1. Introduction

Native potatoes, also known as Andean potatoes (Solanum tuberosum andigenum), have the potential to become highly valued products of agro-industrial interest. These potatoes are tubers of various colors and pleasant organoleptic qualities [¹] that can be grown under different environmental conditions than commercial varieties, allowing for diversification of production as well as food security in low-resource areas [²].

In the Andean region of South America, native potato cultivars have been preserved ancestrally by indigenous people and small farmers, mainly for household consumption. These products, in a lesser sense, are also used for trade in local markets and squares [³]. In spite of its favorable characteristics, cultivated area has been reduced and some varieties have been lost due to both the lack of knowledge of consumers and the introduction of improved varieties [⁴].

There is growing interest in the study and conservation of these potato varieties for many reasons, including: to understand of their evolutionary process in a certain region [⁵], to contribute to the conservation of biodiversity and reduction of the loss of genetic diversity in native or introduced varieties [⁶], to generate technological innovations in production and post-harvest, to introduce them in value chains and fortify producers [⁷], to increase the visibility of native potatoes, and highlight their characteristics to promote greater consumption [⁸,⁹].

To improve competitiveness in the potato value chain, an investigation into post-harvest behavior is required [¹⁰], and changes that occur during storage prior to processing must be understood. Potato tubers in storage present physiological and chemical changes due to their metabolism, which directly impact the shelf life and quality traits of potatoes and their products [¹¹].

This study records changes during post-harvest storage of two native potato varieties from the department of Nariño, exposed to three temperature conditions, to evaluate changes in weight, moisture, glucose and ascorbic acid. The two potato varieties were stored for 52 days in refrigerated storage (4°C), room temperature (18°C), and incubation (27°C).

This research aims to provide methods to improve the storage processes of a tuber like the potato, which is a very important food in the national trade and domestic consumption. The adequate conservation of this vegetable, based on the results of this research, will significantly help to minimize the post-harvest losses of this starch.

2. Materials and methods

In this investigation, two potato tubers varieties were considered: UdenarST Cr29 (common name Curiquinga or CUR), cultivated in the experimental farm Botana of the University of Nariño, and UdenarST Cr16 (common name Ratona Morada or RM), grown in the Obonuco Research Center of the Colombian Corporation of Agricultural Research (AGROSAVIA). Both farms are located in the city of Pasto, capital of the department of Nariño, Colombia.

CUR according to [⁵, ¹²] is characterized as compressed, round-shaped tubercle with very thick, dark purple and yellow skin. The pulp is white and violet with scattered spots. Storage time at room temperature lasted until the detection of deterioration signs such as rot, softening and/or sprouting is about 23 days. This variety is ideal for frying, has floury consistency and good flavor. In addition to being used for chips it can be served as mashed potatoes, soups and for stewing.

The RM has an elliptical shape, its skin is thick, deep purple, and the color of the pulp is cream with purple in scattered areas. Its storage period is 15 days, it has good aptitude for frying as chips, in addition to possible culinary uses in soups, purées, stewing and lapingachos (potato tortillas typical of the region). Its consistency is also floury, and it is flavorful.

All tests were carried out in the Food Conservation and Quality Research Laboratory of the Faculty of Agroindustrial Engineering of the University of Nariño, seat Torobajo, in Pasto, which is located at an altitude of approximately 2527 masl and has an average summer temperature above 18ºC. The tubers were specifically selected so that they would not present evident damages or diseases. After the harvesting, they were cleaned with a brush, and for curing and pre-conditioning [¹³], they were kept at room temperature in a dark, ventilated environment for 3 days. Prior to storage under controlled conditions, they were washed, and the surface water was removed. Five control tubers of the sample were randomly selected, their weight was then recorded during the experiment for each storage condition.

A ventilated and dark cabinet inside the laboratory was used for storage at room-temperature conditions, a laboratory fridge with automatic temperature control for cooling conditions, and an incubator with temperature control was used to simulate high temperature conditions. Together with the control samples, 60 tubers were stored at each temperature. Approximately every 4 days, the control samples were taken to determine their weight loss with an analytical balance (Ohaus, Pioneer PA214). Another 5 tubers were selected at random from each storage condition in order to perform moisture measurements with a moisture analyzer balance (Radwag, PMR 50) and to determine the glucose and ascorbic acid content by means of the Reflectoquant® system (Merck, Darmstadt, Germany). The environmental conditions were registered with a Datalogger thermo-hygrometer (CEM, DT-172).

An unrestricted random sampling (URS) was used with a factorial arrangement of 12x3x2, where the first factor corresponded to the Day of measurement (D), with 12 levels (0, 2, 7, 10, 14, 17, 21, 29, 31, 35, 48 and 52). The second factor consisted of the storage temperature (S), whose levels were 4°C, 18°C and 27°C. The third factor (C) were the cultivars of native potatoes Curiquinga and Ratona Morada, for a total of 72 treatments with 5 repetitions; 360 experimental units were counted. An analysis of variance was performed by means of the F-test (at 5% probability) for each of the response variables (moisture, weight loss, glucose and ascorbic acid content), and in the case of statistical differences between treatments, the Tukey multiple means comparison test was applied at a 95% confidence level.

The determinations made were total weight loss with respect to the initial weight, expressed as a percentage [¹⁴]. For this determination, a record of the weight of each of the five control tubers in each storage environment was taken on the days where the analyses were performed. The moisture content was determined through drying about 2g of potato (with peel) cut into thin slices at 105°C on a moisture analyzer.

The method used to determine the content of glucose and ascorbic acid is based on the principle of reflectometry (reflectance photometry), where diffuse reflected light is measured on a test strip using the Reflectoquant system tests on an RQFlex plus10 Reflectometer. As in classical photometry, the difference in the intensity of the emitted and reflected light allows the quantitative determination of the concentration of specific analytes. In glucose quantification the method recommended by the manufacturer is used [15, 16]. Through the catalytic action of glucose oxidase on the test strips, the glucose is transformed to gluconic acid lactone. The hydrogen peroxide formed here reacts in the presence of peroxidase with an organic redox indicator, producing a bluish green dye that is determined reflectometrically.

The analysis of ascorbic acid content was carried out through the extraction of the material in the potato juice using trichloroacetic acid. The hydrogenation of dehydroascorbic acid takes place on the test strips, and through the action of the ascorbic acid, the yellow molybdophosphoric acid is reduced to phosphomolybdenenum blue analyzed by reflectometry [¹⁷].

3. Results and discussion

The tests were carried out during the second semester of 2017, and due to both the availability of the plant material and the capacity of the equipment, the cultivar CUR was first worked on, followed by the RM. Situations were presented during the first trial, making measurements difficult between days 21 to 29 and 35 to 48. As a result, the RM variety was treated following the same measurement scheme. The environmental conditions registered in each state of storage indicated an average temperature in refrigeration, room-temperature, and incubation of 4°C, 18°C and 27°C respectively, the average humidity level recorded was 62%.

According to the analysis of variance (ANOVA), significant differences were found (p <0.001 and p <0.05) for the simple effects of storage time or day, storage temperature and variety, in the same way for their double and triple interactions, as seen in Table 1. The analysis of each of the evaluated variables was carried out according to three-way interaction.

Table 1 Sum of Squares and Averages of the variables Moisture, Weight Loss, Glucose and Ascorbic Acid, of the influence of storage conditions on the quality of two native potato varieties. 

* = Significant Differences (p-value <0.05); ** = Highly Significant Differences (p-value <0.01); VC = Variation Coefficient; R² = Determination Coefficient; SD = Standard Deviation; Min = Minimum Value; Max = Maximum Value.

Source: The Authors.

It is observed that the varieties presented a general average of 75.88% moisture throughout the trial. The variety CUR presents an initial value of 82.09% while RM has 76.99%. A minimum value of 57.88% is found in CUR in one of the repetitions made on day 35 of the test in refrigerated storage. CUR also presented a maximum value of 84.05% after 2 days of incubation testing. For weight loss of the tubers, a maximum of 66.67% was recorded at 48 days of testing at 4°C in CUR. The maximum glucose value is reached with CUR at 4°C after 17 days of storage.

Ascorbic acid content reaches a maximum value of 199.6mg/L for the cultivar RM in refrigerated storage at 9 days of testing, and in CUR the minimum is recorded at 21 days and at 27°C when it is no longer detectable by the analysis method.

The adjustment, according to the calculation of the percentage of the coefficient of determination (R²) for the models is greater than 85%, which means that the models explain more than 85% of the real variables.

3.1. Effect of storage at different temperatures on moisture and weight loss of two potato cultivars

Fig. 1 show the averages of moisture variation; for the CUR variety, the moisture at 4°C decreases from 82% to 67%, at 18°C storage there are no statistical differences, while slight differences are shown at 27°C. The change in moisture in CUR shows a descending order of 4°C> 27°C> 18°C.

Source: The Authors.

Figure 1 Effect of storage conditions on cultivars. (A) Change in moisture content in CUR; (B) Change in moisture content in RM; (C) Percentage of weight loss in CUR and (D) Percentage of weight loss in RM. 

For RM there are no significant changes in the average moisture in refrigeration. At 18°C and 27°C moisture goes from 77% to 71% and 68% respectively. This indicates a change in the moisture in RM in a descending order of 27°C > 18°C > 4°C.

The interaction of the three factors is observed, indicating that CUR at 18°C and RM at 4°C present similar behaviors, where there is no significant change in moisture.

The percentage of weight loss is shown in Fig. 1; at 4°C, CUR reaches 45%, without presenting major statistical differences from day 21 to 52. At room temperature, a weight loss of 22% is reached and in incubation 23%. The behavior of this response variable at the end of the test is similar in both conditions. A descending order is presented for weight loss of 4°C > 27°C ≥ 18°C.

In the RM variety, the values of means of percentage of weight loss were 6% at 4°C. It can be said that there is no significant difference between the start and end of the test. At 18°C, the percentage reaches 18% and there are no significant differences from the beginning until day 17. At 27°C, a stable behavior is noted, without differences from day 0 to 17, reaching a maximum of 33% at the end. A descending order of 27°C> 18°C> 4°C is observed, which corresponds to the change in moisture. It is observed that the CUR variety has an opposite behavior to RM in moisture content and weight loss in the different storage conditions.

Some authors [¹³, ¹⁸] consider that the loss of water and weight in storage are related to differences in the physiological characteristics of the varieties, location, crop conditions, post-harvest management and the state of maturity of the tubers in harvest. Due to the potato's metabolism, the physiological activity continues in storage causing biochemical changes that affect the quality of the tubers and the temperature can affect characteristics such as moisture, dry matter, reducing sugars, among others [¹⁹].

The processes of respiration and transpiration during storage influence weight loss, loss of moisture and spoilage of the potato [²⁰]. The conversion of starch into sugar and vice versa are catalyzed by enzymatic action that can be regulated by temperature [²¹] as well as by other factors. As a result, different treatments have been studied to prolong the life of the tubers, that include pre-curing, conditions of controlled atmospheres in temperature, humidity and CO₂ content, bud break inhibitors and reconditioning for processing [¹³,²¹,²²-²⁵].

3.2. Effect of storage at different temperatures on the glucose and ascorbic acid content of two potato cultivars

Table 2 and Fig. 2 show the averages of the results for variation in glucose content of the CUR and RM cultivars at the storage conditions established during the test.

Table 2. Glucose Content Averages in two native potato cultivars under different storage conditions. 

Different letters in the same column imply differences between averages, according to Tukey's test at 95% confidence. Comparator DMS = 0.03.

Source: The Authors.

Source: The Authors.

Figure 2.  Effect of storage conditions on cultivars. (A) Change in glucose content in CUR; (B) Change in glucose content in RM; (C) Change in ascorbic acid content in CUR and (D) Change in ascorbic acid content in RM. 

For CUR at 4°C, glucose levels increase until day 17 to 0.65% and then return to the initial value of 0.12%. At 18°C and 27°C, it drops to values of 0.04% and 0.03% respectively. Although the tendency shown is a decrease in levels, there is much variation in the results.

In addition, for cold-induced sweetening processes [¹¹], an order of glucose variation of 4°C > 18°C ≥ 27°C can be established.

In refrigeration, the RM reaches maximum glucose content on day 31 (0.58%). In the other storage conditions, several variations are recorded although, graphically, it is observed that it remains relatively close to the initial value.

In this case, the variation of the glucose content is similar to CUR, 4°C > 18°C ≥ 27°C, but the maximum value is reached 14 days later at 4°C.

The averages of the ascorbic acid content and its variations during storage under the different conditions are detailed in Table 3 and Fig. 2. In CUR, it decreases from 96mg/l to 44mg/l at 18°C. At 27°C, it decreases faster and it is not detected after day 21, while at 4°C it increases until day 10, reaching 144mg/l and then decreases to 72mg/l by the end of the trial. The decrease in ascorbic acid content can be organized decreasingly as 27°C > 18°C > 4°C.

Table 3. Average Ascorbic Acid Contents in two native potato cultivars under different storage conditions. 

Different letters in the same column imply differences between averages, according to Tukey's test at 95% confidence. Comparator DMS = 20.95.

Source: The Authors.

The ascorbic acid graph shows similar behavior for RM in the three storage conditions, although the differences are significant. Levels start at 136mg/l and reach their maximum on day 17, followed by a decrease until the end of the test. The decrease in content can be arranged in a decreasing order: 27°C > 4°C > 18°C.

The CUR variety shows similar tendencies to RM in its glucose content; cold-induced sweetening is comparable with studies carried out in other varieties [¹¹,¹³,²³,²⁶-²⁸]. The physiological and varietal differences influence behavior and it was noted that RM shows longer resistance time to cold sweetening than CUR. Minimum weight loss and moisture at low temperature, as well as ascorbic acid content also decreases at a higher rate at 4°C in RM than in CUR.

On the other hand, moisture and to some extent deterioration, is lower at room temperature in the CUR cultivar. The weight loss is also lower at 18°C than at 4°C and the content of ascorbic acid at 18°C indicates lower losses than at 27°C, though higher than at 4°C, exhibiting behaviors similar to those reported by [²⁹].

The metabolic processes that affect the response variables and their behavior can vary for each variety crop [²³]. As such, it is recommended that the most promising genotypes of each region are identified, and that factors of both crop management and post-harvest treatment are standardized in order to obtain potatoes with desirable agro-industrial characteristics. These factors include: genotype, environmental growth factors such as temperature, soil moisture, mineral nutrition, tuber maturity, mechanical stress during harvest and transport, and storage conditions such as temperature, oxygen concentration, carbon dioxide, dormancy and sprouting [²⁶,²⁷]

The levels of reducing sugars, related to glucose, ideal for processing are around 0.1%; to obtain chips of acceptable colors, levels above 0.33% are unacceptable. Vitamin C, or ascorbic acid, is the vitamin with the greatest presence in the potato. Its initial content along with the maturity and variety of the crop determine losses during storage [¹³].

The post-harvest storage and treatment of the tubers is the key to successfully connect production and agro-industry. This implies detailed engineering studies to supply the abundant demand of processed and fresh potato products, guarantee their potential added value and reduce the losses that occur in these operations [³⁰,³¹].

Deepening the study on the post-harvest behavior of these and other native varieties to achieve longer storage possibilities can be useful to organize transformation processes that generate added value, contribute to the conservation of biodiversity and food security. In addition, the characteristics studied can be considered in genetic improvement processes with other varieties.

4. Conclusions

There are significant differences in behavior of Curiquinga and Ratona Morada potato varieties during storage. The results of the investigation outline the necessary conditions for post-harvest management that allow for the introduction of these varieties in a new value chain.

The RM variety tolerates cold storage and its qualities are not affected significantly in such conditions. On the other hand, CUR conserves its characteristics better at temperatures above 4°C; at 18°C it presents less variation in the response variables.

The physiological differences between CUR and RM varieties along with the differences in sowing, growth, maturity and harvest, have an effect on the behavior of the tubers during storage. The results of this study outline the conditions necessary to prolong the life of these varieties for later commercialization or processing. The recommendation is made to continue studies that include the aforementioned factors, applying this knowledge to other native varieties in order to improve the conditions of these tubers and generate alternatives for small farmers.

Storage conditions can be adjusted to minimize moisture and weight loss, in addition to maintaining the content of glucose and ascorbic acid at certain levels so as not to affect their subsequent processing. In the RM cultivar, cold storage becomes a viable alternative, while for CUR, conditions between 4°C and 18°C must be considered.