The Effect of Thermal Treatment on Selected Properties and Content of Biologically Active Compounds in Potato Crisps

Abstract

The aim of this study was to determine the effect of blanching and frying on selected properties of potato crisps of yellow- and purple-fleshed cultivars. The material used was yellow flesh cv. Ismena and purple flesh cv. Provita. Potato slices were blanched (1–5 min at 80–90 °C) and then fried (155–175 °C). The control sample was comprised of crisps that were not blanched. The fat content and color parameters were determined in crisps, while in defatted samples, the following were determined: the content of total polyphenols; antioxidant capacity, measured by the ability to scavenge the ABTS^•+ radical cations (TEAC ABTS) and by the FRAP method; and the level of fluorescent intermediary compounds (FICs) and browning index (BI), as indicators of the progress of the Maillard reaction. The content of kynurenic acid (KYNA) was examined in the raw material, in slices, in the water after blanching and in the crisps. Blanching affected the fat absorption, with time being more critical than temperature. The color of crisps from yellow flesh potatoes after blanching lightened, while the color in the purple samples darkened. The content of total polyphenols was higher in purple crisps. Increasing the temperature and shortening the time of blanching increased the polyphenol content and the antioxidant capacity. Blanching decreased the level of FICs, while frying increased FICs. Higher BI values characterized the crisps from cv. Provita. Blanching reduced BI values by 50%, while frying at highest temperature increased these values. The content of KYNA in purple potatoes was almost three times higher than in yellow ones. Blanching and frying decreased the KYNA content in potatoes and fried crisps.

1. Introduction

Crisps are one of the most popular potato-based snack products. They are thin potato slices fried in hot fat for a short time [1]. Potato crisps are characterized by low moisture (about 2%), relatively high fat content (30–39%), golden-yellow color, a specific oil–potato flavor and aroma, as well as a crispy and delicate texture [2]. The raw material for the production of crisps should be characterized not only by the appropriate shape and size, but also by the suitable chemical composition (21–25% dry weight, 16–20% starch, and less than 0.25% reducing sugars) [3]. Blanching is a thermal process that is carried out in water, usually at 65–85 °C for 3–5 min. This treatment aims to reduce the content of reducing sugars in the raw material, thus obtaining the appropriate light color of the finished product. During blanching, enzymes may also be inactivated and other groups of water-soluble compounds removed, which would contribute to the formation of a too-dark color of the product [4]. Blanching also has a negative effect on other characteristics of potato crisps, mainly fat content and consistency. Although it was possible to select classic varieties with yellow flesh, characterized by a low content of reducing sugars and thus not requiring blanching, varieties with colored flesh usually have a higher content of these compounds and tend to burn. Therefore, it seems advisable to use blanching in their processing. This process also reduces the content of polyphenolic compounds (namely phenolic acids and anthocyanins), which results from their good solubility in water as well as their sensitivity to high temperature [5]. The key solution in the technological process of producing crisps from potatoes with colored flesh may therefore be the selection of such blanching parameters that will allow crisps to be obtained with the desired color, and at the same time, the highest possible content of bioactive compounds. In the literature, there is no information about the effect of blanching on the content of kynurenic acid in crisps, for cultivars of potatoes with colored as well as for the classic color of flesh. However, so far, the use of cooking as a thermal process has not been shown to change the content of this acid in potatoes [6]. The next most important step in the production of potato crisps is frying. Vegetable oils are used for frying, and the process is carried out most often at a temperature of 175–185 °C for approximately 3–5 min. During the frying process, typical sensory features of the crisps are shaped, such as color and specific taste and smell, which are also dependent on the formation of Maillard reaction products [7,8]. During frying, the product is almost completely dehydrated, and the frying fat takes the place of the evaporated water [2]. Frying, however, significantly reduces the content of phenolic compounds. Kita et al. [3] showed that during the frying of crisps from the red flesh cultivars, the anthocyanin content was reduced by 40–60%, while purple ones, it was reduced in the range of 50 to 80%. In another experiment, Turski et al. [9], analyzing the content of kynurenic acid, a tryptophan metabolite with health-promoting properties [10,11,12], in the raw material and finished products, found that potato crisps were characterized by a lower content when compared to raw potatoes. KYNA exhibits pro-health properties, including antioxidant, anti-inflammatory, and anti-convulsant properties. It was also reported that KYNA can have protective properties on the human brain and an inhibitory effect on the proliferation of colon cancer cells. It can be assumed that the use of a high-temperature process, such as frying, reduces the content of this acid in the finished product. However, the studies only concerned cultivars with yellow flesh [6].

Thermal treatments play a key role in the technological process of obtaining potato crisps. Both blanching and frying shape the quality of the final product. When processing the raw material with colored flesh, blanching may contribute not only to the reduction of reducing sugars but also to a significant decrease in the content of bioactive compounds, most of which are characterized by good solubility in water. Previous studies on the impact of this technological stage on the quality of potato crisps have focused on issues related to color and consistency. Losses of selected groups of bioactive compounds were also shown. There are no reports in the literature on the influence of various blanching parameters on the content of bioactive compounds and the antioxidant capacity of crisps, especially in relation to potatoes with colored flesh. There is also no information on the influence of the technological process on the content of kynurenic acid. Hence, this study aimed at the evaluation of the effect of blanching on the composition and properties of potato crisps obtained from cultivars with classic and colored flesh. The purpose of the study was to determine the effect of heat treatment (blanching and frying) on the selected properties of potato crisps from varieties with yellow and purple flesh, with particular emphasis on the content of kynurenic acid.

2. Materials and Methods

2.1. Material

The material used for the research was potato tubers of two cultivars from the growing season 2020: ZAH 25618 (Provita), with a purple flesh color, and Ismena, with a yellow flesh color. The potato tubers of both cultivars (3 × 8 kg samples) came from the same growing site and were shared for testing by Research Centre For Cultivar Testing (COBORU). The frying medium was “Wielkopolski” refined rapeseed oil, purchased in Wrocław.

2.2. Potato Crisp Processing

The potato tubers were used for laboratory potato crisp production. After washing, potatoes were cut into slices of 2 ± 0.1 mm thickness (slicing machine, Brown, Germany), washed in cold water, blanched in hot water (80–90 °C for 1–5 min), superficially dried (using paper towels), and fried in a fryer (Beckers, Italy) until the moisture content was below 2%. The crisps were fried in refined rapeseed oil heated to 155, 165, or 175 °C. The blanching and frying parameters are listed in

Table 1. Thermal processing parameters of potato crisps.

Blanching Temperature (°C)	Blanching Time (min)	Frying Temperature (°C)
-	-	155
-	-	165
-	-	175
80	2.5	175
82	2.5	175
85	2.5	155
85	2.5	165
85	1	175
85	2.5	175
85	4	175
85	5	175
88	2.5	175
90	2.5	175

. Photos of the obtained crisps are presented on Figure 1 and Figure 2. After discharging the oil and cooling, 100 g of crisps was packed in aluminum foil packages and taken for laboratory analysis. All experiments were performed in duplicate, and the results shown are the mean values obtained in the investigation.

2.3. Determination of Product Quality Attributes

The fat content of the potato crisps was estimated using the Soxhlet procedure following AOAC method 963.15 [13]. Fat was extracted using a Büchi B-811 Universal Extraction System (Büchi Labortechnic AG, Flawil, Switzerland). A 2 g sample was extracted for 180 min, with diethyl ether used as a solvent. Measurements were made in at least three repetitions. A Minolta CM-5 spectrophotometer (Konica-Minolta, Osaka, Japan) was used to evaluate the color of fried products using the Hunter system. The spectrophotometer was calibrated using standard white and black tiles. Five ground samples were evaluated, and the five readings were recorded as an average reading. The mean values of the coordinates L* (lightness–darkness), a* (redness–greenness) and b* (yellowness–blueness) were used to determine the color of the product through reflectance mode. The hue angle and chroma color space parameters were calculated from the a* and b* values.

2.4. Total Polyphenol Compounds

The content of total polyphenolic compounds was examined in 70% methanol extracts (v/v) using the Folin-Ciocalteu colorimetric method as described by Gao et al. [14]. Results (n = 2) were expressed as µg gallic acid equivalent (GAE) per 100 g of crisps.

2.5. Antioxidant Capacity

The antioxidant capacity of 70% methanolic extracts (v/v) was determined using the Trolox equivalent antioxidant capacity (TEAC) with ABTS^•+ radicals and FRAP. The TEAC assay with ABTS^•+ radicals was carried out according to Re et al. [15], while the TEAC with FRAP was performed according to the procedure described by Benzie and Strain [16]. The TEAC results were expressed as mM TE per 100 g of crisps. Data were reported as mean value plus/minus standard deviation (SD) for four measurements.

2.6. Fluorescent Intermediary Compounds (FICs)

The level of fluorescent intermediary compounds in aqueous extracts (0.4 g of defatted sample in 5 mL of deionized water, followed by ultrasounds for 20 min and centrifugation; n = 2) was determined using a procedure modified from Matiacevich and Buera [17]. Aliquots of 200 μL of the extract were placed into 96-well plates. The fluorescence intensity was measured at the excitation wavelength λ = 353 nm and the emission λ = 455 nm on a Synergy H1 spectrofluorimeter (BioTek Instruments Inc., Winooski, VT, USA). The determinations were made in four replications, and the results were expressed as AU (arbitrary units).

2.7. Browning Index (BI)

The browning index was determined in aqueous extracts prepared as described above. A total of 200 μL of the extract was placed into 96-well plates. Measurements were made using a Synergy H1 spectrophotometer (BioTek Instruments Inc., USA). The analyses were performed at two wavelengths, λ = 420 nm and λ = 360 nm, selected from the literature data [18,19]. The browning index was expressed as absorbance units (A420) and (A360) per extract.

2.8. Kynurenic Acid (KYNA)

The determination of kynurenic acid content was performed for raw slices, slices after blanching, the water remaining after the blanching process, and fried crisps. The analysis was carried out using high performance liquid chromatography (HPLC) with fluorescence detection, according to the method described previously [9]. In short, material was homogenized and sonicated in distilled water (1:4, w/v). Supernatant was deproteinated with trichloroacetic acid and centrifuged. Samples were extracted with Dowex 50 ion exchange resin. The eluate containing kynurenic acid fraction was injected into the HPLC system for analysis (ESA catecholamine HR-80, 3 μm, C18 reverse-phase column, mobile phase: 250 mM zinc acetate, 25 mM sodium acetate, 5% acetonitrile, pH 6.2, flow rate of 1.0 mL/min; fluorescence detector: excitation 348 nm, emission 398 nm). Original kynurenic acid (Sigma-Aldrich, St. Louis, MO, USA) was used as internal standard. All reagents used were of HPLC grade.

2.9. Statistical Analysis

Statistical analyses of all data were performed using a one-way analysis of variance (ANOVA). Duncan’s range test was used to determine differences between samples, with a probability level of p ≤ 0.05. Statistical analyses and standard deviations were determined using Statistica v.13.3 software (StatSoft, Kraków, Poland).

3. Results and Discussion

3.1. Fat Content and Color

Crisps from Provita had a higher fat content than crisps from Ismena (

Table 2. Fat content in potato crisps, depending on technological parameters.

Technological Parameters			Fat Content (g/100 g)
Blanching Temperature (°C)	Blanching Time (min)	Frying Temperature (°C)	Ismena	Provita
-	-	155	41.41 ± 0.28 e	41.65 ± 0.1 g
-	-	165	40.24 ± 0.04 fg	39.70 ± 0.01 h
-	-	175	39.31 ± 0.01 h	38.57 ± 0.01 i
80	2.5	175	42.42 ± 0.35 c	50.01 ± 0.06 d
82	2.5	175	39.96 ± 0.28 g	52.11 ± 0.11 a
85	2.5	155	41.63 ± 0.04 de	52.20 ± 0.21 a
85	2.5	165	42.09 ± 0.18 cd	50.22 ± 0.02 d
85	1	175	37.39 ± 0.46 i	47.39 ± 0.02 f
85	2.5	175	40.61 ± 0.00 f	48.62 ± 0.03 e
85	4	175	43.56 ± 0.45 a	50.66 ± 0.11 c
85	5	175	42.70 ± 0.20 bc	52.03 ± 0.04 a
88	2.5	175	42.35 ± 0.35 c	51.01 ± 0.01 b
90	2.5	175	43.08 ± 0.21 ab	48.73 ± 0.26 e

Values are means ± standard deviation (n = 3); different letters (a, b, c, d, e, f, g, h, i) in columns represent significant differences (p ≤ 0.05).

). Furthermore, the use of blanching resulted in an increase in the fat content of the crisps. Among all variants of thermal treatment, the crisps blanched at 85 °C for 4 min and fried in oil at 175 °C were characterized by the highest fat content (43.56%) when the Ismena cultivar was used as the raw material. The lowest fat content (37.39%) was found for crisps blanched and fried at the same temperatures, when the blanching time was 1 min. In the case of the cv. Provita, the highest fat content was seen for crisps blanched at 85 °C for 2.5 min and fried at 155 °C (52.20%), and the lowest was found for those with no blanching and fried at 175 °C (38.57%). Extending the blanching time increased the fat content of potato crisps. The effect of the blanching temperature was ambiguous—in the crisps of the cv. Ismena, the fat content increased with the rise of the blanching temperature, while the opposite relationship was observed in the crisps of the cv. Provita. The frying temperature also influenced the fat content of the potato crisps. As the frying temperature increased, the crisps absorbed less fat.

The results of the instrumental measurement of the color of potato crisps from potatoes with different flesh color, obtained with the use of different thermal treatment parameters, are presented in

Table 3. Color parameters of potato crisps (cv. Ismena), depending on technological parameters.

Blanching Temperature (°C)	Blanching Time (min)	Frying Temperature (°C)	L*	a*	b*	C*	h°
-	-	155	58.81 ± 0.26 f	6.11 ± 0.12 c	26.30 ± 0.39 a	27.00 ± 0.39 a	76.92 ± 0.25 i
-	-	165	58.37 ± 0.12 gh	7.25 ± 0.03 b	24.92 ± 0.14 c	25.95 ± 0.14 b	73.78 ± 0.11 j
-	-	175	55.36 ± 0.08 i	8.83 ± 0.14 a	22.01 ± 0.13 h	23.71 ± 0.17 h	68.14 ± 0.26 k
80	2.5	175	58.16 ± 0.27 h	4.00 ± 0.04 e	22.63 ± 0.14 g	22.98 ± 0.13 i	79.98 ± 0.14 f
82	2.5	175	58.49 ± 0.37 g	4.63 ± 0.04 d	23.51 ± 0.31 f	23.97 ± 0.29 g	78.87 ± 0.24 h
85	2.5	155	62.76 ± 0.28 a	2.05 ± 0.13 k	25.44 ± 0.32 b	25.52 ± 0.33 d	85.39 ± 0.23 a
85	2.5	165	59.82 ± 0.14 e	2.88 ± 0.02 i	24.99 ± 0.04 c	25.16 ± 0.04 e	83.43 ± 0.05 c
85	1	175	58.41 ± 0.09 gh	3.98 ± 0.05 e	21.19 ± 0.11 i	21.56 ± 0.11 j	79.37 ± 0.15 g
85	2.5	175	61.43 ± 0.25 b	3.71 ± 0.09 f	25.63 ± 0.06 b	25.90 ± 0.06 bc	81.77 ± 0.20 e
85	4	175	60.94 ± 0.24 c	2.88 ± 0.07 i	24.29 ± 0.06 d	24.46 ± 0.06 f	83.23 ± 0.15 c
85	5	175	60.62 ± 0.26 d	2.41 ± 0.05 j	23.97 ± 0.22 e	24.09 ± 0.21 g	84.26 ± 0.17 b
88	2.5	175	61.51 ± 0.20 b	3.34 ± 0.05 h	25.47± 0.12 b	25.69 ± 0.12 cd	82.53 ± 0.14 d
90	2.5	175	59.95 ± 0.12 e	3.45 ± 0.01 g	24.24 ± 0.11 d	24.49 ± 0.11 f	81.89 ± 0.03 e

Values are means ± standard deviation (n = 6); different letters (a, b, c, d, e, f, g, h, i, j, k) in columns represent significant differences (p ≤ 0.05).

and

Table 4. Color parameters of potato crisps (cv. Provita), depending on technological parameters.

Blanching Temperature (°C)	Blanching Time (min)	Frying Temperature (°C)	L*	a*	b*	C*	h°
-	-	155	40.73 ± 0.28 a	1.20 ± 0.04 g	−0.42 ± 0.04 fg	1.28 ± 0.03 h	340.15 ± 1.06 e
-	-	165	39.42 ± 0.18 c	1.35 ± 0.03 f	−0.35 ± 0.06 e	1.39 ± 0.03 g	345.29 ± 0.90 c
-	-	175	39.23 ± 0.15 c	0.93 ± 0.07 i	−0.37 ± 0.02 ef	1.00 ± 0.06 i	339.26 ± 0.93 e
80	2.5	175	39.28 ± 0.30 c	1.99 ± 0.03 a	0.58 ± 0.09 a	2.08 ± 0.05 a	15.14 ± 0.86 i
82	2.5	175	40.21 ± 0.17 b	1.91 ± 0.04 b	0.11 ± 0.01 b	1.92 ± 0.04 b	3.37 ± 0.38 j
85	2.5	155	39.38 ± 0.15 c	1.48 ± 0.04 e	−0.46 ± 0.04 g	1.55 ± 0.03 e	342.03 ± 0.85 d
85	2.5	165	37.93 ± 0.09 f	1.65 ± 0.08 d	−0.16 ± 0.04 c	1.66 ± 0.06 d	355.16 ± 0.79 a
85	1	175	40.71 ± 0.13 a	1.81 ± 0.02 c	−0.29 ± 0.03 d	1.83 ± 0.01 c	350.85 ± 0.84 b
85	2.5	175	37.28 ± 0.19 g	1.61 ± 0.03 d	−0.60 ± 0.04 h	1.71 ± 0.04 d	339.21 ± 0.92 e
85	4	175	40.13 ± 0.07 b	1.39 ± 0.03 f	−0.55 ± 0.03 h	1.50 ± 0.03 ef	337.66 ± 0.85 f
85	5	175	38.53 ± 0.24 de	1.03 ± 0.06 h	−0.76 ± 0.05 i	1.28 ± 0.08 h	322.84 ± 0.42 g
88	2.5	175	38.77 ± 0.25 d	1.20 ± 0.05 g	−0.85 ± 0.05 j	1.47 ± 0.05 f	323.35 ± 0.88 g
90	2.5	175	38.48 ± 0.32 e	1.06 ± 0.03 h	−0.99 ± 0.04 k	1.45 ± 0.04 f	317.23 ± 0.54 h

Values are means ± standard deviation (n = 6); different letters (a, b, c, d, e, f, g, h, i, j, k) in columns represent significant differences (p ≤ 0.05).

. The color was determined using the parameters L*, a*, and b*, which define the brightness (L*), as well as the share of red and green (a*) and yellow and blue (b*). The color saturation (C*) and hue (h°) were also determined. Among the crisps of the cv. Ismena variety with the yellow flesh color, the darkest color was that of the unblanched crisps fried at 175 °C (L* = 55.36), and the lightest were crisps blanched at 85 °C for 2.5 min and fried at 155 °C (L* = 62.76). Purple flesh potato crisps were the darkest for the sample blanched at 85 °C for 2.5 min and fried at 175 °C (L* = 37.28), and the brightest for the unblanched sample, fried at 155 °C (L* = 40.73). Increasing the temperature and lengthening the blanching time lightened the color of the crisps of cv. Ismena and darkened the color of the potato crisps with the purple flesh color. On the other hand, the increase in the frying temperature affected the darkening of the crisps (both yellow and purple). The value of the a* parameter in the crisps of cv. Ismena ranged from 2.05 (blanching at 85 °C/2.5 min; frying at 155 °C) to 8.83 (variant without blanching; frying at 175 °C). The use of the blanching process reduced the share of red color (parameter a*) in potato crisps of yellow flesh. In potato crisps with purple flesh, the relationship was opposite—blanching the crisps increased the share of red color in the finished product. The highest red color share was recorded for blanching crisps at 80 °C for 2.5 min and frying at 175 °C (a* = 1.99), and the lowest for unblanched crisps fried at 175 °C (a* = 0.93). Extending the time and increasing the blanching temperature resulted in a decrease in the share of red in the finished product. On the other hand, increasing the frying temperature increased the proportion of red in the crisps from yellow flesh potatoes. Frying had no effect on the changes of this parameter in potato crisps of cv. Provita. The b* parameter values were positive for the crisps of cv. Ismena, which is related to the yellow color of the potato flesh. The highest yellow color share was found for unblanched crisps fried at 155 °C (b* = 26.30), and the lowest for crisps blanched at 85 °C for 1 min and fried at 175 °C (b* = 21.19). In crisps made out of cv. Provita, the blue color was the highest in the crisps blanched at 90 °C for 2.5 min and fried at 175 °C (b* = −0.99), while the yellow color was predominant in the crisps blanched at 80 °C for 2.5 min and fried at 175 °C (b* = 0.58). Extending the blanching time and increasing the blanching temperature resulted in an increase in the proportion of blue in cv. Provita crisps. The frying temperature did not change the b* parameter. The parameter C* (chroma) describes the degree of color saturation. The highest color saturation among the crisps of cv. Ismena was characteristic of unblanched crisps fried at 155 °C (C* = 27.00), and the lowest was found for crisps blanched at 85 °C for 1 min and fried at 175 °C (C* = 21.56). The saturation of the color of the crisps from the purple flesh cultivar was the highest for the sample blanched at 80 °C for 2.5 min and fried at 175 °C (C* = 2.08), and the lowest for the unblanched sample fried at 175 °C (C* = 1.00). Blanching changed the color saturation in potato crisps. The longer the blanching time and the higher the blanching temperature, the greater the saturation of the color of the crisps from the yellow flesh potatoes. The inverse relationship was noted for purple-colored crisps. On the other hand, increasing the frying temperature decreased the color saturation. The h° (hue) color tone defines the dominant wavelength in the color spectrum. This length determines the shade of a given color. Changes in this color parameter were observed in the crisps of both cultivars as a result of using different blanching and frying parameters. The values of the h° parameter increased with increasing temperature and extending the blanching time. On the other hand, increasing the frying temperature decreased the h° parameter.

3.2. Total Polyphenol Content and Antioxidant Capacity

Processing, especially thermal treatment, during the production of potato crisps, provokes significant alterations in their quality. In the current study, non-blanched crisps obtained from cv. Ismena had, on average, a 1.6-fold lower content of total polyphenols than products from cv. Provita (with purple flesh) (

Table 5. Total polyphenol content and antioxidant activity in potato crisps, depending on technological parameters.

Technological Parameters			Total Polyphenols (mg GA/100 g)		ABTS•+ (mmol Trolox/100 g)		FRAP (mmol Trolox/100 g)
Blanching Temperature (°C)	Blanching Time (min)	Frying Temperature (°C)	Ismena	Provita	Ismena	Provita	Ismena	Provita
-	-	155	53.90 ± 2.66 c	92.26 ± 9.68 b	0.48 ± 0.01 c	0.50 ± 0.02 f	0.34 ± 0.04 c	0.50 ± 0.04 b
-	-	165	58.88 ± 1.98 b	96.68 ± 5.92 b	0.56 ± 0.05 b	0.80 ± 0.03 b	0.40 ± 0.04 b	0.55 ± 0.03 a
-	-	175	76.47 ± 2.78 a	107.73 ± 13.17 a	0.72 ± 0.04 a	0.81 ± 0.02 a	0.48 ± 0.04 a	0.56 ± 0.06 a
80	2.5	175	23.42 ± 1.12 g	48.33 ± 1.76 ef	0.22 ± 0.00 f	0.39 ± 0.00 de	0.14 ± 0.01 gh	0.28 ± 0.02 fgh
82	2.5	175	22.89 ± 0.38 g	41.58 ± 1.55 fg	0.24 ± 0.01 f	0.36 ± 0.01 de	0.15 ± 0.01 fg	0.24 ± 0.01 h
85	2.5	155	24.46 ± 2.55 fg	45.72 ± 3.39 efg	0.23 ± 0.01 f	0.40 ± 0.02 de	0.17 ± 0.02 ef	0.29 ± 0.01 efg
85	2.5	165	29.21 ± 1.80 e	52.58 ± 1.66 de	0.28 ± 0.01 e	0.42 ± 0.01 d	0.19 ± 0.02 de	0.30 ± 0.02 ef
85	1	175	26.87 ± 1.78 ef	63.40 ± 3.95 c	0.26 ± 0.03 f	0.58 ± 0.02 b	0.16 ± 0.01 fg	0.37 ± 0.01 c
85	2.5	175	11.34 ± 0.62 i	60.55 ± 2.44 cd	0.14 ± 0.01 h	0.51 ± 0.03 b	0.10 ± 0.02 i	0.36 ± 0.03 cd
85	4	175	16.58 ± 1.01 h	46.28 ± 0.36 efg	0.18 ± 0.01 g	0.40 ± 0.01 d	0.13 ± 0.01 gh	0.29 ± 0.02 efg
85	5	175	13.13 ± 0.56 i	38.63 ± 3.00 g	0.15 ± 0.01 gh	0.35 ± 0.02 ef	0.12 ± 0.01 hi	0.25 ± 0.01 gh
88	2.5	175	27.23 ± 1.56 e	54.58 ± 7.26 de	0.26 ± 0.01 e	0.49 ± 0.03 b	0.19 ± 0.03 de	0.33 ± 0.03 de
90	2.5	175	32.58 ± 0.79 d	54.22 ± 5.9 de	0.30 ± 0.06 d	0.48 ± 0.02 c	0.21 ± 0.01 d	0.32 ± 0.03 ef

Values are means ± standard deviation (n = 2); different letters (a, b, c, d, e, f, g, h, i) in columns represent significant differences (p ≤ 0.05).

). This could be connected with a higher content of phenolic acids and anthocyanins than for boiled potato tubers; the content of polyphenols in purple flesh potatoes was 10 times higher when compared to those with creamy flesh [20]. A similar observation was also made by Nemś et al. [21] for snacks with the addition of dried purple potatoes. In case of non-blanched samples (controls), the temperature of frying had a significant impact of the content of total polyphenols: the higher temperature, the higher the content of TPC, regardless the cultivar used (Table 5), which might be connected with the different thermostability of dominant polyphenolic compounds [3]. Crisps obtained from cv. Provita exhibited rather higher total polyphenol content comparable to such purple fleshed cultivars as Valfi and Salad Blue, used in previous studies [3]. On the other hand, the non-specificity of this spectrophotometric method, especially interference of other substances present in the extracts examined, including newly formed sugar-based compounds formed during the Maillard reaction, might influence the levels of TPC [22].

In general, the application of blanching before frying led to a 64% and 49% decrease in the content of polyphenols in potato crisps from cv. Ismena and cv. Provita products, respectively. This was in line with the study of Nemś et al. [5], where a significant decrease in the content of total polyphenols and anthocyanins was demonstrated in potato crisps with purple flesh that were blanched in water and in organic acid solutions. Nowacka et al. [23] observed similar relationships in apple slices. It was explained that during blanching, to a large extent, polyphenols, including anthocyanins, were washed into the water, and prolonged blanching time intensified this process. The difference between the degradation of polyphenols in blanched crisps from the analyzed potato cultivars showed that the compounds present in cv. Provita were more stable than in cv. Ismena. When the products were. compared after blanching and frying, it was noted that cv. Provita had 2.2-fold higher content of total polyphenols than cv. Ismena. This was connected with an initial composition of raw materials, especially biologically active components that react with the Folin reagent.

The higher blanching temperatures, above 85 °C, favored the preservation of polyphenolic compounds in cv. Ismena, used for production of potato crisps. No such relation was observed in case of cv. Provita. When considering the influence of blanching time at 85 °C, the highest retention of total polyphenols was indicated for the shortest time (Table 5), regardless the cultivar used. As the time of blanching was extended, the linear degradation of total polyphenols was noted in case of cv. Provita but not observed in products from cv. Ismena. An increase in frying temperature increased the content of total polyphenols only for crisps from cv. Provita, pointing a strong impact of the frying temperature on selected polyphenol groups, depending on their thermolability.

The antioxidant capacity in the crisps varied depending on the blanching and frying parameters used. Similar to TPC, non-blanched samples from cv. Provita had 1.2 and 1.3 times higher values of antioxidant capacity measured by TEAC ABTS and FRAP assays, respectively, compared to samples from cv. Ismena (Table 5). Nemś et al. [24] observed a similar relationship in potato tubers with different flesh color, with purple flesh varieties characterized by a 4–5 times higher ability to scavenge ABTS^•+ radical cations compared to potatoes with yellow flesh. When considering the effect of frying temperature, it can be seen that as the temperature increased, the antioxidant capacity values for unblanched potato crisps of both cultivars increased, which was correlated with TPC values (r = 0.9506). This may be due to the possible formation of compounds, other than those natively present in the processed raw material, that exhibit antioxidant properties. Blanching caused on average a 2.6- and 1.7-fold decrease in the antioxidant capacity of the analyzed products for crisps form cv. Ismena and Provita, respectively, regardless of the method applied. The same relationship was previously reported by Nemś et al. [5]. Comparing products blanched at 85 °C for different durations, it was observed that crisps obtained from cv. Provita were characterized by decreasing antioxidative capacity with increasing blanching time, which was not observed for crisps obtained from cv. Ismena. When considering blanching temperature, the lowest antioxidant capacity values, regardless of the method used, were recorded at 85 °C, while the highest values were recorded for products blanched at 88 and 90 °C, for crisps obtained from both potato cultivars. When analyzing the effect of frying temperature for crisps blanched using the same temperature and time parameters (85 °C; 2.5 min), a similar trend as for TPC was observed. Of the frying temperatures used (155, 165, and 175 °C), the antioxidant capacity values, regardless of the method used, were highest for a frying temperature of 165 °C, while the lowest for 175 °C. One of the explanations may be that, at higher temperatures, the intensity of the Maillard reactions increases, during which new compounds with antioxidant and pro-oxidative properties are formed. On the other hand, Serpen and Gökmen [25] observed an inverse relationship. The researchers, analyzing potato crisps from the raw material with different content of reducing sugars, observed that the antioxidant activity in the crisps fried at 160, 170 and 180 °C gradually decreased with increasing process temperature.

3.3. FIC and BI

Thermal treatment during the production of potato crisps is of particular importance due to the Maillard reactions (non-enzymatic browning). This sequence of complex reactions leads to the formation of compounds responsible for the specific taste, smell, and color, being key factors in terms of the consumers’ acceptance of crisps. During the advanced stage of the Maillard reaction, among others, fluorescent intermediary compounds (FICs) are formed. The intensity of their fluorescence is a useful indicator of changes in food products caused by thermal processes [26].

In non-blanched samples, the FIC levels were influenced by the frying temperature. In the case of both cultivars, the higher the temperature, the higher the FIC level (

Table 6. FIC and BI in potato crisps, depending on technological parameters.

Technological Parameters			FIC		BI (λ = 360)		BI (λ = 420)
Blanching Temperature (°C)	Blanching Time (min)	Frying Temperature (°C)	Ismena	Provita	Ismena	Provita	Ismena	Provita
-	-	155	33.06 ± 1.97 d	11.14 ±1.33 bc	0.52 ± 0.04 c	1.26 ± 0.07 b	0.17 ± 0.00 b	0.22 ± 0.04 b
-	-	165	45.23 ± 3.45 b	11.68 ± 0.49 b	0.65 ± 0.02 b	1.22 ± 0.02 b	0.18 ± 0.03 b	0.20 ± 0.01 c
-	-	175	58.95 ± 4.84 a	18.82 ± 0.95 a	0.85 ± 0.04 a	1.39 ± 0.07 a	0.25 ± 0.02 a	0.26 ± 0.03 a
80	2.5	175	30.63 ± 0.53 d	7.95 ± 0.43 gh	0.33 ± 0.02 d	0.79 ± 0.02 d	0.10 ± 0.00 d	0.16 ± 0.04 de
82	2.5	175	39.32 ± 1.28 c	8.32 ± 0.75 fg	0.30 ± 0.01 de	0.74 ± 0.05 de	0.09 ± 0.01 de	0.16 ± 0.06 de
85	2.5	155	18.14 ± 0.82 f	5.79 ± 0.19 j	0.24 ± 0.00 fg	0.75 ± 0.06 de	0.08 ± 0.02 fg	0.16 ± 0.04 de
85	2.5	165	22.17 ± 1.66 e	6.30 ± 0.29 ij	0.26 ± 0.01 fg	0.74 ± 0.03 de	0.08 ± 0.01 ef	0.15 ± 0.02 ef
85	1	175	40.48 ± 0.81 c	9.49 ± 0.72 de	0.32 ± 0.04 d	0.94 ± 0.03 c	0.10 ± 0.04 d	0.17 ± 0.02 d
85	2.5	175	32.81 ± 1.06 d	10.29 ± 0.49 cd	0.26 ± 0.01 fg	0.81 ± 0.03 d	0.06 ±0.01 hi	0.14 ± 0.05 f
85	4	175	24.20 ± 1.89 e	7.06 ± 0.38 hi	0.23 ± 0.02 gh	0.71 ± 0.03 ef	0.07 ± 0.03 hi	0.15 ± 0.01 ef
85	5	175	22.31 ± 2.62 e	7.25 ± 0.39 hi	0.20 ± 0.01 h	0.66 ± 0.03 f	0.06 ± 0.02 i	0.15 ± 0.02 ef
88	2.5	175	30.09 ± 1.84 d	8.36 ± 0.45 fg	0.26 ± 0.03 fg	0.78 ± 0.02 d	0.07 ± 0.03 hi	0.15 ± 0.03 ef
90	2.5	175	30.77 ± 1.62 d	8.97 ± 0.76 ef	0.28 ± 0.01 ef	0.76 ± 0.05 de	0.07 ± 0.01 gh	0.14 ± 0.02 f

Values are means ± standard deviation (n = 2); different letters (a, b, c, d, e, f, g, h, i, j) in columns represent significant differences (p ≤ 0.05).

). Crisps made out of potatoes with purple flesh were characterized by a 3.3 times lower FIC level when compared to products from cv. Ismena with yellow flesh. This can be linked to the properties of polyphenolic compounds, including anthocyanins, that might prevent the formation of Maillard reaction products during thermal treatment [27]. In other words, the advanced Maillard reactions in the purple-colored crisps were less intense than in the yellow-colored crisps.

Blanching applied before frying significantly diminished the FIC levels in crisps produced from both cv. Ismena and Provita. On average, the FIC levels in crisps, regardless of the time and temperature of blanching and frying, were 36% and 43% lower in crisps made from cv. Ismena and Provita, respectively, when compared to the level in the control samples (non-blanched). The blanching, as well as its duration, reduced the level of fluorescent intermediary compounds in the crisps. This could be connected with the effect of partial removal of the reducing sugars from the surface of the potato slices during blanching, which might reduce the amount of substrates involved in the Maillard reactions. As a result, this process might halt the formation of fluorescence intermediary compounds in potato crisps. When the crisps from both cultivars were compared, the FIC levels were lower in products gained from cv. Provita. This strongly showed that the higher the polyphenol content, the less likely the formation of Maillard reaction products during thermal treatment. Similar to the control samples, crisps fried at higher temperatures were characterized by a higher level of FIC, which was also observed by Carciochi et al. [28].

One of the most common indicators of non-enzymatic browning reaction that occurs in sugar-rich products is the browning index. In processed foods, browning is linked to the formation of melanoidins—compounds that are formed during the final stage of the Maillard reaction [29]. In the current study, the values of browning index were influenced by the frying temperature of non-blanched samples. The browning index was higher for products from cv. Provita with purple flesh when compared to samples from cv. Ismena, regardless of the wavelength used for their determination (Table 6). This might be connected with the presence of components that are present in a raw, non-processed purple-fleshed potato and at the maximum absorption wavelength of 420 nm, e.g., selected polyphenols and their complexes [30]. The application of blanching before frying led to a decrease in the BI values (420 nm) of, on average, 61% and 32% for cv. Ismena and cv. Provita, respectively. Blanching of potato slices in water might result in a partial dissolution of anthocyanins in water. Such a pre-treatment method probably mitigated the course of color reactions during thermal treatment. It was observed that the browning index also decreased with the increase of the temperature and the duration of blanching. A similar relationship was observed in the studies of Abano and Amoah [31]. It can be concluded that blanching applied before frying of potato crisps significantly modified the initial chemical composition of potato slices, which influenced the formation of browning pigments in the final products. Taking the above into consideration, it can be stated that blanching had an impact of the final quality of potato crisps, regardless of the cultivar used for their preparation.

3.4. KYNA

The content of kynurenic acid was measured in raw potatoes, potato slices before and after blanching, the water remaining after blanching, and potato crisps. Such analysis has not been conducted so far. In their studies, Turski et al. [6,9] determined KYNA content in various food products, including potatoes. However, the research was performed on potatoes with yellow and white flesh. In the current research, it was shown that the content of kynurenic acid in Provita, a purple flesh potato cultivar, was higher (almost three times) than in cv. Ismena tubers, similar to the content of other bioactive compounds (

Table 7. Kynurenic acid content changes with potato crisp processing.

Sample	Technological Parameters			Potato Cultivar
	Blanching Temperature (°C)	Blanching Time (min)	Frying Temperature (°C)	Ismena	Provita
Raw potato (μg/g)	-	-	-	0.226 ± 0.176	0.683 ± 0.307
Potato slices (μg/g)	-	-	-	0.124 ± 0.011	0.658 ± 0.062
Potato slices after blanching (μg/g)	80	2.5	-	0.052 ± 0.005	0.343 ± 0.045
	85	2.5	-	0.042 ± 0.006	0.257 ± 0.019
	90	2.5	-	0.033 ± 0.003	0.305 ± 0.046
Water after blanching (μg/mL)	80	2.5	-	0.002 ± 0.000	0.012 ± 0.002
	85	2.5	-	0.002 ± 0.000	0.013 ± 0.003
	90	2.5	-	0.002 ± 0.000	0.012 ± 0.001
Potato crisps (μg/g)	80	2.5	175	0.077 ± 0.006 a	0.431 ± 0.024 a
	85			0.082 ± 0.011 a	0.360 ± 0.016 b
	90			0.077 ± 0.018 a	0.409 ± 0.020 a

Values are means ± standard deviation (n = 3); different letters (a, b) in columns represent significant differences (p ≤ 0.05).

). The content of kynurenic acid in yellow flesh potato tubers differed depending on the potato variety [9]. The KYNA content in potatoes from the cv. Ismena variety (0.226 μg/g) determined in the experiment was average in relation to the potato cultivars previously reported by Turski et al. [9]. In the current study, it was found that the slicing process resulted in a reduction of KYNA content in Ismena but not Provita potato slices. Thermal treatment, i.e., blanching, caused additional losses in the content of this compound in potato slices of both cultivars—almost by half. Moreover, it was demonstrated that the loss of kynurenic acid during the blanching of potato slices is temperature dependent. The effect of blanching on the content of kynurenic acid has not been directly demonstrated by researchers so far. It was shown that the cooking of whole tubers in boiling water did not change the content of this acid in potatoes [6]. It was shown that kynurenic acid was also present in the water after blanching, which confirms the good solubility of this compound in the aqueous medium. However, the content of KYNA in water after blanching, regardless of the temperature used, was at a similar level. The frying process also affected the content of kynurenic acid in potato crisps. This is confirmed by the research of Turski et al. [9], who analyzed the content of kynurenic acid in raw material and finished products. It was found that potato crisps had a lower content of kynurenic acid compared to raw potatoes. It can be assumed that the use of a high-temperature process, i.e., frying, affected the content of this acid in the finished product.

Recently, kynurenic acid has drawn attention due to its involvement in fat metabolism. Agudelo et al. [10] demonstrated that kynurenic acid improves the metabolic consequences of a high-fat diet (HFD). It was proven that kynurenic acid had a role in the significant reduction of body weight and decrease of subcutaneous/inguinal white and visceral/epididymal white adipose tissues of mice that were treated with HFD containing 34.9 g% fat, which corresponds to the content of fat in crisps. Moreover, lower serum triglyceride level and improved glucose homeostasis were found [10]. All these effects were attributed to the activation of G protein-coupled receptor 35 (GPR35) by kynurenic acid [10,32]. Similar results were reported by Li et al. [12]. Intriguing properties of kynurenic acid were reported by Lima et al. [33], who found that kynurenic acid treatment results in a better functional recovery of injured ischemic-reperfusion tissue, probably due to tissue repairing and control of neurotransmitter release [33]. Similarly, Lee et al. [34] reported that kynurenic acid attenuates LPS-induced atherosclerosis by suppression of inflammation. Accumulated evidence points to the beneficial effect of kynurenic acid in various health conditions linked to excessive ingestion of fat. Therefore, the rationale for preserving high kynurenic acid content in fatty food appears to be justified. Taking the above into consideration, the selection of potato cultivars with a relatively high content of this acid should be considered along with process parameters that protect its content in ready-to-eat snacks.

4. Conclusions

Crisps produced from cv. Provita with purple flesh were characterized by a significantly higher content of biologically active compounds, polyphenols, and kynurenic acid. These products had higher antioxidant capacity values compared to the yellow crisps. The blanching time and temperature, as well as the frying temperature, influenced the selected properties of the crisps. Crisps produced after blanching purple flesh potatoes resulted in a reduction in the intensity of the brown color in the product, but the blanched crisps absorbed much larger amounts of fat (even up to 52%). Blanching also reduced the content of polyphenols, kynurenic acid, and the antioxidant capacity of crisps from both cultivars analyzed. Blanching at a higher temperature for a shorter time for cv. Provita reduced losses of total polyphenols in the obtained crisps. Blanching applied before frying significantly diminished the FIC levels in the crisps, while in non-blanched samples, the FIC levels were influenced by the frying temperature. The use of different blanching temperatures did not have a significant effect on the content of KYNA in yellow-colored crisps, while blanching at 80 °C for purple slices allowed the largest amount of this acid to be retained in the finished product. Therefore, the appropriate selection of thermal treatment parameters can be a key point in the technological process of producing purple crisps to obtain high-quality products, and at the same time, minimize the loss of biologically valuable compounds.