Sample preparation

The potato of the Yungay species was purchased from the farmers of Tarma because the solar dryer system was installed within the facilities of the Faculty of Applied Sciences of the National University of Central Peru (UNCP)-affiliate Tarma, which it is located at an altitude of 3 040 masl (latitude of 11° 25’ 12” S and longitude of 75° 41’17” W). Its climate is arid and semi-cold with an average wind speed of 1.1 m/s, average annual temperature ranging from 13 °C to 18 °C, and average solar radiation of 5.40 kW h^-1 m^-2 day^-1 as reported by ^{Camayo-Lapa et al. (2017}).

Potatoes of similar diameter and size were selected, washed and cut into 1 cm x 1.5 cm cubes. The pieces were then scattered inside in a drying room. In total 850 g of weight was used for each experiment. The initial relative humidity measured in the potato samples was 67.0%.

Physical, physicochemical and chemical characterization of the sample

Color was evaluated by means of a colorimeter (Lovibond, RT100, Germany) with illuminant D65 and a 10º standard observer in which the reflection spectra of the samples were determined with the coordinates of the CIE-L^*a^*b^*, where the luminosity (L^*), the physicochemical characteristics such as total acidity following the titration method; 942.15 (^{AOAC, 1996}), the result was expressed as a percentage of sulfuric acid. The pH was determined by the potentiometric method, using a pH meter (Schott, PH11, Germany) and the proximal chemical composition with the methods recommended by the Association of Official Agricultural Chemists - AOAC International (^{AOAC, 2006}). In ash (total ash method) and humidity (stove drying method).

Autonomous indirect solar dryer

In Figure 1, the solar collector and its two reflectors are observed, which are flat mirrors and as a whole, it forms the thermal capture system, they are responsible for capturing solar radiation and transforming it into thermal energy (heat) to heat the air that it goes to the drying chamber where the product to be dried is.

In this study, the drying chamber allows up to 20 kg of product to dry and has a built-in ventilation system, humidity and weight sensors, both monitored by means of an electronic display attached to the dryer. Through the display, a system allows the temperature in the drying chamber to be manipulated, according to the required drying temperature, for each product according to technical quality standards. This temperature programming has a range from 20 °C to 70 °C according to the recommendations of (^{Infoagro, 2020}).

Figure 1 Parts of the autonomous indirect solar dryer. 

Installation of the automated solar dryer

The solar collector and its reflectors were oriented to the geographic north, since Tarma is in the southern hemisphere of the earth, thus making better use of incident solar radiation. In addition, the thermal collection system, oriented to the north, was installed with a 22-degree inclination to favor the collection of solar energy during the winter months. The solar collector and the drying chamber were connected through a rectangular profile stainless steel duct, which prevents the leakage and loss of hot air during the passage of hot air into the drying chamber.

Design, simulation and elaboration of the control system

To have control of the system, a control circuit board was designed and implemented using Proteus software (Version 8.9, England). In addition, sensors were installed in the drying cabin, in the chimney and in the solar collector. Measurements of the operating system of the solar dryer were made, for this an algorithm was created with the objective of registering the temperature and humidity on the display in real time. The flow chart of the stages considered in the system to control/manipulate the temperature and humidity is shown in Figure 2.

Context for performance evaluation of the drying process

A few meters away from the solar dryer, a meteorological station was installed where meteorological variables such as ambient temperature (°C), relative humidity (%) and solar radiation (W/m²/h) were measured and recorded, in addition of the internal temperature (°C) of the drying chamber for three consecutive days between 8:00 am to 5:00 pm local time in the city of Tarma (FACAP-UNCP).

Figure 2 Flow chart of the temperature and humidity control system. 

Analysis of data

The data from the physical, physical-chemical and proximal chemical analysis were reported as mean ± standard error, determined in triplicate. Differences between treatments (drying temperature and sulfite concentration) were analyzed by two-way analysis of variance (Anova) and posterior Tukey (p< 0.05). Measurements of the behavior of solar radiation, internal and external temperature in the solar dryer were plotted using SPSS Software V.24 and R Project (free software), version 3.3.6 (^{R Team Core, 2019}).

Automated no-load solar dryer measurements

Preliminary tests without load were carried out with the aim of verifying the operation of the solar dryer. The average results ± standard deviation (DS) of the measurements made (ambient temperature (Ta), solar radiation (RS), relative humidity (HR) and internal temperature of the dryer (Ti)) without load and every hour (8:00 h -17: 00 h) during the three days of tests, in the automated solar dryer are shown in Table 1. The results showed an ambient temperature range of 7.3 °C to 19 °C (13:00 h), while the internal temperature of the internal dryer varied between 10.7 °C to 50.7 °C (12:00 h).

In turn, solar radiation ranged from 72 W m^-2 h^-1 to 880 W m^-2 h^-1. A similar behavior was observed among the three variables (Ta, RS and Ti) previously mentioned, with an increase in their value in the early hours (8:00 am to 1:00 pm) in the morning and a decrease in the evening (2:00 pm to 5:00 pm), reaching its maximum values between 11:00 h and 14:00 h. In contrast, a decrease in relative humidity was observed as the hours passed, being more intense in the hours from 9:00 to 12:00 h. The global radiation and ambient temperature recorded by the Davis meteorological station at the faculty in Tarma are approximately similar to those estimated by ^{Camayo-Lapa et al. (2017}) of 5.47 kW m^-2 day average monthly monthly, which makes the use of any equipment for water or air heating very profitable. ^{Seveda and Jhajharia (2012}) reported a 75% reduction in humidity, showing results similar to that of this work.

This is in agreement, since the highest solar radiation energy (13:00 hours showed the highest radiation of 880 ±144 W m^-2 h^-1) was captured during this period. This is explained because, if a higher concentration of solar radiation reaches an area or surface, the temperature of this area will be increased (^{Daut et al., 2012}). These results showed a higher internal temperature of the chamber (12:00 h showed the highest internal temperature of 50.7 ±7 °C) within this period, thus demonstrating that the installed solar dryer system and photovoltaic panel work perfectly.

Figure 3 shows the average ambient temperature vs. the average internal temperature of the drying chamber (average of the three days). The internal temperature of the drying chamber showed a notable increase in its temperature up to a maximum of 50.7 ± 7 °C (this observed at the time corresponding to 12:00 h) due to the action of the control system that was configured at this temperature as the maximum. This result corroborates the good operation of the control system, made up of two fans that come into operation if required. It is noted that the internal temperature of the drying chamber is to provide room temperature and solar radiation since higher chamber temperature, higher temperatures and solar radiation values were recorded.

Figure 3 Ambient and internal temperatures of the average dryer, controlled at 55 °C. 

Testing of the automated solar dryer with potato Yungay

To evaluate the viability of drying the Yungay potato in the automated solar dryer, the potatoes were first peeled and cut into cubes of 1 cm x 1.5 cm, in total 50 cubes with a weight per sample tested of 850 g. These cubes were immersed for 5 min in blanching using a solution of sodium bisulfite at two different concentrations: 0.02% and 0.03% w/v, once removed they were drained for 5 minutes and they were dried at three temperatures: 50 °C, 55 °C and 60 °C for an approximate time of 72 h, 3 consecutive days, obtaining a 70% reduction (from 850 g to 250 g, from 67% to 13%) in the weight of the sample. Table 2 shows the physicochemical characteristics analyzed after the drying process.

From Table 2, it is noted that when evaluating the interaction of pretreatment with bisulfite and drying temperatures, they did not show a significant difference (p> 0.05) in their physical, physical-chemical and proximal chemical characteristics: humidity, acidity, ash, pH and luminosity. Inferring that a temperature of 50 and a concentration of 0.02% of sodium bisulfite would be optimal for obtaining the dried potato. At these drying temperatures the measured quality parameters confirm that the final product is suitable; also, when evaluating the final appearance of the product with respect to the color attribute, all the samples have a light yellow + brown color.

Testing of the automated potato solar dryer

For the validation of the automated solar dryer, pretreatment tests were carried out on the peeled potato (potato cubes) with bisulfite to obtain dry potato as presented in Table 2. In this regard, ^{Ahmed et al. (2010}) evaluated the effect of peeling, drying at temperatures between 55 °C-65 °C and a pre-treatment of its physicochemical properties and nutritional quality.

They noted that bisulfite inhibits enzymatic browning by causing a reaction between sulfite ions and quinines by inhibiting polyphenoloxidase activity, oxygen depletion, and offering color stability of the final product. They concluded that a better quality product was obtained when the samples are pretreated with sulfite before drying at any temperature. Likewise, drying in the automated dryer helps to preserve the color in its luminance parameter (L^*) and this variable allows to directly evaluate the color degradation of the potato to be dried (^{Espinoza, 2016}).

The NTP 011.119 ‘potato and its derivatives: potato, definitions and requirements’, establishes as quality parameters in dehydrated potato a maximum of maximum 15% humidity and 2.5% ash (Indecopi, 2010). From Table 2, a maximum humidity of 1.89% (0.02% bisulfite concentration) and ash of 2.49% (0.03% bisulfite concentration) are observed. From this it is established that the characteristics (humidity and ash) of the dried Yungay potato obtained in the automated solar dryer is within the quality parameters of the 011.119 standard. It can be affirmed that using the solar dryer and controlling the parameters, a good quality product was obtained and allows us to go on to the development of an industrial automated solar dryer.

^{Cheftel (2000}) pointed out that the availability of water (as a solvent) induces reactions and provides favorable conditions for the development of microorganisms, which could deteriorate the product. In this case, the low percentage of humidity (13.89%) compared to the Peruvian standard (15%) guarantees that our product (Yungay dried potato) will have a longer shelf life, since the probability of propagation of microorganisms is minimal.

^{Waterschoot et al. (2016}) point out that the ash content is often related to the presence of minerals such as phosphorus, sodium, potassium, magnesium and calcium. In this work, such minerals were not measured, but we assume the presence of them because the potato is cultivated in soils where their presence exists and that many times they are absorbed through the roots. A report with the content of different minerals in different potato varieties is reported by the Ministry of Health (^{MINSA, 2017}). ^{Peña (2017}), for his part, commented that the variability of the ash content basically depends on the maturity of the product and its origin. Ash values in different types of potatoes (for example, bitter, yellow, chuño, white frost, etc.) are in the range of 1.1% to 3.7% (^{MINSA, 2017}).

^{Obregón and Repo (2013}) reported pH values of 6.90, 6.60, 6.30 and 6.70 for the Peruvian potato varieties, yellow runtus, huayro and huamantanga, respectively. These reported values are in agreement with the values obtained in this work (Table 2). The pH has an inversely proportional correlation to the acidity values (^{Salazar et al., 2008}). This behavior is shown in Table 2, where at higher pH concentrations, lower acidity concentrations.

This behavior is due to the fact that the organic acid content of the potatoes varies according to their state of maturity. ^{Ojeda et al. (2010}) evaluated thirteen different potato clones under different environments (areas). Results showed differences in the number of stems, dry matter and leaf area index. Likewise, differences were detected in its physical variables (specific gravity, dry matter, and polar and equatorial diameters) and chemical variables (acidity, pH, starch, and sugars). These results agree with those found in this work.