Monitorization of Mineral Content and Location after 3 Months of Storage of Naturally Enriched Potato (Solanum tuberosum L.) with Calcium ^†

Abstract

Potato (Solanum tuberosum L.) is one of the most important staple food crops and one of the most consumed food crops worldwide. As such, it is a suitable food matrix for biofortification studies, namely, with Ca, as it is an essential mineral for plant growth and development, it being required for several structural issues. In this context, this study aimed to monitor the mineral content and location of Ca and other essential minerals (K, P, S, Fe, and Zn) and assess some quality parameters (color of the pulp, total soluble solid, and dry weigh content) in tubers of Solanum tuberosum L. (Agria variety), after three months of storage, submitted to a Ca biofortification process with four foliar sprays with three concentrations of calcium nitrate (0.5, 2, and 4 kg·ha⁻¹) and two concentrations of calcium chloride (3 and 6 kg·ha⁻¹). It was found out that, in most treatments, Ca, K, P, S, Fe, and Zn have higher contents in the epidermis region and that control tubers showed a lower dry weight content compared to the biofortified ones. Moreover, after three months of storage, naturally enriched tubers maintain a preferential accumulation of Ca in the epidermis region (as seen in the harvest) and showed a decrease in the dry weight content in the control and biofortified tubers (compared to the harvest data). Additionally, no significant differences were observed in the colorimetric parameters of the pulp tubers and in the total soluble solid content, presenting similar data to the harvest ones. In conclusion, the storage process of biofortified tubers affected a quality parameter—dry weight content—that is relevant for industrial processing and a criterion for potato tubers classification. In this context, only the Ca(NO₃)₂ 2 kg·ha⁻¹ and CaCl₂ 3 and 6 kg·ha⁻¹ treatments were suitable for industrial processing after 3 months under storage conditions.

1. Introduction

The worldwide population is expected to reach 9 billion by 2050 [1], and to feed the future population, food production must increase by 25 to 70% [2]. As such, it is important to reduce the post-harvest food losses and to avoid waste, which is especially high in perishable crops. Potato is a perishable crop, and correct post-harvest management is required to reduce food losses [3]. In fact, potato is considered the third most important (non-grain) food crop worldwide, and more than a billion people around the world eat potato [4], it being widely grown [5]. In Portugal, the consumption per capita is around 85 kgs of potato per year [6]. As a perishable commodity, to allow for the later use of potatoes, it is important to understand the factors that affect potato storage and to ensure a correct storage for 3 to 10 months. Therefore, potato tubers have an active metabolism during post-harvest, leading to losses in mass and quality. The loss in quality during storage depends on the storage conditions and on the culture management since the early stage (seed storage) and during the growth and harvest [5]. Due to the metabolic process of respiration, potatoes must be under refrigeration or cool temperatures during the post-harvest stage to slow down this process and maintain tuber quality [7]. Additionally, considering that potato tubers are mainly water (about 80%) and dry matter (around 20%, mostly starch) [8], low external vapor pressure or even relative humidity can lead to losses in the internal water and shrinking [7]. Therefore, two of the major causes of post-harvest losses are the water loss and sprouting that can decrease the nutritive quality of potato tubers [7].

Nevertheless, the lack of essential nutrients in individuals who are attaining healthy levels of calories is a major worldwide problem [9], so it is important to have more food production with quality (i.e., able to provide the daily nutritional dietary requirements). In this context, the aim of this study is to monitor the mineral content and location of Ca, K, P, S, Fe, and Zn and analyze the important quality parameters (namely, the color of the pulp, total soluble solids, and dry weight content) in tubers of Solanum tuberosum L. (Agria variety), after three months of storage (under low temperatures), submitted to a Ca biofortification process with four foliar sprays with three concentrations of calcium nitrate (0.5, 2, and 4 kg·ha⁻¹) and two concentrations of calcium chloride (3 and 6 kg·ha⁻¹) in order to verify if the storage process has any effect on the mineral content, the location of tuber tissue, or the quality parameters.

2. Materials and Methods

2.1. Biofortification Intinearary

The experimental potato-growing field used to grow the Agria variety (Solanum tuberosum L.) is described in [10]. After harvest, the potatoes were stored in cold rooms between 6–8 °C for 3 months.

2.2. Mineral Content in Potato Tissues through Fluorescence Detection

The content and location (in three regions: center, middle, and peel) of Ca, K, P, S, Fe, and Zn in the tissues of the tubers after 3 months under storage conditions were determined using the µ-EDXRF system (M4 Tornado™, Bruker, Germany), as previously described in [10], after being sliced transversely (with 4 mm) at the equatorial region and dried at 60 °C. The values of the minerals content were obtained through the average of four readings taken by the device.

2.3. Quality Parameters

The dry weight and total soluble solids content were determined considering four randomized tubers, as described in [10]. Colorimetric parameters using fixed wavelength, following [11], were carried out in the pulp of four randomized fresh tubers.

3. Results

After three months of harvest, the mineral content and location of the tuber tissues under the storage conditions were assessed (Figure 1 and Figure 2).

Regarding most of the treatments, Ca, K, P, S, Fe, and Zn showed a higher content in the epidermis/peel region (zone 3) (Figure 2). Calcium showed a higher content relative to the control in the CaCl₂ 6 kg·ha⁻¹ and Ca(NO₃)₂ 4 kg·ha⁻¹ treatments, respectively. Additionally, the Ca(NO₃)₂ 0.5 kg·ha⁻¹ treatment had a higher content of Ca in zone 1—center region. In the epidermis region, K presented a higher content in the Ca(NO₃)₂ 4 kg·ha⁻¹ and CaCl₂ 6 kg·ha⁻¹ treatments regarding control. Regarding P, this treatment showed oscillations, despite all the remaining treatments (control included) having a similar distribution in the tuber tissues. However, as seen in Ca and P, the Ca(NO₃)₂ 0.5 kg·ha⁻¹ treatment showed a higher content in the center region and CaCl₂ 6 kg·ha⁻¹ treatment in the epidermis. Moreover, regarding S, in zone 3, the CaCl₂ 6 kg·ha⁻¹ treatment demonstrated high levels compared to the other treatments and the control. Additionally, as seen in P, in S, despite some oscillations, the remaining treatments showed a similar distribution in the three zones. Iron showed the highest contents in zone 3, both in the control and in the biofortification treatments, compared to the remaining tuber tissue regions. However, only the Ca(NO₃)₂ 4 kg·ha⁻¹ treatment showed a higher content than the control. As observed in S and Fe, Zn also showed higher contents in zone 3. However, only the Ca(NO₃)₂ 4 kg·ha⁻¹ and CaCl₂ 6 kg·ha⁻¹ treatments presented higher levels than the control. A higher Zn content (in the epidermis region) in the Ca(NO₃)₂ 4 kg·ha⁻¹ treatment was verified.

Some quality parameters were determined in the Agria variety tubers after three months under storage conditions. The dry weight, total soluble solids, and colorimetric parameters (L, a*, and b*) were analyzed, and no significant differences were detected between the treatments (

Table 1. Mean values ± S.E. (n = 4) of dry weight, total soluble solids, and colorimetric parameters (L, a*, and b*) in tubers of Solanum tuberosum L., cv. Agria, 3 months after harvest (under storage conditions). There were no significant differences, in terms of each parameter, between treatments (p ≤ 0.05). Foliar spray was carried out with three concentrations of Ca(NO₃)₂ (0.5, 2, and 4 kg·ha⁻¹) and two concentrations of CaCl₂ (3 and 6 kg·ha⁻¹). The control was not sprayed.

Treatments	Dry Weight (%)	Total Soluble Solids (°Brix)	Colorimetric Parameters
			L	a*	b*
Control	18.3 ± 0.75	5.17 ± 0.14	54.4 ± 1.00	−2.53 ± 0.19	20.5 ± 0.41
Ca(NO3)2 (0.5 kg·ha−1)	19.8 ± 0.39	5.15 ± 0.12	55.7 ± 1.56	−2.74 ± 0.16	21.5 ± 0.89
Ca(NO3)2 (2 kg·ha−1)	20.8 ± 0.60	5.17 ± 0.14	56.3 ± 1.34	−3.08 ± 0.24	20.9 ± 0.23
Ca(NO3)2 (4 kg·ha−1)	19.8 ± 0.84	5.33 ± 0.27	54.8 ± 0.85	−2.51 ± 0.22	20.9 ± 0.24
CaCl2 (3 kg·ha−1)	20.6 ± 0.34	5.33 ± 0.14	54.2 ± 0.02	−2.54 ± 0.13	20.3 ± 0.21
CaCl2 (6 kg·ha−1)	20.3 ± 0.30	5.67 ± 0.36	56.8 ± 0.83	−3.07 ± 0.09	22.2 ± 0.27

). Additionally, the control showed the lowest dry weight content, and the Ca(NO₃)₂ 2 kg·ha⁻¹ treatment showed a higher dry weight content and a lower a* parameter. Regarding the total soluble solids and the L and b* parameters, the CaCl₂ 6 kg·ha⁻¹ treatment showed higher values compared to the remaining treatments.

4. Discussion

To allow for the later use of potatoes, storage plays a vital role to ensure year-round supplies (both for fresh consumption and for the processing industry) [12]. However, not only is good management during the post-harvest stage (cool temperatures or even refrigeration) important; the maintenance of the quality of the tubers, mainly in terms of nutrients—nutritive quality, is as well [7]. As such, after three months under storage conditions (under low temperatures), the mineral content and location of some mineral elements were determined (Figure 2). In our previous research, which was carried out at harvest [10], we observed that, for the Agria variety, the highest contents of Ca were obtained close to the epidermis, indicating that the Ca deposition in the tuber tissues prevailed in the peel/epidermis region independently of the treatments applied (the control, Ca(NO₃)₂ 4 kg·ha⁻¹, and CaCl₂ 6 kg·ha⁻¹ treatments). After three months of storage (Figure 2), the data obtained suggest that Ca accumulation remained in the same regions of the tuber tissue verified at harvest [10]. However, compared to the harvest data [10], the Ca content (Figure 2) in the control, Ca(NO₃)₂ 4 kg·ha⁻¹, and CaCl₂ 6 kg·ha⁻¹ treatments showed an increase in the epidermis region of the tubers (zone 3). In zone 2, equal values in the control and a decrease in the biofortification treatments and in the center region (zone 1) were also verified. Additionally, there was an increase in the Ca content in the control and a decrease in the higher treatments applied with calcium nitrate and calcium chloride. A comparison of the data obtained at harvest [10] and after three months of storage (Figure 2) suggests that there was a migration of Ca to the epidermis of the tubers, probably due to the beginning of sprouting in the tubers (which implies cellular growth), and consequentially, minerals that are stored/accumulated in the tuber can be transported to the sprout, leading to a decrease in the inner regions of the tuber (zone 1 and 2). In fact, with the onset of sprouting, tubers turn into a source organ for the growth of the developing sprout [13]. Additionally, it is important to note that Ca plays a vital role in maintaining the structural integrity of the cell walls and in intracellular adhesion [14] (it being implicated in cell growth) and that sprouts, and Ca have a close relationship in tubers; sub-apical necrosis on sprouts is an indirect indicator of low calcium [15]. Additionally, this tendency, verified in our data, of a greater concentration of Ca in the peel compared to other tissues, was already mentioned [16]. Regarding the remaining elements assessed (K, P, S, Fe, and Zn), there was a greater concentration prevailing in the epidermis/peel region than in the flesh regions (zone 1 and 2) (Figure 2), as also reported by [17]. However, it was also reported by [17] that most phloem mobile elements (phosphorus, sulfur, and potassium) remain in a greater concentration in the tuber flesh; it is assumed that this was the reason why these elements showed similar distributions (despite some oscillations) in the three zones analyzed. However, in most treatments, the highest content (as seen in Ca) was obtained in zone 3, also suggesting that (after three months of storage) it is due to tubers turning into a source in the sprouting process [13]. There were greater quantities of Ca, Fe, and Zn in the epidermis region than in the middle and center tissues (which had quite lower contents), probably due to them being less immobile [18]; as such, this preferential location can be associated with the fact that these elements are delivered (at least) by direct movement across the epidermis during the development of the tuber [17].

Despite potato tubers being stored at low temperatures (mainly to inhibit rapid sprouting and decay) [19], sprouting can occur (as it did in our study). In fact, the onset of sprout growth can adversely affect not only the nutritional characteristics but the processing characteristics of the tubers [20]; as such, some quality parameters were also carried out (Table 1). Although there were no significant differences in dry weight, total soluble solids, and colorimetric parameters, overall, there was a decrease compared to the harvest data [10,21]. Regarding the dry weight content, only the Ca(NO₃)₂ 2 kg·ha⁻¹ and CaCl₂ 3 and 6 kg·ha⁻¹ treatments were suitable for industrial processing (most were higher than 20%) [22] after 3 months under storage conditions, and this is probably due to the fact that vegetables that have undergone treatments with Ca (usually calcium chloride) and have been stored under low temperatures have the potential not only to improve the nutritional quality of vegetables (considering that Ca can increase the postharvest shelf life due to its role that in maintaining cell wall stabilization and integrity) but also to reduce the respiration rate, which has an effect on vegetable preservation/conservation [23,24].

Nevertheless, regarding most of the treatments, in the soluble sugars (mostly, sucrose, glucose, and fructose) content, there was a decrease compared to the harvest data [10] (mainly in control tubers). However, regarding the Ca(NO₃)₂ 4 kg·ha⁻¹ and CaCl₂ 3 and 6 kg·ha⁻¹ treatments, there was a higher content of total soluble sugars, which is in accordance with [19], reporting that, under low temperatures, the storage soluble sugars content increases due to the function of granule-bound starch synthase 1, beta-amylase, invertase inhibitor, and fructokinase [19]. Color (colorimetric parameter) is an important factor for consumers that affects its acceptability [25]. Regarding the L parameter, there was a decrease compared to the harvest data [10], showing less bright tubers. However, it was possible to verify a mix of greenish (negative a* parameter) and yellowish tones (positive b* parameter).

5. Conclusions

Through the monitorization of the mineral content and location of Ca, K, P, S, Fe, and Zn and the assessment of some quality parameters (color of the pulp, total soluble solids, and dry weigh content) in Solanum tuberosum L. (Agria variety), after three months of storage, submitted to a Ca biofortification process with four foliar sprays with calcium nitrate or calcium chloride, it was possible to conclude that the deposition in the tubers tissue of Ca, K, P, S, Fe, and Zn was close to the epidermis/peel region. Furthermore, there was a decrease in the dry weigh content in the control and all the biofortified treatments (compared to the harvest), indicating that only the Ca(NO₃)₂ 2 kg·ha⁻¹ and CaCl₂ 3 and 6 kg·ha⁻¹ treatments are suitable for industrial processing after 3 months of storage under low temperatures. Additionally, there was not a clear tendency regarding the total soluble solids due to opposite behaviors (an increase in half of the treatments and a decrease in the other half).