Introduction

Bamboo is the generic name given to plants of the Poaceae family, Bambusoideae subfamily, distributed in practically all ecosystems of the world. Today, 121 genera and 1 662 species of bamboos are recognized (^{Canavan et al., 2017}). In Mexico, there are eight genera and 61 species of woody bamboo have been identified, 42 of which are endemic to Mexico (^{Ramírez-Ojeda et al., 2021}; ^{Ruiz-Sánchez et al., 2022}); the subtribe Guaduinae is the most important culturally and economically, and the Guadua genus is the most representative (^{Londoño et al., 2002}), as due to its large dimensions (diameter and length) and excellent physical and mechanical properties, it has great economic potential, particularly for housing construction in tropical regions (^{Ordóñez et al., 2013}).

Within the Guadua taxon, one species that stands out for its anatomical, physical and mechanical characteristics is Guadua aculeata Rupr. ex E. Fourn. (tarro, caña brava), reaching 25 m in height and 18 cm in diameter at the base, with hollow internodes 20 to 30 cm long and a thickness of 2 cm (^{Cedeño and Irigoyen, 2011}). The most commonly used part of the bamboo is the culm (main stem) of the plant, whose traditional shape is cylindrical; however, its diameter decreases with height. It is internally separated crosswise by solid nodes, and its internodes are hollow (^{Chaowana et al., 2021}). Anatomically, it consists of a cortex (cuticle), fibers and parenchyma cells. The proportions and dimensions of each component vary between species, within species, and within the same individual, this generates a great heterogeneity in their behavior during the transformation processes (^{Yuan et al., 2022}).

The drying process is key to the industrial and commercial utilization of the culms, since it improves both their structural properties and their aesthetic appearance (^{Tang et al., 2012}; ^{Burger et al., 2017}; ^{Wang et al., 2019}), expanding their area of application and improving their utilization value (^{Lv et al., 2022}). However, producers, traders, and users of culms have not given it the required importance, as they consider it a natural process (^{Montoya and Jiménez, 2006}) that can take weeks to several months to attain a moisture content close to 12 % (^{Liese and Tang, 2015}), which is considered to be a percentage suitable for various uses.

Cracking and splitting along the fibers are the main flaws that occur during drying of culms (^{Lv et al., 2018a}; ^{Lv et al., 2021}) as an effect of low transverse tensile strength due to a lack of radial cells (^{Liese and Tang, 2015}; ^{Lv et al., 2018b}).

Up to the present day, little research has been done on the solar drying of bamboo culms (^{Ong, 1996}; ^{Montoya and Jiménez, 2006}; ^{Morales-Pinzón et al., 2012}; ^{Vetter et al., 2015}; ^{Hossain et al., 2021}; ^{Kaba et al., 2022}), in all of which sections less than 2 m long were utilized. Given the scarce importance that has hitherto been attached to the solar drying process of 6 m bamboo culms for structural use, the present study tests this drying method for this size of culms as a more efficient option than open air drying. The objective of this study was to evaluate the drying process of Guadua aculeata culms in an active solar tunnel dryer in the Hueytamalco region, state of Puebla, Mexico.

Materials and Methods

The collection of the material consisted in obtaining 6 m long pieces of mature culms, which diameter at breast height was measured at a height of 1.3 m above ground level, with a Forestry Suppliers^® 5 m-long measuring tape, model 283D/5m-CSE, accurately graduated to the millimeter. The normal diameter at breast height of the culms ranged between 10.7 and 13.8 cm and averaged 12.4 cm. These pieces were obtained in natural stands of G. aculeata located in the Las Margaritas site in the Hueytamalco municipality, Puebla, Mexico, between the geographical coordinates 19°52' and 20°12' N, and 97°12' and 97°23' W, at an average altitude of 430 m.

A solar dryer built at the Las Margaritas Experimental Site of the Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP) was used to dry the culm sections.

The drying process was carried out from August to November 2018. The solar dryer used is of the tunnel type, in the form of a rectangular prism, it consists of a semi-parabolic roof and rectangular walls covered with polyethylene with ultraviolet protection, a PTR structure, and a concrete floor. The dryer has an internal solar collector with the shape of a false ceiling, built with smooth aluminum sheets painted matte black. The dimensions of the dryer are 10 m in length, 4 m in width, and 2.8 m in height (Figure 1). The main modifications made to the original design were the replacement of the fans with a single 20 cm-diameter wind extractor fan positioned on the rear wall and centered at the bottom, whose purpose is to extract the humid air from the interior and expel it out of the dryer.

Figure 1 Diagram of the solar dryer used for the drying of Guadua aculeata Rupr. ex E. Fourn. culms. 

The temperature and relative humidity were recorded with two HOBO^® UX100-003 Data Loggers, with an error of 3.5 %; one was placed inside the dryer, and the other outside, at a height of 2 m from the ground, in order to record the external environment condition. Daily and monthly average solar radiation data were obtained from the Global Solar Atlas provided by the World Bank Group (^{Solargis, 2023}) database for the geographic coordinates 20.000610, -97.307414 (20°00'02", -097°18'27"), corresponding to the location of the Las Margaritas Experimental Site facilities.

In order to assess the solar drying process, a pile was formed with 50 pieces each 6 m long, collected in a proportion of 80 % from the basal part and 20 % from the middle third of the total length of the culm. At the time of the felling, four 6.1 m pieces were selected to be used as drying samples, and from each piece two 5 cm long moisture probes were obtained, one from each end.

The pieces were stacked horizontally in five beds of 10 pieces each, and between them were placed, as separators, three 2.5 m long culms of a smaller diameter than those stacked, with a separation of 3 m approximately. Two drying samples were placed on each side of the pile, one at the bottom, and the other in the middle (Figure 2).

Figure 2 Stacking of Guadua aculeata Rupr. ex E. Fourn. culms inside the solar dryer. 

The initial moisture content (MC) in green condition of the dried samples was determined using the gravimetric method, for which purpose the specimens were weighed on an Ohaus^® PR Series Precision PR1602/E digital electronic balance with an accuracy of 0.01 g in order to determine their wet weight; then they were placed in a HDP 334 TECSA^® laboratory oven, for drying at 103±2 °C for 24 hours, the period required to reach their dry weigh. The MC of each specimen was calculated as the weight loss, expressed as a percentage of its dry weight, by means of Equation 1 (^{International Organization for Standardization, 2019}):

Where:

MCi = Moisture content of each test specimen expressed as a percentage of its respective dry weight

 IWw = Initial wet weight of specimen before drying

Dw = Oven dry weight of the test specimen

The drying process was monitored by estimating the calculated dry weight (CDW) of the drying samples, using the average MC obtained from the aforementioned specimens and the weight of each sample at the time of the felling and cutting of the culm (Equation 2) (^{Simpson, 1991}; ^{Aquino-González et al., 2010}):

Where:

CDW = Calculated dry weight for the sample

IWw = Initial wet weight of the drying sample

MCi = Average moisture content of the test specimens, determined with the Equation (1)

The drying process was monitored using the gravimetric method, by estimating the weight loss of the drying samples, which were weighed every two days; the MC was determined by means of Equation 3 (^{Baranski et al., 2021}):

Where:

MC _ds = Moisture content of each drying sample at the time of weight recording through the drying process

Ws = Weight of samples at any time during the drying process

CDW = Calculated dry weight of the drying samples

The drying rate (DR) was determined with Equation (4), which describes the variation of moisture content measured at a time (t) with respect to the previous time (t-1) (^{Montero and Rozas, 2019}):

Where:

DR = Drying rate (% MC day^-1)

MC∆ = Moisture content variation (%)

t∆ = Time between measurements (day)

Results

Solar drying curve of culms

The drying process of G. aculeata was monitored during a period of 1 920 hours (80 days); the initial average MC of the drying samples was 106.71 %, and the average observed MC reached at day 80 was 29.84 %, which is within the range of the MC corresponding to the fiber saturation point (^{Liese and Tang, 2015}). Figure 3 shows the average fitted drying curve and the respective standard deviations represented by the exponential Equation (4), whose highly significant regression coefficients values ―with a probability level of 95 %― were α=95.02205 and β=0.00064 (α=0.05, p value=2e-16), along with the observed drying trends. The values obtained for the goodness-of-fit statistics of the model were 0.9856, 2.4996, 985.80, 1005.41, and -486.90 for R ² _adj , RMSE, AIC, BIC, and logLik, respectively.

Figure 3 Observed graphical behavior of the solar drying process of culms and trend of the average drying curve based on exponential expression. 

In order to estimate the drying time (t) in hours required for an MC value of interest considered suitable for the intended use of the culms, it was necessary to find the value of t in Equation (5), which gave rise to the Equation (6). However, in order to estimate the drying time in days, the result of Equation 5 should be divided by 24.

t=-ln⁡MCαβ (6)

Where:

t = Drying time (hours)

ln = Natural logarithm

MC = Moisture content (%)

α and β = Regression coefficients used to generate the drying curve

Drying rate of Guadua aculeata in a solar dryer

The moisture loss per day, as well as throughout the drying period, was different in each of the sample culms. Figure 4 illustrates the behavior of the two samples that had the slowest and fastest drying rates during the drying process. In both cases, it can be seen that at the beginning of the process, when their moisture contents were high, more than 1.0 % moisture was lost per day. Drying sample 1, with an initial MC of 108.36 %, exhibited a maximum moisture loss rate of 1.86 % per day and maintained the drying rate above 1.0 % for the first 16 days, until it reached approximately 78 % moisture. From that point on, the rate decreased, ranging between 1.0 and 0.6 % until day 40, when it reached a MC of 60 %, after which the drying rate remained below 0.6 % every 24 hours.

Figure 4 Drying rate of two bamboo samples with solar drying, every 24 hours. 

Drying sample 3 had an initial MC of 114.52 %, it started with a moisture loss of 2.9 % every 24 hours, with a constant drying rate above 1.5 %, until the MC was reduced to approximately 66 % in the first 17 days; subsequently, the drying rate was maintained above 1.0 % until the MC reached 44 % after 38 days; from this point on, the average drying rate was constant at 0.5 % every 24 hours.

Discussion

The initial MC on a dry basis of the different bamboo species cited in the specialized literature is above 100 %, although there are variations depending on the species, growth area, and cutting season (^{Hossain et al., 2021}). For the G. aculeata culms studied, the average initial MC was 106 %, which is higher than the value recorded by ^{Wang et al. (2019)} for Phyllostachys heterocycla fo. pubescens (Pradelle) D. C. McClint., of 90 %. Even though there is a clear difference in MC between the different heights of a bamboo culm, ^{Hartono et al. (2022)} document average moisture contents of 223.4 % in the lower part of the culms of Bambusa vulgaris var. vittata Rivière & C. Rivière. As for the drying process, according to ^{Tang et al. (2012)}, bamboos with solid culms are easier to dry than species with hollow culms, such as G. aculeata. ^{Montoya and Jiménez (2006)} indicate that the drying period depends on species-inherent factors such as culm diameter, wall thickness, initial moisture content, age, position in culm height, length of the sample to be dried, and growth site.

A fundamental factor to be considered in determining the drying time of bamboo culms is the length of the pieces, longer pieces require longer periods because the movement of moisture through the anatomical structure of bamboos only occurs in the longitudinal direction. In the absence of anatomical elements in the transverse direction such as vessels or parenchyma, moisture movement in this direction is secondary (^{Huang et al., 2015}; ^{Lv et al., 2021}). Thus, ^{Morales-Pinzón et al. (2012)} dried 6 m-long preserved sections and needed at least four months to attain a MC of 19 to 27 %.

In order to dry in a solar dryer 2.5 m-long culms of three bamboo species with an average initial MC of 60 % to 14 %, ^{Hossain et al. (2021)} required 30 days for Bambusa balcooa Roxb., 27 days for Bambusa vulgaris Schrad. ex J. C. Wendl., and 24 days for Schizostachyum Nees. While, in order to conduct studies on drying of 4 m long Yushania alpinia (K. Schum.) W. C. Lin culms stacked horizontally under a shed without exposure to humidity, rain, or sunlight, ^{Kaba et al. (2022)} took 97 days to dry from an initial MC of 110.81 % to a final moisture of 13.90 %. In another study on air drying and shed drying, by ^{Yan et al. (2022)}, it is reported that 6 m long sections of Phyllostachys edulis (Carrière) J. Houz. culms with an initial MC of 76 % attained a final MC of 11 % after six months.

In the present study, we started with an average MC of 106.71 % and required 80 days to reach a final MC of 29.84 %. However, with the exponential function generated, 4.7 months are estimated to be required to attain a final MC of 11 to 12 %; this proves, by comparison, that solar drying is 22 % faster than traditional drying, with a low proportion of defects due to cracking, and improves the quality of the product (^{Chen et al., 2023}).

When the fitted exponential function is utilized, a period of 109 days is estimated to be required for the drying of 6 m-long canes of G. aculeata (^{Seduvi, 2017}) at a MC value of less than 18 %, a percentage suggested for use in structural design. This is similar to the 106-day period calculated by ^{Montoya and Jiménez (2006)} in Pereira, Colombia, for Guadua angustifolia Kunth to attain a moisture content of 17.5 %; their study used the exponential function MC=132.42×exp⁡(-0.0188×t) with an R ² of 0.969. In this sense, ^{Vetter et al. (2015)} states that drying of bamboo culms in their round form requires long periods of time; this is an indispensable condition to achieve the minimum of defects due to cracking, which, according to ^{Chen et al. (2023)} usually occur at the nodes, at the internodes and at both ends of the sections of the culms.

The drying rate at which the G. aculeata culms lost moisture during the solar drying process was above 1.0 % per day during the first 20 days, going from an initial moisture content of 106.71 % to an average moisture content of 67 %, i. e., on average they lost approximately 40 % moisture. After this period, the drying rate was variable, averaging a 37.41 % loss over the next 60 days, with an average rate of 0.62 % per day, although some drying samples reached a daily drying rate of close to 1.0 % during this second period. The values obtained for the drying rate are higher than those recorded by ^{Yan et al. (2022)} for the open-air and shed drying of Phyllostachys edulis culms, which exhibited a daily drying rate of 0.50 % when reducing its MC from 76 to 35 % in 81 days and of 0.22 % when going from 35 to 11 % in a period of 108 days. However, although these are average drying rates, the height of the culm from which the section to be dried is obtained must be considered in the process, since pieces from the middle part have a higher drying rate than those coming from the lower part, as shown in the results of the two samples analyzed in relation to the drying rate, sample 1 was obtained from the basal part and exhibited a lower drying rate than sample 3, which was taken from the second third of the culm; this is due to the chemical, physical and anatomical structure differences that exist in bamboo culms (^{Tang et al., 2012}; ^{Chaowana et al., 2021}), as well as to the greater wall thickness of the lower section of the culm (^{Vetter et al., 2015}; ^{Chen et al., 2023}).

Conclusions

The evaluation of the solar drying process of commercial Guadua aculeata culms from Hueytamalco, Puebla, Mexico, using sections of 6 m in length, allows us to conclude that the drying time of Guadua aculeata culms under solar drying conditions in Hueytamalco, Puebla, can be estimated with the equation: t=-ln⁡MC95.022050.00064. Using the adjusted exponential equation, a period 109 days is estimated to be required to dry G. aculeata culms to a moisture content of 18 %, which is the percentage suggested for structural use. The G. aculeata culms exhibit a drying rate of over 1 % during the first 20 days of processing, and of 0.62 % on average during the remaining 60 days.