defect is among the challenges of legume crops during storage. Hence, this study aimed to determine the effects of storage temperature and relative humidity (RH) on cooking quality and moisture uptake of the selected recently released common bean varieties. Different storage conditions combinations were used for the analysis: ambient storage at 25°C and 65% RH as control storage; storage conditions of 35°C and 75% RH; and 35°C and 85% RH were explored. Zoasha, Nasir, Awash Weten and control bean varieties show reduced moisture uptake and increment in cooking time. The moisture uptake of Zoasha reduced from 127% immediately measured after harvesting to 79% (45°C/85%RH). Nasir, Awash Weten and the control sample varieties of beans reduced their moisture uptake from 121% to 74%, 124% to 75% and 142% to 90%, respectively. Those results show that accelerated storage conditions had a significant (P ≤ 0.01) effect on the moisture uptake. The initial cooking time of Zoasha, Nasir, Awash Weten and control samples were 101 minutes, 104 minutes, 101 minutes and 89 minutes, respectively. But, at accelerated storage conditions, the cooking time increased significantly (P ≤ 0.01) by 120%, 117%, 95% and 115%, respectively, for Zoasha, Nasir, Awash Weten and control sample varieties. At accelerated storage condition, the control sample should have the lower energy consumption by decreasing the cooking time by 25%, 22% and 20% from Zoasha, Nasir and Awash weten, respectively. The result could give an insight for food processors, breeders and policymakers to consider the hard-to-cook effects for the aforementioned common bean varieties.


PUBLIC INTEREST STATEMENT
Common beans hard-to-cook (HTC) effect issue is a challenge for food scientists and technologists to control.The postharvest storage conditions, such as temperature and relative humidity, affect the HTC phenomena.The HTC-affected beans demand more cooking energy, which costs a lot.The adverse effects of the storage conditions, such as temperature-RH combinations of ambient storage (25°C and 65% RH) as a control, 35°C and 75% RH storage condition, 35°C and 85% RH storage conditions were affected HTC effect on the bean varieties.As a result, at accelerated storage conditions, the cooking time increased significantly by 88%, 93%, 94% and 63%, respectively, for Zoasha, Nasir, Awash weten and control sample varieties.The result can give insight into the food processors and the scientific community about the HTC effects of the specified common bean varieties.accelerated storage condition, the control sample should have the lower energy consumption by decreasing the cooking time by 25%, 22% and 20% from Zoasha, Nasir and Awash weten, respectively.The result could give an insight for food processors, breeders and policymakers to consider the hard-to-cook effects for the aforementioned common bean varieties.

Introduction
Common bean, which is one of the most consumable legumes produced for direct human consumption, has the best commercial value exceeding all the other legumes combined (Brigide et al., 2014).It has a good source of dietary protein for people especially incapable of consuming animal products, even though they are poor in methionine and cysteine (Wacu et al., 2015).It is also a valuable source of the amino acid lysine and tryptophan; the minerals iron, copper and zinc; and beneficial phytochemicals, antioxidants and flavonoids (Petry et al., 2015).
Legumes like common beans play a vital role in achieving development goals (Akibode & Maredia, 2012).Nutritionally dry beans are nutritious and complement cereal crops as a source of proteins and minerals (Petry et al., 2015).In addition to nutritional value, they are important legumes agronomically.It provided essential nitrogen for cereal crops and was used as human food and animal feed (Vanlauwe et al., 2019).
Due to temperature and relative humidity fluctuation in arid and semi-arid parts of the world, grain storage becomes a more significant problem compared to areas with cold and moderate climates and subtropics (Njoroge et al., 2015).Legumes are subjected to storage for various periods of time before consumption.They are more difficult to preserve as compared to cereal crops due to susceptibility to developing hard-to-cook defects during storage in warm conditions, temperature (>25°C) and relative humidity (>65%; Coelho et al., 2007).In Ethiopia, about 25-40% of the Agricultural product is lost due to inappropriate storage temperature and relative humidity (Tefera, 2016).Knowing the optimum temperature and relative humidity is important to minimize the loss during storage (Kumar & Kalita, 2017).
Food malnutrition is the most concerning issue in a developing country like Ethiopia.The malnutrition problem in a developing country may be partly inadequate amount and undiversified diet.Even though common beans are one of the staple foods that can solve this malnutrition problem, they are susceptible to hard-to-cook defects while stored at accelerated storage conditions (Wacu et al., 2015).The hard-to-cook defects of common bean lead to prolonging the cooking time.The prolonged cooking time translates the high processing (cooking cost) due to high energy consumption.In addition to high energy costs, the prolonged cooking time may lead to some important components of common beans being lost due to their susceptibility to heat.This leads to malnutrition problems due to hard-to-cook defects, which result in deteriorating physical properties and cooking qualities, which may directly affect nutritional attributes.Due to the reduction of the nutritional quality, the appetizing ability and the consumer's acceptance become reduced (Pirhayati et al., 2011).
In Ethiopia, hard-to-cook phenomenon (HTC) is one of the major contributors to the malnutrition problem.This HTC phenomenon results from inappropriate storage conditions, mainly higher temperature and relative humidity, which legumes can store without significantly affecting their nutritional quality (El-Refai et al., 1988).These prolonged cooking times lead to spending more energy costs.Due to high energy costs, poor people further limit their consumption of nutritious food and shift to even lower balanced diets and less frequent meals, thereby affecting health and nutrition in the short and long term (Sawaya et al., 2009).
Storage of common beans under adverse conditions of high temperature (T > 25°C) and high humidity (RH >65) renders them susceptible to a hardening phenomenon, also known as the hardto-cook (HTC) defect.The most determinant factor of this phenomenon is the extended cooking time and resistance of moisture uptake.Mechanisms involved in the HTC defect have not been elucidated satisfactorily.Most researchers have reported that the defect develops in the cotyledons.Recently, some authors have suggested that the seed coat plays a significant role in the process of common bean hardening.A better knowledge of cotyledon and seed coat microstructure may lead to a better understanding of the causes of seed hardness.
There is limited literature on the hard-to-cook properties of recently released common bean varieties cultivated in Ethiopian climates and stored under different storage conditions.Therefore, the optimum storage condition (T and RH) to prevent HTC development and quality retain is required to be investigated for three recently released varieties from the Ethiopian research institute, namely, Zoasho (molted), Nasir (small red) and Awash weten (small white) and one commonly used common bean variety as local check, which is commonly known as SAB-632.These varieties are a recently released by Ethiopian Agricultural Research Institute.These varieties are good in yield and nutrient content, hence needed to be characterized for hard-to-cook effect and moisture uptake as affected by storage conditions.In addition to quality, the capability of each variety to develop HTC phenomena has been investigated.

Experimental site
The experiment was conducted at the Ethiopian Institute of Agricultural Research in Food Science and nutrition laboratories.The experimental samples were collected from Melkasa Research Center, which is located 115 km southeast of Addis Ababa with a longitude 39°21' E and latitude 8°24' N and an altitude of 1550 m.a.s.l.The soil type was classified as andosol.The mean maximum and minimum temperatures are 28°C and 14°C, respectively.The storage, cooking time and physical properties of common bean were done at Werer Agricultural Research Center, which is located 272 km from the capital Addis Ababa.

Sample collection and sample preparation
Three recently released common bean varieties and one commonly consumed common bean were collected from Melkasa Agricultural Research Center.All the varieties were stored in the dark room.Those varieties were Zoasho (molted), Nasir (small red) and Awash weten (small white) and one locally used variety (SAB 632).All samples were released in 2016, and the representative sample from each variety was packed in a polythene bag to prevent any mixing among samples when transported to Werer Agricultural Research Center, where the storage and analysis were done.All the representative samples were stored under various storage conditions for six-months of storage periods.
After collection, all the varieties were dried using an oven.The dried sample was subjected to soak with distilled water at the ratio of 1:5 (weight volume).After soaking, the sample was treated with heat at 96.5°C in the shaking water path.Beans are considered to be fully cooked at this temperature when the cotyledon could disintegrate on pressing between the thumb and the forefingers.The number of cooked beans was recorded continuously for 5 hours (Wood, 2017).

initialThe initial condition of the samples
The common bean sample varieties were collected and stored under dark roconditionsion immediately after harvesting.The initial determination of moisture uptake and cooking time wasdone and subjected to storage at temperaturestemperatures variable storage temperature and relative humidity.condition conditions

Experimental design
The experimental design was CRD.The factors were temperature, relative humidity, three recently released, and one commonly used common bean.The optimum temperature runs up to 25OC, whereas the relative humidity is up to 65%.But, if those storage conditions increase to 45oC and 85%, common beans could quickly develop hard-to-cook defects.Two factors (T and RH) had three levels.The temperatures were 25 O C, 35 O C and 45OC, whereas the relative humidity were 65%, 75% and 85%.Among those, 35 O C and 75%RH were the centre points.The optimum storage temperature and relative humidity for legumes are 25 O C and 65%RH.But, if the condition becomes accelerated storage condition up to 45 O C and 85%RH, the storage develops hard-to-cook phenomena.These conditions were selected to ensure the complete hardening of the bean in a relatively short amount of time.The third factor (variety) had four levels.Thus, Zoasha (Molted), Nasir and Awash Weten (small white) and one locally consumed bean that used as a control were used in this study.Those varieties were expected to develop HTC defects (Kinyanjui et al., 2015;Njoroge et al., 2015) at accelerated storage conditions.Therefore, the total number of experiments was thirtysix (36), and each experiment has done in triplicate.Analysis of the physical and cooking time was done for the bean varieties stored at the interval of two, four and six months in the triplicate runs to determine the quality loss during the accelerated storage conditions.The storage time was recorded immediately after harvest and the sample was subjected to store in the storage chamber.

Experimental designThe experimental design was CRD.
The factors were temperature, relative humidity, three recently released, and one commonly used common bean.The optimum temperature runs up to 25°C, whereas the relative humidity is up to 65%.However, if those storage conditions increase to 45°C and 85%, common beans could quickly develop hard-to-cook defects.Two factors (T and RH) had three levels.The temperatures were 25° C, 35°C and 45°C, whereas the relative humidity was 65%, 75% and 85%.Among those, 35°C and 75centerre the centre points.The optimum storage temperature and relative humidity for legumes are 25°C and 65%RH.However, if the condition becomes accelerated storage condition up to 45° C and 85%RH, the storage develops hard-to-cook phenomena.These conditions were selected to ensure the complete hardening of the bean in a relatively short amount of time.The third factor (variety) had four levels.Thus, Zoasha (Molted), Nasir, and Awash Weten (small white) and one locally consumed bean was that used as a control were used in this study.Those varieties were expected to develop HTC defects (Kinyanjui et al., 2015;Njoroge et al., 2015) at accelerated storage conditions.Therefore, the total number of experiments was 36, and each experiment was done in triplicate.Analysis of the physical and cooking time was done for the bean varieties stored at the interval of, two, four and six months in the triplicate runs to determine the quality loss during the accelerated storage conditions.The storage time was recorded immediately after harvest, and the sample was subjected to store in the storage chamber.

Sage time determination
The storage time was recorded immediately after harvest, and the sample was subjected to store in the storage chambertreatmenta treatment, combination of Temperature and Relative Humidity, the dependent variables (moisture uptake and cooking time) were analyzed in two-, four-and sixmonth storage periods starting from the initial storage periods.

Cooking time determination
In thermostatic water baths at different time intervals, 100 seeds from each variety were subjected to standard cooking at 96°C.The cooked samples were withdrawn from the water bath and cooled in a cold water bath for 1 min before determining the beans' cooking status (softness/hardness) or cook ability.This was determined subjectively by pressing the cooked beans between the thumb and forefinger.The beans were classified as cooked when the cotyledons disintegrated on pressing (Kinyanjui et al., 2015;Vindiola et al., 1986).The percentage of cooked beans in the batch was determined as a function of time.These data were then used to generate cooking curves by plotting the storage condition versus cooking time.These cooking curves define the cooking quality of the different bean varieties.

Moisture uptake determination
Twenty seeds from each variety were weighed and placed in the perforated bags for 30 minutes.After that, the bags were removed and reweighed, and the weight gain was calculated.The weight gain during soaking was considered as the moisture uptake, and the moisture uptake curve was plotted.

Statistical analysis
Statistical analyses of data were conducted using the Genstat 18th edition statistical software package.Comparisons between treatments were made using a two-way analysis of variance (twoway ANOVA) with a probability level of P ≤ 0.01.Significant differences between the treatments were made by fisher-protected LSD test.

Effects of temperature and relative humidity on moisture uptake of common bean varieties
The moisture uptake curve of the experimental sample is clearly shown in Figure Figure 1.According to this figure, the moisture uptake shows significant (P ≤ 0.01) degradation at increasing storage temperature and relative humidity over six-month storage periods.This may be due to the development of hard-to-cook defects and microstructural differences.At this condition, each cotyledon becomes strong, and water cannot segregate through.The result is in line with Njoroge et al. (2015).Zoasha had the lowest loss of a percentage of moisture uptake capability from the selected varieties at the maximum of a six-month storage period, followed by Awash weten.However, Nasir had the highest percentage reduction, followed by SAB_632.This shows that Nasir is highly susceptible to the development of hard-to-cook phenomena.Immediately after harvesting all varieties, the moisture uptake differs slightly.However, at accelerated storage conditions with increasing storage periods, the moisture uptake is significantly reduced (P ≤ 0.01).This may be due to the fact that stored beans develop stone cotyledon, which resisted the segregation of water.With the consideration of storage period (months) as the independent variable, Figure Figure 2 shows the moisture uptake curve of all the treatment samples at ambient (25°C/65%RH), mid (35° C/75%RH), and accelerated storage conditions (45°C/85%RH).According to Figure Figure 2, the moisture uptake for each variety with increasing storage periods at constant storage conditions shows a significant reduction at accelerated storage conditions at six-month storage periods.This may be due to the fact that because at the maximum storage periods, each variety becomes high water resistance, and the capability of holding water becomes highly reduced.The result of the experiment is in lin Wacoth Wacu et al. (2015).

Interaction effects of varieties and treatments on moisture uptake of common bean over six-month storage periods
The interaction effect of varieties and treatments (T and RH) on moisture uptake is represented by the data shown in Table 1.According to this table, the interaction effects of varieties with treatments show significant differences (P ≤ 0.01) in moisture uptake.The control sample shows the smallest reduction in moisture uptake (50%), followed by Zoasha (52%), Nasir (55%) and Awash weten (60%).This shows that at the end of six-month storage periods, the control sample is the best variety in terms of moisture uptake, and Awash wetena the high resistance for water intake capacity.This may be because all the varieties develop the hard-to-cook defects with different levels.The result is in line with Njoroge et al. (2015).

Effects of storage temperature and relative humidity on cooking qualities of common beans
According to Figure Figure 3, the cooking quality in terms of cooking time for all varieties shows a significant (P ≤ 0.01) increment at accelerated storageconditionconditions.Zoasha increased the cooking time by 31% at midpoint (35°C/75% RH) and by 88% at accelerated (45°C/85%RH).Within the same storage temperature and relative humidity, the Nasir variety has increased the cooking time by 32% and 93% from the normal storage condition.Figure Figure 3 also shows the increasing trends of cooking time for Awash Weten and SAB-632 significantly (P ≤ 0.01).According to the curve in the figure, the Awash Weten variety has extended the cooking time by 33% and 94% at 35°C/75% RH and 45°C/85%RH, respectively, whereas the local check (SAB-632) has prolonged the cooking time by 23% and 63% from the initial cooking time at the same storage condition for other varieties with the increasing temperature and relative humidity; the cooking time of common beans extends significantly (Njoroge et al., 2015).This may be due to the development of hard-to-cook phenomena and become high resistance to heat and are directly associated with the soaking characteristics of common beans.Moreover, the varieties have shown a visible difference in cooking time as affected by storage conditions (Figure Figure 3).
The cooking time of each common bean variety is highly dependent on storage periods.According to Figure Figure 4, the cooking time shows a significant (P ≤ 0.01) increment from the initial cooking time.This indicates that the common bean varieties become difficult to cook with increasing storage periods, develop hard-to-cook defects and become resistant to heat and water absorption.The trends and results of the experiments are inline with Shimelis and Rakshit (2005).Awash, Nasir and Zoasha and control varieties were shown from the highest to lowest cooking time, respectively, along all the storage T/RH and storage periods.

Interaction effect of varieties with treatments on the cooking time of common bean over six-month storage periods
The interaction effect of varieties and treatments (T and RH) cooking time is represented by the data shown in Table 2.According to Table 2, the interaction effects of varieties with treatments show significant differences (P ≤ 0.01) in cooking.All the varieties show the increasing trends of cooking time over six-month storage periods.From all varieties, the control sample is the best varieties in terms of cooking time, which increase its cooking time by 95% from initial followed by Nasir 102%, Zoasha 106% and Awash Weten 109%.This shows that at the end of six-month storage periods, the control sample is the best variety in terms of cooking time, and Awash Weten is highly resistant to heat and prolonged cooking time.This may be due to the fact that all the varieties develop the hard-to-cook defects with different levels.The result is in line with Kamau and Wacu (2016).

Conclusions
The experiment was carried out at high temperatures (35°C and 45°C) and relative humidity (75% and 85%) led to lower moisture uptake and extended cooking time.From recently released varieties, the control sample is the best variety in terms of the resistance of moisture uptake and shorter cooking time at the accelerated storage condition, followed by Zoasha, Nasir, and Awash varieties in terms of cooking time and moisture uptake.The shorter cooking time is the indicator of the resistance of the bean variety to the development of hard-to-cook phenomena.
During the storage of each variety under high temperature and relative humidity, the moisture uptake was significantly decreased, which shows that each variety had developed hard-to-cook phenomenally in a six-month storage period.The increasing temperature and relative humidity also significantly increase the cooking time of the common bean.This shows that all the varieties become susceptible at accelerated storage conditions to develop hard-to-cook phenomena and are proofed by extended cooking time; with increasing storage periods, the moisture uptake decreases with a negative correlation where the cooking time correlates positively with storage periods.From the experiments carried out, the higher relative humidity (85%) leads to significant change in both the moisture uptake and cooking time when it is combined with temperature.This shows that optimizing relative humidity to the optimum range (65%) up to 75% will preserve common beans for longer periods with minimal change, even at a higher temperature.

Figure
Figure 2. Moisture uptake curve of four varieties over six month storage periods at a constant storage temperature and relative humidity (00/00 = moisture uptake immediately after harvest).

Figure
Figure 4. Effects of storage conditions on four common bean varieties over six months of storage periods at constant storage temperature and relative humidity (00/00 = cooking time immediately after harvest).