Effects of different pretreatment methods on the drying characteristics and quality of potatoes

Abstract The effects of different pretreatments on the vitamin C content of peeled fresh potato, the drying characteristics, and several quality attributes of dehydrated potatoes were investigated. Citric acid pretreatment (0.1%–0.3%, 10–30 min), steam blanching (100ºC, 1–2 min), and water blanching (95°C, 1–2 min) were found to have no obvious effect on the drying rate of potatoes, whereas temperature was the main influencing factor. In terms of quality of dehydrated diced potato, 20 min of citric acid pretreatment resulted in the highest vitamin C retention and better color. Furthermore, dehydrated potato pretreated with citric acid all showed similar dynamic moisture adsorption curves, namely type II sorption isotherm. The moisture adsorption curves can be well fitted using the Guggenheim–Anderson–deBoer model with R 2 higher than .97.


| INTRODUC TI ON
Potato is one of the most popular foods and ranks the fourth among the five major crops (after wheat, rice, and corn, before soybean).
Fresh potato is rich in nutrients including a variety of minerals, and its vitamin C content can reach to 42 mg/100 g among the five major crops (Burlingame, Mouillé, & Charrondière, 2009). Fresh-cut fruits and vegetables are convenient and easy to prepare for consumers (Brecht, 1995;Luo & Barbosa-Canovas, 1996;Saltreit, 1997).
However, the peeled potato is very prone to browning and has a short storage time compared to the whole one. Therefore, it is essential to find efficient and simple approaches to extend the storage time and stabilize the color of potato after peeling.
Among the various pretreatment technologies for fresh-cut produce, the most common ones are blanching, chemicals, freezing, ultrasound, and high pressure (Jambrak, Mason, Paniwnyk, & Lelas, 2007;Yu, Ma, & Bi, 2010). In particular, the addition of chemical inhibitors could suppress browning (Friedman, 1996). Sulfites can effectively control enzymatic browning (Sapers, 1993). However, alternatives are needed due to consumers' health awareness and government bans on their use in fresh fruits and vegetables (Food & Drug Administration, 1989). Citric acid was found to inhibit the browning of fresh Chinese water chestnuts and extend the shelf life (Jiang, Pen, & Li, 2003).
Water blanching and steam blanching have remained as popular commercial pretreatment methods, since they are easy to establish and implement. In general, water blanching is carried out at 80-100°C for 20 s to 20 min. Moisture sorption/desorption isotherms are essential for predicting stability during storage of foods, which influenced the shape and quality of foods.
In this paper, the effects of pretreatment with citric acid, steam blanching, and water blanching on the vitamin C content of fresh potato, the drying characteristics, and the quality and dynamic moisture adsorption characteristics of dried diced potato were investigated, in order to identify the best pretreatment technology.

| Materials
Fresh, whole, and bud-free potatoes were purchased from the Hanlin Neighborhood Fresh Market in Suzhou Industrial Park, Jiangsu Province, China. The potatoes were peeled and cut into cuboids (1.2 × 1.2 × 1.1 cm) using a stainless steel knife.

| Pretreatment of potatoes
The cut potato was blanched for a shorter time according to the size of the potato pieces. The following pretreatments were tested, using three samples in each one.
1. Water blanching (WB): Boiling in 50 ml water at 95°C for 1 or 2 min, then placing in crushed ice for 2 min to cool. The surface moisture was dried with a paper towel.
2. Steam blanching (SB): Treatment in a steam box (100°C) for 1 or 2 min, then placing in crushed ice for 2 min to cool. The surface moisture was dried with a paper towel.

| Hot air drying experiments
The hot air drying experiments were carried out at air temperatures of 50, 60, and 70°C in a hot oven. Drying experiments were proceeding until sample weight became constant. When this condition was reached, there is a dynamic equilibrium between the sample equilibrium content moisture Me and drying air humidity. The sample moisture and solid contents were determined using the hot air at 150℃ for 24 h. Sample weight was recorded at an interval of 15min using an electronic balance (SATORIUS, BASA2202S, China) with a range of 2,200 g and system error of ±0.001g.
The drying kinetics was represented by the moisture ratio (MR,

Equation 1) as a function of time:
where M t is the moisture content at time t, M 0 is the initial moisture content, and M e is the equilibrium moisture content.

| Determination of vitamin C
The ascorbic acid was analyzed according to Toor & Savage (2005) with modification. Sample with 2 g, extracted, was mixed with 25 ml oxalic acid dehydrate (OAD) (purity ≥ 99.5%., Sinopharm Chemical Reagent) solution (2 g/100 g OAD) by using a mechanical homogenizer. After 20 min at 10,000 rpm at 4°C, the supernatant was collected in a new tube. The vitamin C was assayed by using 2, 6-dischlorophenolindophenol method. The results are expressed in milligrams of vitamin C per 100 g of fresh potatoes. The retention of vitamin C in fresh pretreated potatoes and dehydrated potatoes was calculated from the ratio of the measured vitamin C content to the fresh potato vitamin C content.

| Determination of color
The color of the sample was measured with a CR-100 colorimeter, and the samples were measured three times, and the color was expressed by the parameters L*, a*, and b* of the color difference meter.

| Dynamic moisture adsorption model
The Guggenheim-Anderson-deBoer (GAB) model mathematically describes the relationship between the relative humidity and the equilibrium moisture content. This model can fit a wide range of isothermal adsorption lines for foods. The corresponding equation is as follow: where X is the moisture content [kg/kg on dry basis], X m is the single-layer number of molecules or single-layer moisture content [kg/kg on dry basis], a w is the water activity, C g is the thermodynamic proportionality constant, and K is a constant.

| Data processing and analysis
Statistical analysis was conducted using Microsoft office software.
Significant differences (p < .05) between means were evaluated by one-way ANOVA and Scheffer test. Graphics were drawn by Microsoft office software and origin 8.5.

| Effect of pretreatment on vitamin C content of fresh potato
Vitamin C is an essential nutrient for humans by preventing diseases like scurvy. Since it is water-soluble, easily oxidized, and thermo-sensitive, vitamin C in foods can be easily degraded depending on the processing and storage conditions (temperature, pH, light, time, and so on). Thus, it is often used as an indicator of the quality of fruits and vegetables (Goula & Adamopoulos, 2006). , which also increased with the pretreatment time (p < .05). Therefore, a small amount of vitamin C must have been leached out of the potato during the pretreatment, although most of it was retained. Figure 2 shows the effect of different blanching methods on the content of vitamin C in fresh potato. The retention of vitamin C was 67% and 62% after water blanching for 1 and 2 min, and 81% and 73% after steaming blanching for 1 and 2 min, respectively. As expected, prolonged blanching in either water or steam reduced the level of vitamin C. The reason is that heating destroyed the cell structure to release a large amount of vitamin C from the cells, a part of which was oxidized by oxygen. Compared to water blanching, steam blanching caused less vitamin C loss, since in this method, the potato was not in direct contact with bulk water.

| Effect of drying temperature and pretreatment on potato drying characteristics
The effect of different drying temperatures on the drying characteristics of potato is shown in Figure 3. MR decreased considerably when increasing the drying time, similar to previous results for tomato (Demiray & Tulek, 2012), red pepper (Akpinar, Bicer, & Yildiz, 2003), and rosehip (Erenturk, Gulaboglu, & Gultekin, 2004). The equilibrium moisture content of the potato was measured after drying for 10, 8, and 6 hr at 50, 60, and 70°C.  Figure 5 shows the vitamin C content in the fresh and dehydrated potato samples. From Figure 5a, it can be found that the duration of CA pretreatment did not significantly influence the vitamin C retention in dehydrated potato (p > .05), which remained at 55%-61.6%. From

| Effect of different pretreatment methods on the color of dehydrated potato
Color is an essential parameter in food quality. The color of dehydrated vegetables may change significantly due to the Maillard reaction, pigment degradation, enzymatic browning, and oxidation of vitamin C during drying.

| Effect of citric acid concentration on dynamic water adsorption characteristic of dried potato
Figure 9a-c show the adsorption/desorption isotherms of potato pieces after pretreatment in CA solution at 50°C for 30 min using 0.1%-0.3% CA, respectively. The water adsorption was characterized by type II isotherm (Brunauer, Derming, Deming, & Troller, 1940). This shows adsorbing water of potato is from a gradual higher moisture uptake to small amounts of water uptake, as relative humidity increased.
When dehumidifying, the moisture content on a dry base gradually decreased with decreasing relative humidity, and a relatively serious lag phenomenon occurred. This may mean that a part of the adsorbed water did not release during the desorption process due to interaction with certain nonaqueous components. And the concentration of CA has no obvious influence on dynamic moisture adsorption.
The GAB model, based on Langmuir single-molecule adsorption and Brunauer-Emmet-Teller (BET) multi-molecular adsorption theory, mathematically describes the relationship between relative F I G U R E 7 Changes in color parameters of dehydrated potato cubes subjected to citric acid pretreatment at different conditions followed by drying at 50°C. a: L* value, b: a* value, c: b* value, and d: ∆E value humidity and the equilibrium water content. This model can fit isothermal adsorption lines of various foods in a wide range. Figure 10 compares the measured desorption isotherm to that desorption isotherm fitted by the GAB model. In terms of the regression coefficient (R 2 > .97) and variance (S 2 ) (Table 1), the GAB model has well goodness of fit, and therefore, it can be used to fit the desorption isotherm.

| CON CLUS IONS
We investigated the effects of CA pretreatment, steam blanching, and water blanching under varying parameters on the vitamin C content of peeled fresh potato, the drying characteristics, and several quality attributes of the dehydrated potato. The vitamin C content did not significantly decline with CA pretreatment time, whereas the F I G U R E 8 Changes in color parameters of dehydrated potato cubes subjected to water blanching and steam blanching followed by drying at 50°C. (a): L* value, a* value and b* value, (b): ∆E value F I G U R E 9 Adsorption and desorption curves of dehydrated potato cubes subjected to citric acid pretreatment at different conditions followed by drying at 50°C. a: 0.1% citric acid, b: 0.2% citric acid, and c: 0.3% citric acid lowest content in fresh-cut potato was 62% after blanching in water for 2 min. The drying temperature is the main factor influencing the moisture ratio. In the dehydrated potato, the best (i.e., lightest) color was observed in the sample pretreated with 0.2% CA for 20 min, which also had the highest vitamin C (61.6%). All dehydrated potato samples pretreated by CA showed similar dynamic moisture adsorption curves (type J curve and type II sorption isotherm), which could be well-fitted by the GAB model with R 2 values higher than 0.97. In summary, this study found that pretreatment in 0.2% CA solution for 20 min produced the best dried potato in terms of the moisture ratio, vitamin C, color, and dynamic moisture adsorption.

| E THI C AL G U IDELINE S
The authors declared that Ethics approval was not required for this research.

ACK N OWLED G M ENTS
The authors thank the National Key Research and Development Program of China (Nos. 2017YFD0400905).

CO N FLI C T O F I NTE R E S T
The authors declared that they have no commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Xiangfeng Sun
https://orcid.org/0000-0002-8770-3918 F I G U R E 1 0 Comparison of desorption curves between experimental curves and GAB models for dehydrated potato cubes subjected to citric acid pretreatment followed by drying at 50°C. a: 0.1% citric acid, b: 0.2% citric acid, and c: 0.3% citric acid TA B L E 1 GAB model parameters for dehydrated potato cubes pretreated by citric acid