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Article

Targeted Primary and Secondary Metabolite Analysis of Colored Potato “Michuñe Negra” Grown in Soilless Culture and during Prolonged Cold Storage: Implications in Acrylamide Formation during Frying

by
Diego García-Ríos
1,
Juan E. Alvaro
1,*,
María Elvira Zuñiga
2,3,
David Campos
4,
Ana Aguilar-Galvez
4,
María Salomé Mariotti-Celis
5,
Franco Pedreschi
6,* and
Romina Pedreschi
1,7
1
Escuela de Agronomía, Pontificia Universidad Católica de Valparaíso, Calle San Francisco s/n, La Palma 2260000, Chile
2
Escuela de Ingeniería Bioquímica, Pontificia Universidad Católica de Valparaíso, Av. Brasil 2085, Valparaíso 2373223, Chile
3
Centro Regional de Estudios en Alimentos Saludables, Av. Universidad 330, Valparaíso 2373223, Chile
4
Instituto de Biotecnología, Universidad Nacional Agraria La Molina, Av. La Molina s/n, La Molina, Lima 12056, Peru
5
Escuela de Nutrición y Dietética, Facultad de Medicina, Universidad Finis Terrae, Pedro de Valdivia 1509, Santiago 7501015, Chile
6
Departamento de Ingeniería Química y Bioprocesos, Pontificia Universidad Católica de Chile, P.O. Box 306, Santiago 6904411, Chile
7
Millennium Institute Center for Genome Regulation, Santiago 7800003, Chile
*
Authors to whom correspondence should be addressed.
Agronomy 2023, 13(5), 1209; https://doi.org/10.3390/agronomy13051209
Submission received: 16 March 2023 / Revised: 17 April 2023 / Accepted: 19 April 2023 / Published: 25 April 2023
(This article belongs to the Section Plant-Crop Biology and Biochemistry)
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Versions Notes

Abstract

:
Colored potatoes have been of interest because of their nutritional and health-promoting properties. However, their aptitude for processing regarding acrylamide formation levels is highly variable and for the most part unknown. In this work, the effect of cultivation season (summer and winter), and postharvest cold storage (1, 2, and 5 months at 5 °C ± 1 °C) on the Maillard reaction precursors (reducing sugars, sucrose, and asparagine), phenolic compounds (anthocyanins, phenolic acids and other flavonoids), and acrylamide after frying (GC-MS) were assessed on the “Michuñe negra” potato cultivar from the southern region of Chile grown under fertigation. Acrylamide levels were surveyed on six samples of commercial colored potato chips for comparison purposes. Cultivation season and cold storage showed an effect on both main primary and secondary metabolites. The amount of reducing sugars such as fructose increased with cold storage. Anthocyanin and phenolic contents were higher in the winter season and increased with cold storage. Acrylamide levels were high, surpassing the EFSA benchmark value (750 µg kg−1) in all cases, notably in chips made after one month of cold storage (2125% higher). Additional measures in both agronomical and processing stages of the purple-fleshed potato need to be implemented to lower the acrylamide levels down to acceptable values.

1. Introduction

Native potato cultivars from the southern regions of Chile are morphologically and compositionally diverse, especially in terms of compounds with health-related properties such as phenolic compounds, including hydroxycinnamic acid derivatives and anthocyanins [1,2,3], and resistant starch [4]. Potatoes with colored flesh have been of interest for the development of new products such as colored potato chips, often considered “healthier snacks” due to their increased antioxidant content compared to that of conventional chips [5]. Nevertheless, both the stability of antioxidant compounds during processing and the formation of Maillard reaction (MR) related to neoformed contaminants (such as acrylamide) on colored potato chips are still matter of research as they both depend on the initial composition of the raw materials as well as the processing conditions [6,7]. Thus, determining the levels of MR precursors (glucose, fructose, and asparagine) is important to assess the neoformed contaminant potential of colored potatoes and to outline agronomy-based mitigation strategies together with postharvest cold storage and processing mitigation strategies.
Climate change represents a challenge for potato production. Under the current scenario, the increase in temperatures and reduction in water availability will cause a significant reduction in potato production worldwide [8,9]. In this regard, the utilization of protected and soilless systems for potato production emerges as an alternative because of their better efficiency in terms of water and nutrient consumption [10].
Cold storage (between 2–10 °C) of potato tubers is a common conservation practice used by the chip industry as it reduces respiration, and inhibits sprouting and tuber rotting [11,12]. The main downside of this practice is the accumulation of reducing sugars [13,14], as the low temperatures induce the activation of amylases that convert starch to monosaccharides, a phenomenon known as cold-induced sweetening (CIS) [15]. This issue has led to the development of CIS-resistant potatoes for the snack (chip) industry [16,17]. Likewise, cold storage has been shown to promote the accumulation of antioxidant compounds in some cases such as total phenolics [18] and anthocyanins [19], and other works have shown that the effect of cold storage strongly depends on the cultivar and growing conditions [20,21].
To our knowledge, the composition of native Chilean potatoes grown on soilless systems and from different cultivation cycles developed for potato chip industry have not been reported. This study aimed (i) to determine the acrylamide level in potato chips made from purple-colored potatoes grown in a protected soilless system and compare it with commercial colored potato chips, (ii) to determine the effects of the growing cycle (summer/winter) on targeted primary and secondary metabolites involved in the Maillard reaction of a native potato cultivar grown in a protected soilless system, (iii) to determine the evolution of such metabolites during prolonged cold storage (5 ± 1 °C up to 5 months) and (iv) to evaluate the acrylamide formation in chips made from potatoes stored under cold conditions.

2. Materials and Methods

2.1. Plant Material and Cultivation Conditions

“Michuñe negra” potatoes were cultivated in a multi-tunnel greenhouse (polyethylene cover thickness: 200 µm) with a natural ventilation system equipped with a fertigator at the Faculty of Agronomical and Food Sciences of the Pontificia Universidad Católica de Valparaíso (PUCV) (32°53′44.04″ S and 71°12′25.34″ O). Two cultivation cycles were assessed: the spring–summer cycle (30 October 2020–20 January 2021) and the autumn–winter cycle in 2021 (31 May 2021–27 September 2021). For the soilless growing system, the unit crop was a 55 L Projar Golden Grow Blend Medium-washed (Valencia, Spain) coir growth pot (500 × 480 mm, H × W). The distance between the unit crop was 1.1 m and that between rows was 1.8 m. Each unit crop was fertigated by one pressure-compensated dripper, with a nominal flow rate of 4.0 L h−1, four manifolds and its corresponding microtubes and spikes. The potato plants were fertigated with a standard nutrient solution adjusted to potato plant requirements and were monitored daily during the experiment. For the experiment, two checkpoints were used for fertigation control: (1) a control dripper (4.0 L h−1) and (2) a drain pan to monitor the supplied fertigation and its absorption response. At these control points, the volume of nutrient solution, pH, and EC of the fertigation input and drainage were measured. The nutrient solution’s pH was maintained at 5.8 ± 0.1 and electrical conductivity (EC) was maintained at 1.7 ± 0.1 dS m−1. Each fertigation pulse was triggered when 10% of the readily available water in the substrate had been used, and when the volume necessary to produce between 20% and 30% of drainage in order to avoid any accumulation of salts was reached. Other cultural management practices (e.g., weed control and the hilling up of tubers) were performed in accordance with commercial practices.

2.2. Postharvest Storage Conditions

Potato tubers were harvested after reaching maturity, which was determined by the decay of the vegetative part of the plants. Harvested potatoes were cured for 8 days at 12 °C and 95% relative humidity (RH) to allow prolonged cold storage. After curing, the potatoes were stored for up to 5 months at 5 °C and 95–98% RH. Samples consisting of three pooled tubers were taken at harvest, after curing, and at 1, 2, and 5 months of cold storage for analysis as described in Section 2.4.1, Section 2.4.2, Section 2.4.3, Section 2.4.4, Section 2.4.5 and Section 2.4.6 (n = 6). The potato tubers were washed, cut into small cubes, frozen in liquid nitrogen, ground in a laboratory mill (A 11 basic Analytical mill, IKA®-Werke GmbH & Co., KG, Staufen, Germany), lyophilized, packed under a vacuum, and stored until analysis.

2.3. Obtention of Potato Chips

Samples for frying were taken after the curing procedure, as well as after cold storage for 1, 2, and 5 months. The potatoes were left at room temperature for one day, washed with tap water, and cut into slices of approximately 1.5 mm (without peeling), and excess starch was removed by submerging the chips in water for 1 min. After drying them with absorbent paper, the potato slices (~40 g per batch) were fried in 4 L of sunflower oil at 180 °C for 2.5 min. Then, the potato chips were left so the excess could be drained, cooled down to room temperature and stored in plastic bags at −20 °C until analysis. Frying experiments were performed in triplicate, each replicate consisting of about 40 g of potato slices. In addition to the chips made from the potatoes cultivated for this study, commercially available colored potato chips (fried with peel) from three different brands were acquired from a local supermarket. Chips were classified by color (red and purple) and stored for acrylamide determination for comparison purposes. Commercial samples were labelled in red and purple with a number indicating their brand.

2.4. Analytical Determinations

All analytical determinations except that of acrylamide (Section 2.4.7) were performed on the potatoes before frying the samples in accordance with the procedure in Section 2.2.

2.4.1. Hydrophilic Antioxidant Capacity

The hydrophilic antioxidant capacity was determined by the inhibition of the DPPH radical in accordance to the method of Fuentealba et al. [22]. Briefly, 50 mg of the lyophilized material was weighed in 2 mL reaction tubes, mixed with 1.4 mL of 80% methanol and kept in the dark at 4 °C overnight. Then, the tubes were centrifuged at 17,000× g at 4 °C for 10 min. The supernatants were collected, 20 μL was mixed with 180 μL of methanol, and 1250 μL of the 60 μM DPPH in the methanol was added. The mixture was homogenized and left to stand in the dark for 30 min. Absorbance at 517 nm was measured and the inhibition of the DPPH radical was calculated. The results were expressed as the μmol of the Trolox equivalent (TE) g−1 dry weight (DW).

2.4.2. Targeted Primary Metabolite Analysis

Free polar metabolites (amino acids, sugars, and organic acids) were extracted and derivatized in accordance to the method of Fuentealba et al. [23]. An amount of 20 mg of the lyophilized sample was weighed in a reaction tube, mixed with 500 μL of cold methanol, and 20 μL of an internal standard (3000 ng μL−1 phenyl-β-d-glucopyranoside). Then, the samples were incubated at 70 °C for 15 min with agitation at 120 RPM, cooled down to −20 °C, and centrifugated at 17,000× g for 10 min at 4 °C. An aliquot (100 μL) of the supernatant was dried under a stream of nitrogen gas at 40 °C. Next, 120 μL of methoxyamine hydrochloride was added and incubated for 90 min at 30 °C with agitation at 120 RPM. After, 120 μL of N, O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) was added and incubated for 30 min at 37 °C with agitation at 120 RPM. The derivatized extracts were stored at −20 °C until GC-MS analysis.
The samples were injected on an Agilent 7890B gas chromatographer equipped with a PAL3 autosampler and coupled to an Agilent 5977A single-quadrupole mass spectrometer (Agilent Technologies, Santa Clara, CA, USA). Separation was performed on a HP-5-MS Ultra Inert column (0.25 μm, 30 m × 0.25 mm). The injection volume was 1 μL, with a split ratio of 1:150, and the injector and interphase temperatures were 220 °C and 280 °C, respectively. The following temperature ramp was used: 120 °C for 1 min, which was then increased up to 300 °C at a 10 °C min−1 rate and kept at 300 °C for 6 min. High-purity helium was used as a carrier gas at a flow of 1 mL min−1. Ions were generated by electron impact ionization (EI). The ionization source's temperature was 230 °C and the quadrupole’s temperature was 150 °C. Mass spectra were acquired in the 50–600 m/z range at a scanning speed of 2.66 scan cycles per second. The chromatographic peaks were identified by comparing the retention times and mass spectra with the previously injected commercial standards (asparagine, fructose, glucose, and sucrose from Sigma-Aldrich, St. Louis, MO, USA). Quantification was performed by the internal standard method. Results are expressed as mg g−1 DW.

2.4.3. Starch Content

Starch content was determined in accordance with the method of Rodríguez et al. [24] with some modifications. Briefly, 30 mg of the lyophilized sample was mixed with 1.8 mL of ultrapure water, homogenized, and incubated at 70 °C with 120 RPM of agitation for 15 min. Then, the samples were centrifuged at 17,000× g for 10 min. After the supernatant was discarded, this procedure was repeated two times to ensure the removal of soluble sugars. The remaining pellet was incubated at 100 °C for 10 min and cooled down to room temperature. Next, 600 µL of 0.2 M sodium acetate (pH 5.5), 120 µL (70 units mL−1) of amyloglucosidase from Aspergillus niger (Sigma-Aldrich, St. Louis, MO, USA) and 120 µL (7 units mL−1) of α-amylase from Aspergillus oryzae (Sigma-Aldrich, St. Louis, MO, USA) was added to the samples and incubated at 37 °C for 16 h. The samples were centrifuged at 15,000× g for 5 min and reducing sugars were quantified on the supernatant by the method of Miller [25] with 3,5-dinitrosalicylic acid (DNS). Absorbance was measured at 540 nm (Multiskan™ SkyHigh, Thermo Scientific, Waltham, MA, USA) in the samples, blank solution, and a positive control (starch). A standard curve of glucose was constructed to determine the amount of reducing sugars after enzymatic hydrolysis. Starch content was determined by multiplying the glucose concentration by 0.9 (the factor to convert glucose to anhydrous glucose) and expressed as g of starch per 100 g−1 DW.

2.4.4. Extraction of Anthocyanins and Other Polyphenols

Extracts were prepared in accordance to the method of Oertel et al. [26]; 100 mg of lyophilized potato was weighed in a 2 mL reaction tube and mixed with 500 µL of 70% methanol, shaken for 30 min at 120 RPM and centrifuged for 10 min at 17,000× g. The supernatant was reserved, and the extraction was repeated on the pellet. Both supernatants were combined and used for the determination of total anthocyanin content and total phenolic compound content.

2.4.5. Total Anthocyanin Content and Profile Determined by LC-MS

Total monomeric anthocyanins were determined by the pH differential method of Giusti and Wrolstad [27] and expressed in mg of cyanidin-3-O-glucoside equivalents per g−1 of the sample DW.
Chromatographic analysis was carried out in accordance to the method of Oertel et al. [26] with some modifications. LC separation and detection were performed on a Dionex UltiMate 3000 UHPLC system coupled to a Quantum Access Max TQMS system (Thermo Scientific, Waltham, MA, USA). Anthocyanins were chromatographically separated on an Acquity UPLC® BEH C18 column (1.7 µm, 100 × 2.1 mm, Waters, Wexford, Ireland) equipped with an Acquity VanGuard BEH C18 precolumn (1.7 µm, 5 × 2.1 mm, Waters, Wexford, Ireland). The column was kept at 30 °C. The mobile phase was composed of water with 5% formic acid (solvent A) and acetonitrile (solvent B). A linear gradient was used: 5% of B for 1 min, from 5 to 30% of B from 1 to 8 min, from 30 to 95% of B from 8 to 8.5 min, 95% of B from 8.5 to 11.5 min, from 95 to 5% of B from 11.5 to 12 min, and 5% of B from 12 to 17 min. The mobile phase’s flow rate was 300 μL min−1. Heated electrospray ionization (H-ESI) in the positive mode was used to generate ions. The spray voltage was 3.5 kV, the vaporization temperature was 240 °C, the sheat gas pressure was 30 (in arbitrary units), the auxiliar gas pressure was 5 (in arbitrary units), the capillary temperature was 270 °C, and the collision gas pressure was 1.5 mTorr. Data were acquired using the single-reaction monitoring (SRM) mode and the peak areas were registered.

2.4.6. Total Phenolic Compounds and Profile by LC-MS

Total phenolic compounds were determined using the Folin–Ciocalteau reagent method [28] with some modifications. Absorbance was measured at 755 nm and the results were expressed as mg of gallic acid equivalents per g−1 of the sample DW.
Chromatographic separation was performed using the method of Oertel et al. [26] with some modifications. The chromatographic and detection system was the same as that used for the separation of anthocyanins. An Acquity UPLC® HSS T3 column (1.8 µm, 100 × 2.1 mm, Waters, Wexford, Ireland) equipped with an Acquity VanGuard HSS T3 precolumn (1.8 µm, 5 × 2.1 mm, Waters, Wexford, Ireland) was used. The column was kept at 40 °C. The mobile phase was composed of water with 0.1% formic acid (solvent A) and acetonitrile with 0.1% formic acid (solvent B). A linear gradient was used: from 5 to 20% of B for 4.5 min, from 20 to 45% of B from 4.5 to 9 min, from 45 to 100% of B from 9 to 11 min, 100% of B from 11 to 14 min, from 100 to 5% of B from 14 to 14.5 min, and 5% of B from 14.5 to 18 min. The mobile phase’s flow rate was 400 μL min −1. Heated electrospray ionization (H-ESI) in the positive mode was used to generate ions. The spray voltage was 3.5 kV, the vaporization temperature was 240 °C, the sheat gas pressure was 30 (in arbitrary units), the auxiliar gas pressure was 5 (in arbitrary units), the capillary temperature was 270 °C, and the collision gas pressure was 1.5 mTorr. Data were acquired using the single-reaction monitoring (SRM) mode and the peak areas were registered.

2.4.7. Acrylamide Content

Acrylamide (AA) content was determined by GC-MS according to the method modified by Pedreschi et al. [29]. Isotope-labeled acrylamide (AA-D3) was used as an internal standard at a concentration of 20 mg L−1. Chromatographic separation was carried out on an Agilent 7890A gas chromatograph coupled to an Agilent 597C XL mass detector and equipped with an Agilent DB-FFAP column (0.25 μm, 30 m × 0.25 mm). The injection volume was 2 μL, it was in the splitless pulsed mode, and the injector and interphase temperatures were 200 °C and 250 °C, respectively. The following temperature ramp was used: 60 °C for 1 min, which then increased up to 190 °C at a 10 °C min−1 rate, followed by an increase to 240 °C at a 50 °C min−1 rate. High-purity helium was used as a carrier gas at a flow of 0.8 mL min −1. Ions were generated by chemical ionization using methane (CI). The ionization source’s temperature was 150 °C and the quadrupole’s temperature was 150 °C. Acrylamide (m/z = 70) and AA-D3 (m/z = 73) areas were measured using the SIM mode in the negative polarity and in triplicate. The results were expressed as μg kg−1 of DW.

2.5. Statistical Analysis

The experiments were performed using a completely randomized model using six independent replicates (each biological replicate was composed of three pooled potatoes). Statistical analysis was carried out using Statgraphics Centurion XV software (Manugistic Inc., Rockville, MD, USA). A two-way ANOVA with cultivation seasons (summer and winter) and postharvest storage treatment (harvest, curing, and cold storage for 1, 2, and 5 months, respectively) as factors was performed. Differences between treatments were analyzed by Tukey’s HSD test using the linear general model with a confidence level of 95%. For the anthocyanin results, a two-way hierarchical cluster analysis was carried out using the Metaboanalyst platform (www.metaboanalyst.ca) accessed on 15 February 2023.

3. Results

3.1. Acrylamide Formation in Chips from Winter Season Potatoes and Comparison with Commercial Colored Potato Chips

Acrylamide formation was investigated by making chips using winter season potatoes. It is important to note that no pre-processing technique aiming to mitigate acrylamide formation was applied other than soaking the potatoes briefly in water to remove excess superficial starch. As shown in Figure 1, acrylamide content was significantly high (p < 0.05) after one month of cold storage (480% higher than the average value obtained from chips made from potatoes after curing) whereas after two and five months of cold storage the acrylamide went down to the level observed after curing. The acrylamide levels observed of the “Michuñe negra” potato chips were compared with those of commercial colored potato chips from three different brands (red- and purple- labeled with a number to indicate their brand) (Figure 1). The acrylamide levels of two commercial purple potato chips (Purple_1 and Purple_2) were significantly lower, at 1256 and 1277 µg kg−1, respectively, while the other samples presented higher values that were similar to the ones obtained in this work (excepting for Cold_1m chips). For all samples, the mean acrylamide level was higher than the EFSA benchmark value of 750 µg kg−1 [30]. Chips made from the “Michuñe negra” purple-fleshed potato had acrylamide concentrations that were from 284% to 2125% higher than the EFSA value. Meanwhile, in the commercial samples they ranged from 67% to 501% higher. These results show that further mitigation strategies must be employed to lower the acrylamide levels of chips made from colored potatoes.

3.2. Targeted Metabolites Related to Neocontaminant Formation

Metabolites related to the Maillard reaction and neocontaminant formation, namely fructose, glucose, sucrose, and asparagine, were measured from the colored potatoes in two cultivation cycles (Figure 2). Regarding reducing sugars, significant differences (p < 0.05) were found for fructose and glucose content between the summer and winter seasons. Overall, higher values of fructose (p < 0.05) were observed for the summer season at harvest and after curing, although no significant differences in fructose levels were observed in the average fructose amounts after one month of cold storage (Figure 2A). In both seasons, fructose levels peaked after one month of cold storage, which could be expected due to cold-induced sweetening (CIS). A similar trend was observed in the glucose levels in potatoes from the summer season, although no significant changes were observed in potatoes from the winter season whose glucose levels remained unchanged from harvest through to storage for up to five months (Figure 2B). As shown in Figure 2C, the sucrose level was significantly higher (p < 0.05) in potatoes from the winter season than in those from the summer, and it decreased after cold storage. On the contrary, the sucrose level during the summer season remained practically unchanged (p > 0.05) even after five months of cold storage. Asparagine (Figure 2E) contents remained on average unchanged between seasons and regardless of the cold storage time.
Starch contents were similar at harvest for both cultivation seasons (Figure 2D). However, the postharvest behavior differed among them. Starch content in the winter season potatoes did not change significantly with cold storage (p > 0.05). In contrast, it decreased significantly even after the curing treatment in the summer season potatoes. Starch is the main sugar reservoir in potato tubers, and under low temperature storage it breaks down into lower-molecular-weight oligosaccharides, and then into maltose, glucose 6-P or glucose in the amyloplasts, and is exported to the cytosol where the maltose becomes sucrose and ultimately becomes glucose and fructose by the action of invertases [31].

3.3. Total Anthocyanin Content and Profile

The average content of total anthocyanin ranged from 0.12 mg C3G eq. g−1 DW (Summer-Harvest) to 1.46 mg C3G eq. g−1 DW (Winter-Cold_1m, Figure 3A). A significant effect of the cultivation season was observed, with higher contents in the potato tubers from the winter season whose values are, on average, 4.4 times higher than the ones from the summer season at harvest. The effect of cold storage on the total anthocyanin content was particularly evident in the winter season potatoes with values reaching a peak after one month of storage and no significant differences observed even after 5 months of storage. In contrast, no significant differences (p > 0.05) were found during cold storage in the summer season tubers, although the total values showed an apparent increase.
Colored potatoes from the southern region of Chile are characterized by their anthocyanin content, especially that of the skin rather than the pulp [2]. In our study, we analyzed the whole potato since it is usually processed and consumed that way, at least locally.
A total of 19 anthocyanins were tentatively identified according to their MS/MS fragmentation pattern (Table 1). The anthocyanin profile was composed by five cyanidin derivatives, four malvidin and peonidin derivatives, three delphinidin derivatives, two petunidin derivatives, and one pelargonidin derivative. The LC-MS profiling of the anthocyanins revealed qualitative and quantitative differences, after applying hierarchical clustering, as shown by the resulting heatmap (Figure 4). By comparing the log10 values of the peak areas, two main clusters separating tubers at harvest and tubers after postharvest storage (horizontal axis) appeared. Four anthocyanins (peaks 4, 5, 7, and 8) were not detected in tubers at harvest and were not considered for the analysis. A notable difference between summer and winter season tubers was observed, showing a differential biosynthetic anthocyanin pattern between seasons.

3.4. Total Phenolic Content and Profile

The total phenolic compound content showed the same trend as the total anthocyanin content did and it ranged from 0.39 mg GAE g−1 DW (Summer-Cold_1m) to 1.91 mg GAE g−1 DW. As shown in Figure 3B, there were significant differences among cultivation seasons and storage treatments. However, similarly to the total anthocyanin content, no significant differences were observed between tubers at harvest and after cold storage in the summer season. In contrast, a significant increase was observed in the winter season reaching a maximum after 1 month of storage (84% higher than the harvest values; p < 0.05).
The phenolic compound profile of the colored potatoes was further investigated by LC-MS. A total of five compounds were identified and quantified (Table 2 and Figure 5), namely neochlorogenic, chlorogenic, cryptochlorogenic, caffeic, and rutin. Most of the phenolic compounds belonged to the caffeoylquinic group, and only one flavonoid was detected (rutin). In general, the content of all phenolics increased as the cold storage time increased. Rutin was an interesting case as it was present only in trace amounts at harvest but increased after curing and cold storage treatments, a behavior observed for some anthocyanins as well.

3.5. In Vitro Antioxidant Capacity

Hydrophilic antioxidant capacity was measured by the DPPH radical, and the results are shown in Figure 3C. Similarly to the total anthocyanin and phenolic contents, it was significantly higher in the winter season at 11.51 μmol TE g−1 DW (Summer-Cold_1m) while 4.52 μmol TE g−1 DW was the lowest antioxidant capacity observed (Summer-Cold_1m). Though no significant differences were observed between storage treatments, the overall behavior of antioxidant capacity was consistent with the total anthocyanin and total phenolic contents.

4. Discussion

Potato cultivars developed for the potato chip industry are characterized by their lower levels of reducing sugars and asparagine such that the resulting products have a reduced acrylamide content [6,32,33]. Indeed, commercial potato crisps from the European market have had their mean acrylamide content consistently reduced to levels below the benchmark value (750 µg kg−1) in the last decade [34]. However, these cultivars do not include colored-flesh potatoes. In recent times, chips made from vegetables other than potato have been developed and marketed as “healthier snacks” [35]. These chips include colored vegetables such as beet, sweet potato, carrot, and parsnip, among others [35,36]. As such, colored potato chips are more closely related to this group of snacks than to conventional potato chips. Studies on colored vegetable chips, although limited due to their smaller market, suggest that these snacks have on average twice the levels of acrylamide found in potato chips [35]. Except the notably high level of acrylamide detected in chips made after 1 month of cold storage, the acrylamide levels found in chips made from the “Michuñe negra” potato and those found in commercial colored potato chips were similar to the values reported for colored vegetable chips such as orange sweet potato (868–4457 µg kg−1; nine samples), purple beet (494–2231 µg kg−1; nine samples) [36], and carrot (700–3090 µg kg−1; seven samples) [35]. In general, studies related to the acrylamide levels in potato products made from colored potatoes are scarce, and the published research focuses on French-fry-type potatoes, the processing conditions of which generally differ from those used for chips.
The content of reducing sugars in potatoes is in general expected to increase after cold storage as a result of CIS, and high variability in the reducing sugar content after cold storage has been reported for different native potato accessions [33,37]. For the “Michuñe negra” potato, the level of reducing sugars after prolonged cold storage was similar to that the observed in the “Innovator” potato, a variety considered CIS-resistant [38]. In general, a low reducing sugar content is desired for chip processing, although the maximum value accepted varies largely and values as low as 0.35 mg g−1 DW and up to 16.5 mg g−1 DW are considered acceptable [33]. Nevertheless, “Michuñe negra” potatoes’ reducing sugar levels were higher (by up to eight times after 1 month of cold storage) compared to those of chipping cultivars while asparagine levels were similar [6,32]. The previously cited works remark that the content of reducing sugars has a stronger correlation with acrylamide formation than that of asparagine does. Indeed, glucose and fructose are considered the main acrylamide precursors of acrylamide formation in heat-processed foods [39]. Interestingly, neither glucose nor asparagine contents were high after one month of cold storage for winter season potatoes, which seems to contradict the acrylamide value observed. On the other hand, it is also noteworthy that both sucrose and fructose were indeed higher in potatoes after one month of cold storage than in potatoes from the other treatments selected for frying. This is important as the fructose-related reaction (caramelization) has also been pointed out as a contributor to acrylamide formation in potato chips in addition to the Maillard reaction involving glucose [40]. Although the aldehyde group from glucose is more reactive than the ketone group from fructose, it has shown less reactivity than fructose has in the Maillard reaction under low-moisture and low-molecular-mobility conditions [41], and a similar case could be occurring in the potato’s starchy matrix where molecular mobility could be lower than that in homogeneous solutions. Since no significant differences were found in the average contents of asparagine during storage, the acrylamide level differences could be attributed to the variations in reducing sugar levels that occurred during cold storage. In fact, better correlations have been reported between reducing sugar and acrylamide levels than between asparagine and acrylamide [42]. These results show that “Michuñe negra” purple-colored potatoes have a rather high capacity to form high levels of acrylamide under the conventional frying method, and thus mitigation strategies at both the agronomical level and the processing level should be employed in order to obtain potato chips with acceptable acrylamide levels.
Colored potatoes have been of interest because of their higher contents of compounds with antioxidant properties such as anthocyanins and other phenolics. Indeed, anthocyanin content in the “Michuñe negra” potato at harvest was within the range reported by Giusti et al. [43] for red- and purple-colored potatoes (0.08 1.52 mg g−1 DW). Eleven anthocyanins were tentatively identified as acylated, which is in accordance with the reported profiles for purple-colored potatoes [26,43]. Regarding the effect of cold storage on anthocyanins, our results are consistent with the findings of Hamouz et al. [20] for certain potato cultivars with a red and purple flesh color with an increase from 13.3% to 77.6% being observed after six months of storage at 4 °C. After one month of storage at 5 °C ± 1 °C, the total anthocyanin content increased by 66.7% in the summer season, whereas it increased by 175.5% in the winter season. This observed increase in anthocyanins has been postulated to be a result of the biosynthesis during cold storage related to cold-induced sweetening that takes place under these conditions because sugars are a fundamental part of the anthocyanin structure [21].
Phenolic compounds have been suggested to be potential mitigators of acrylamide formation in foods, although the exact mechanisms of action are hard to predict and in some cases may even promote acrylamide formation. In general, the results are inconclusive both in model systems and food matrices [44]. No conclusions regarding the participation of phenolics in acrylamide formation/mitigation can be drawn from our study. The work from Kalita et al. [6] on potato chips, a slight negative correlation between total phenolic content (r = −0.3671), total chlorogenic acid content (r = −0.3590) and acrylamide formation was shown for flesh-colored potatoes. Orsák et al. [45] found positive correlations between chlorogenic acid content and acrylamide formation, and they suggested a possible contribution from anthocyanins which could have decomposed under the high frying temperatures, increased the sugar pool available for the Maillard reaction and thus increase acrylamide formation. However, although both works are based on potatoes with red- and/or purple flesh, none of them investigated the role of anthocyanins on acrylamide formation in model solutions or food matrices. Further studies with colored potatoes should be designed to study the role of anthocyanins and other important phenolics in acrylamide formation.

5. Conclusions

The “Michuñe negra” potato native to the southern region of Chile and grown under a protected, fertigated system in the winter cultivation season showed high acrylamide-forming potential when processed for chip production. Seasonality and cold storage have a quantitative effect on the levels of reducing sugars but not on asparagine levels and thus may influence acrylamide formation potential directly. The factors studied also showed an effect on anthocyanins and other phenolics both at the quantitative and qualitative levels, but no conclusions can be drawn from this study regarding the role of phenolics on acrylamide mitigation. Acrylamide levels detected on both commercial and experimental samples show that both agronomical strategies (regarding the selection of appropriate cultivars for processing and nutritional management, among others) and processing strategies (pretreatments to lower the precursors levels) still need to be employed to reduce the acrylamide levels in colored potato chips.

Author Contributions

Conceptualization: J.E.A., F.P. and R.P.; funding acquisition: J.E.A., R.P. and F.P.; methodology: J.E.A. and F.P.; formal analysis: D.G.-R. and R.P.; supervision: J.E.A., M.E.Z., D.C., A.A.-G. and R.P.; resources: J.E.A., D.C., F.P. and R.P.; investigation: D.G.-R.; writing—original draft: D.G.-R.; writing—review and editing: D.G.-R., J.E.A., D.C., M.E.Z., A.A.-G., M.S.M.-C., F.P. and R.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by ANID-FONDECYT N°1190080, ANID-FONDEQUIP EQM140070 and ANID-PCI REDES BIO0001, Chile. D. García thanks ANID-Subdirección de Capital Humano/Doctorado Nacional/2022-21220912. R. Pedreschi and D. García acknowledge ANID—Millennium Science Initiative Program—ICN2021_044.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are not publicly available due to privacy reasons.

Acknowledgments

The authors would like to thank Sebastián Veas, Excequel Ponce, Juan Vidal, Juan Pablo Gómez, Ignacia Hernández, and Paz Delgado for their technical and/or logistical assistance.

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 13 01209 g001 550
Figure 1. Acrylamide content in potato chips obtained from “Michuñe negra” potato chips and commercially available colored potato chips. Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). MN: Michuñe negra.
Figure 1. Acrylamide content in potato chips obtained from “Michuñe negra” potato chips and commercially available colored potato chips. Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). MN: Michuñe negra.
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Agronomy 13 01209 g002 550
Figure 2. Main sugar, starch, and asparagine contents in “Michuñe negra” potato. (A): Fructose, (B): Glucose, (C): Sucrose, (D): Starch, (E): Asparagine. Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). Color legend is indicated in (A).
Figure 2. Main sugar, starch, and asparagine contents in “Michuñe negra” potato. (A): Fructose, (B): Glucose, (C): Sucrose, (D): Starch, (E): Asparagine. Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). Color legend is indicated in (A).
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Figure 3. Secondary metabolites and antioxidant capacity in “Michuñe negra” potatoes. (A): Total anthocyanins, (B): Total phenolics, (C): Hydrophylic antioxidant capacity (DPPH). Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). Color legend is indicated in (A).
Figure 3. Secondary metabolites and antioxidant capacity in “Michuñe negra” potatoes. (A): Total anthocyanins, (B): Total phenolics, (C): Hydrophylic antioxidant capacity (DPPH). Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). Color legend is indicated in (A).
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Agronomy 13 01209 g004 550
Figure 4. Heatmap showing the two-way hierarchical clustering results of the anthocyanins in the “Michuñe negra” potato from two cultivation seasons. Anthocyanins are depicted on the vertical axis (A₋peak number; refer to Table 1 for their identification). The horizontal axis represents the corresponding class of each sample (1M: 1₋month cold storage, 2M: 2₋month cold storage, 5M: 5₋month cold storage, C: curing, H: harvest, S: summer season, W: winter season).
Figure 4. Heatmap showing the two-way hierarchical clustering results of the anthocyanins in the “Michuñe negra” potato from two cultivation seasons. Anthocyanins are depicted on the vertical axis (A₋peak number; refer to Table 1 for their identification). The horizontal axis represents the corresponding class of each sample (1M: 1₋month cold storage, 2M: 2₋month cold storage, 5M: 5₋month cold storage, C: curing, H: harvest, S: summer season, W: winter season).
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Figure 5. Phenolic acid and rutin content on “Michuñe negra” potato from two cultivation seasons. (A): Neoclorogenic acid, (B): Chlorogenic acid, (C): Cryptochlorogenic acid, (D): Caffeic acid, (E): Rutin. Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). Color legend is indicated in (A).
Figure 5. Phenolic acid and rutin content on “Michuñe negra” potato from two cultivation seasons. (A): Neoclorogenic acid, (B): Chlorogenic acid, (C): Cryptochlorogenic acid, (D): Caffeic acid, (E): Rutin. Different letters indicate significant differences (Tukey’s HSD test; p < 0.05). Color legend is indicated in (A).
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Table
Table 1. Anthocyanin profile of “Michuñe negra” potato by LC-TQMS.
Table 1. Anthocyanin profile of “Michuñe negra” potato by LC-TQMS.
Peak N°RT (min)m/z [M]+Fragments 1Tentative ID
15.17771301, 463, 609Peonidin-3-rutinoside-5-glucoside
25.49801331, 493, 639Malvidin-3-rutinoside-5-glucoside
35.85595287Cyanidin-3-rutinoside
46.22773303, 465, 611Delphinidin-3-rutinoside-5-glucoside
56.35627303, 465, 521Delphinidin-3-glucoside-5-glucoside
66.80757287, 449, 595Cyanidin-3-rutinoside-5-glucoside
77.10611287, 449, 565Cyanidin-3-(3″-glucosylglucoside)
87.38611303, 465Delphinidin-3-(6-p-coumaroyl)-glucoside
97.60933301, 463, 771Peonidin-3-caffeoyl-rutinoside-5-glucoside
107.73963331, 492Malvidin-3-caffeoylrutinoside-5-glucoside
117.74903287, 449, 742Cyanidin-3-p-coumaroylrutinoside-5-glucoside
127.75933317, 479, 771Petunidin-3-p-coumaroylrutinoside-5-glucoside
137.92933287, 449, 772Cyanidin derivative
148.03963317, 479, 801Petunidin-3-feruloylrutinoside-5-glucoside
158.17887271, 433, 725Pelargonidin-3-p-coumaroylrutinoside-5-glucoside
168.23917301, 463, 755Peonidin-3-p-coumaroylrutinoside-5-glucoside
178.24947331, 493, 785Malvidin-3-p-coumaroylrutinoside-5-glucoside
188.39947301, 463, 785Peonidin-3-feruloylrutinoside-5-glucoside
198.40977331, 493, 815Malvidin-3-feruloylrutinoside-5-glucoside
1 Ions used for area measurement are shown in bold.
Table
Table 2. Phenolic profile of “Michuñe negra” potato by LC-TQMS.
Table 2. Phenolic profile of “Michuñe negra” potato by LC-TQMS.
Peak N°RT (min)m/z [M–H]−Fragments 1Identification
13.53353179, 191, 201Neochlorogenic acid
25.25353191Chlorogenic acid
35.50353173, 179, 191Cryptochlorogenic acid
45.65179135Caffeic acid
57.80609301Rutin
1 Ions used for quantitation are shown in bold.
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García-Ríos, D.; Alvaro, J.E.; Zuñiga, M.E.; Campos, D.; Aguilar-Galvez, A.; Mariotti-Celis, M.S.; Pedreschi, F.; Pedreschi, R. Targeted Primary and Secondary Metabolite Analysis of Colored Potato “Michuñe Negra” Grown in Soilless Culture and during Prolonged Cold Storage: Implications in Acrylamide Formation during Frying. Agronomy 2023, 13, 1209. https://doi.org/10.3390/agronomy13051209

AMA Style

García-Ríos D, Alvaro JE, Zuñiga ME, Campos D, Aguilar-Galvez A, Mariotti-Celis MS, Pedreschi F, Pedreschi R. Targeted Primary and Secondary Metabolite Analysis of Colored Potato “Michuñe Negra” Grown in Soilless Culture and during Prolonged Cold Storage: Implications in Acrylamide Formation during Frying. Agronomy. 2023; 13(5):1209. https://doi.org/10.3390/agronomy13051209

Chicago/Turabian Style

García-Ríos, Diego, Juan E. Alvaro, María Elvira Zuñiga, David Campos, Ana Aguilar-Galvez, María Salomé Mariotti-Celis, Franco Pedreschi, and Romina Pedreschi. 2023. "Targeted Primary and Secondary Metabolite Analysis of Colored Potato “Michuñe Negra” Grown in Soilless Culture and during Prolonged Cold Storage: Implications in Acrylamide Formation during Frying" Agronomy 13, no. 5: 1209. https://doi.org/10.3390/agronomy13051209

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