Acrylamide content in dried coloured-flesh potato products: Effects of drying

ABSTRACT

The aim of this study was to determine the impact of the chemical composition of coloured-flesh potato tubers and different drying temperatures on acrylamide content in semi-finished and finished products. The dehydrated dice obtained after pre-drying contained around 67–190 μg/kg dry matter (dm) of acrylamide. The dice pre-dried at 120°C contained six times less acrylamide than those pre-dried at a higher temperature (160°C). Further drying of potato dice at 50°C increased the acrylamide content in the finished products by approximately 40% on average. The finished products contained 236–1539 μg/kg dm of acrylamide.

KEYWORDS:

• Acrylamide
• Blue-flesh potatoes
• Drying temperature
• Free amino acids
• Potato dice
• Reducing sugars

Introduction

The discovery made by Swedish researchers^[1] regarding the presence of acrylamide in food subjected to heat treatments has led to a discussion regarding the impact of this compound on food safety. As a result, interest in acrylamide from both researchers and food producers has increased significantly in recent years. In 1994, acrylamide was classified by the International Agency for Research on Cancer^[2] as a probable carcinogenic compound to humans and animals.^[2–4] Acrylamide enters the human body mainly through the gastrointestinal tract, but other routes of its penetration include also the respiratory tract and skin.^[5] Furthermore, it is metabolised with the food and then partially excreted from the body with urine. Acrylamide is synthesised in high-carbohydrate foods that are exposed to temperatures above 120°C, as is done in most technological processes, such as baking, frying, roasting, drying, and grilling.^[6–10] In particular, high contents of this compound may be found in fried potato products (i.e., French fries and chips). Only a few reports may, however, be found concerning acrylamide content in dehydrated potatoes.^[11] Dehydrated potatoes are often used as components of semi-finished products to obtain frozen or delicatessen products, such as noodles, dumplings, pancakes, and pancake rolls, which require thermal processing before being consumed (i.e., roasting or frying). These processes lead to temperatures in excess of 120°C, which promotes further increases in acrylamide contents.

The mechanism of acrylamide formation in carbohydrate foods subjected to high temperatures has been thoroughly described in the scientific literature.^[12] Furthermore, acrylamide is formed as a result of the Maillard reaction, where reducing sugars (glucose and fructose) and amino acids (asparagine) contribute to its formation.^[13–16] As a result of the high temperatures experienced during heat treatments (roasting, frying, etc.), the amino group of a free asparagine reacts with the carbonyl group of a sugar to make a Schiff base, which is decarboxylated under the influence of temperature. Later, the base can be hydrolysed to yield a 3-aminopropionamid, which is then converted by elimination of the ammonia molecule to acrylamide or a direct decomposition of the decarboxylation product of the Schiff base used to separate the imine and acrylamide may occur.^[14,17] Additionally, the amount of acrylamide formed in potato products depends mainly on the content of precursors in the tubers. In particular, the reducing sugars play a larger role than asparagine. Conversely, in grain products, asparagine is the more important precursor in toxic compound formation, as opposed to the reducing sugars.^[12,17–19]

In recent years, an increasing number of raw materials and food products with high organoleptic and nutritional values have been brought on the market, such as potatoes with purple and red flesh. They are characterised, above all, by an attractive colour and a higher nutritional value than traditional, light-fleshed tubers. Noteworthy is also the higher content of polyphenolic compounds in the coloured tubers, which has beneficial effects on the human body.^[20] When marketing new raw products and products derived from coloured-flesh potatoes, however, attention should also be paid to the amount of anti-nutrients or toxic compounds they contain. In view of the above, the aim of this study was to determine the impact of the chemical composition of coloured-flesh potato tubers and different drying temperatures on acrylamide formation in semi-finished and ready-to-eat products.

Materials and methods

Raw material

The material used for this study included four varieties of potatoes with blue flesh, namely, Vitelotte, Blaue Annelise, Salad Blue, and Blaue St. Galler, from experimental plots belonging to the testing station of The Central Institute for Supervising and Testing in Agriculture at Přerov nad Labem (Czech Republic). The research was conducted during the growing seasons in 2013 and 2014. Potato tuber samples were harvested after reaching full maturity. The potatoes were collected four times over two years of research (in 2013 and 2014). From all of the potato tuber varieties, dehydrated dice were prepared using laboratory methods as described below.

Dehydrated dice production in laboratory conditions

The potatoes were washed and then peeled using a laboratory carborundum peeler. They were then diced into 10 × 10 × 10 mm cubes by laboratory manual cutting equipment and were rinsed with distilled water at 20°C. Then, the diced potatoes were blanched in water at 75°C for 5 min and dried in a laboratory oven at 120°C, or 140°C or 160°C for 1 h. Afterwards, the drying temperature was decreased to 55–60°C to obtain a final moisture content in the range of 8–11% (approximately 8 h).

Potato sample preparation for analysis

After being pre-dried at different temperatures, the samples of raw materials and semi- finished products were frozen and freeze-dried using a lyophiliser (apparatus of Edwards firm). Then, all samples freeze-dried (raw materials, semi-finished products) and finished products were ground in an electric grinder (apparatus of Retsch firm) and used to determine the concentrations of acrylamide.

Acrylamide analysis

Reagents

All reagents were of analytical grade unless otherwise stated. Acrylamide (2-propene amide) and d₃-acrylamide (standard solution) were purchased from Fluka, Switzerland. HPLC-grade methanol was purchased from Merck (Darmstadt, Germany), deionised Milli-Q water was obtained from a Millipore purification system (Millipore, Bedford, MA, USA), and HPLC-grade acetonitrile was purchased from Lab-Scan (Ireland). Two Supelco SPE columns (Bellfonte PA, USA) were used to purify the samples. The upper column (MCAX) of 300 mg and 3 mL in volume was placed directly on the lower column (DSC C-18 of 1 g and 6 mL in volume). The two columns were connected to each other by an adapter.

Standard solutions and calibrated standard solutions

Preparation and calibration of a standard solution were performed by modifying the method described by.^[6,21,22] The stock solutions of the acrylamide and d₃-acrylamide-standard solutions and the calibrated standard were prepared in acetonitrile at a concentration of 500 mg/L (500 ppm) (Fluka Co.). The standard solution was protected from light and stored in a refrigerator. Stock solutions of the standards with a 500 mg/L (500 ppm) concentration were prepared in acetonitrile and refrigerated. A working standard solution, for spiking samples and for generating a standard curve, was obtained by dilution in acetonitrile. The concentrations for the standard curves were 0, 1, 5, 10, 30, 50, 100, 500, 1000, 1500, and 2000 ng/mL, all with acrylamide-d₃ at 50 ng/mL for the potato products.

Apparatus

The acrylamide content of the dehydrated potatoes was determined using the LC-MS-MS system (1200 L, Varian, Walnut Creek, CA., USA), and the experiment was performed using a triple quadruple with interfacial electrospray (ESI) on an HPLC system (ProStar Model, Varian, Walnut Creek, USA). A 1200 L HPLC-MS-MS system was equipped with a ProStar 210 Pump and a ProStar 430 Autosampler (Nitrogen generator, Domnick Hunter, model G 4510 E, UK). The samples were separated in a Pursuit XRs 3 u C-18 column (150 × 2.0 mm) with a Metaguard Pursuit XRs 3 u C18 column (2.0 mm, Varian, Walnut Creek, CA, USA) using 0.5% methanol in aqueous 0.1% acetic acid as the mobile phase at a flow rate of 0.2 mL/min. The volume of each injected sample was 20 μL.

LC-MS-MS conditions for acrylamide determination

The LC-MS-MS system was operated in a positive electrospray mode, with a needle voltage of 5000 V, nebulising gas (compressed air or N₂) at 54 psi, a capillary voltage of 25 V, and drying gas (N₂, 99.5%, 400°C, 22 psi). The collision cell gas (Ar 99.999%) pressure was 1.86 mTorr, and the detector voltage was set to 1300 V. Acrylamide content was determined by multiple reaction monitoring (MRM). The MRM mode was performed by monitoring the 72 → 54.9 m/z transition for acrylamide (collision energy 10 V) and 75 → 58 m/z transition for acrylamide-d₃ (collision energy 10 V). In all of the MRM transitions, the dwell and inter-scan delay times were 1 s with a SIM width of 0.7 atm. The limit values of the detection method were calculated signal-to-noise = 3:1 for the summed signals of both fragment ions.

Sample preparation for the LC-MS-MS analysis

The samples were prepared by modifying the methods described by.^[6,21–23] Two-gram (0.0001 g) samples of the dehydrated potatoes were weighed out and ground using a GM 200 cutting mill (Retsch, GmbH, Germany) at a speed of 2000 rev·min⁻¹ for 16 s (2 × 16 s). The samples were then placed in 50-mL Falcon centrifuge tubes, and 40 mL of deionised water was added. Simultaneously, the samples containing 200 µg/L of the internal standard (akrylamid-d₃) at a concentration of 10 µg/mL were prepared. The samples were shaken using a Multi Reax shaker (Heidolph Instruments GmbH & Co.KG. Germany) for 10 min. Next, they were centrifuged in a 3 KI 5 centrifuge (Sigma) at a speed of 9500 rev·min⁻¹ and 4°C for 15 min.

Solid phase extraction (SPE) was performed by modifying the method described by Aurand and Trinh.^[21] One millilitre of a transparent (clarified) solution (IA) was placed in SPE columns that had previously been conditioned with 1 mL of methanol and 1 mL of deionised water, and vacuum-dried (Vac Elut 20 Manifold, Varian, Walnut Creek, CA, USA). The samples in the columns were washed with 1 mL of deionised water. The upper SPE columns (MCAX) and the filtrate were removed. Acrylamide from the lower SPE columns (C-18) was eluted with 2 mL of methanol, and the filtrate obtained was nitrogen-dried at 30°C. The dry residue was dissolved in 500 µL (0.5 mL) of deionised water and placed in an autosampler using liquid chromatography-tandem mass spectrometry (LC-MS-MS). The final acrylamide content of the finished product was expressed in [μg/kg] = [ppb].

Free amino acid analysis

The content of free amino acids was determined by high-performance liquid chromatography using an AAA-400 amino acid analyser (INGOS, Prague, Czech Republic). The analytical procedure was applied in accordance with the recommendations of the producer. A 220 mm long column was filled with Ostion LG FA ionex (Czech Republic). The temperatures of the column and reactor were 55–74°C and 120°C, respectively. For the detector, a two-wavelength photometer (440 and 570 nm) was employed. The prepared samples were analysed using the ninhydrin method. Finally, the amino acid composition was expressed as grams of amino acid per 16 g of N.^[24] The calculations were carried out relative to the external standard.

Analytical methods in raw materials

The dry matter (dm) of the fresh potato samples and freeze-dried material was determined by the reduced weight after drying at 105°C until a constant weight was achieved.^[25] The contents of total and reducing sugar and of starch were determined in the raw potato tubers with the chemical method.^[26] The colour of raw potatoes and ready products was examined by Minolta Konica CR- 200 spectrophotometer tristimulus colorimetry (Hunter Lab).

Statistical analysis

Study results were subjected to statistical calculations using Stat Soft Statistica v. 10.1. We applied a multiway analysis of variance and Duncan’s test (P ≤ 0.05) to determine the significance of the differences between mean values. All experiments were performed in three technological replications from two years of investigation, and the presented results show the mean values of the combined data.

Results and discussion

Effect of chemical composition of raw potatoes on acrylamide content

Particular attention should be paid to the quality of tubers introduced into food processing of the new potato varieties. The potatoes intended for the production of fried products, such as French fries and chips, or dehydrated products, such as diced potatoes, potato flakes, and potato granules, need to meet strict requirements that affect their ‘technological value’. This term is related to the external features of the tubers (i.e., size and shape, depth and number of points, defects and thickness of the skin, etc.) as well as to internal features (i.e., chemical composition and flesh properties). These features are then used to decide upon their usefulness and application. Potatoes intended for the production of dehydrated dice should contain 21–25% of dm, 15–19% of starch, and less than 0.5% of reducing sugars.^[27] The content of dm in the analysed purple-fleshed potatoes ranged from 20.1% (Blaue St. Galler var.) to 24.9% (Vitelotte var.) and that of starch from 17.0% (Blaue St Galler var.) to 22.5% (Vitelotte var.) (Table 1). Both the total and reducing sugar contents in the analysed potato varieties did not exceed the recommended values: below 1% for total sugars and 0.5% for reducing sugars (Table 1). The chemical composition of potatoes affects the efficiency and quality of dried or fried products. If dm and starch contents are too low, the texture of dried potatoes is poor (they are characterised by a rough, overly firm texture), whereas contents that are too high contribute to the disintegration of the potato dice during blanching or drying treatments. The reducing sugars (glucose and fructose) are equally important as dm and starch. The content of reducing sugars in potatoes varies widely. The content of glucose ranges from 0.05 to 1.5%, whereas that of fructose may vary from 0.15 to 1.5%.^[27] Their content in potato tubers is strongly correlated with the potato variety but also depends on cultivation or storage factors.^[28] Low reducing sugar contents in potatoes lead mainly to incorrect coloration of the dried and fried products, as well as to acrylamide formation.^[29–32] The reducing sugars and asparagine form, under the influence of high temperatures and low water activity, the acrylamide.^[4] The content of asparagine in potatoes is relatively high and reaches on average 93.9 mg/100 g f.m.^[32] Additionally, it constitutes the largest share of all of the free amino acids in potatoes.^[33,34]

Table 1. Chemical compounds and the colour of the flesh of raw potatoes.

Chemical compounds	Potato variety
	Vitelotte	Blaue Annelise	Salad Blue	Blaue Sant Galler
Dry mass (g/100g f.m.)	24.9^c ± 0.14	22.8^b ± 0.15	22.9^b ± 0.13	20.1^a ± 0.16
Starch (g/100g f.m.)	22.5^d ± 0.13	19.0^b ± 0.12	20.0^c ± 0.14	17.0^a ± 0.11
Total sugar (g/100g f.m.)	0.3^a ± 0.01	0.7^c ± 0.03	0.4^b ± 0.02	0.8^d ± 0.02
Reducing sugar (g/100g f.m.)	0.1^a ± 0.01	0.5^c ± 0.02	0.2^b ± 0.01	0.5^c ± 0.01
Color (Lab Hunter)
L	40.2^d ± 0.20	28.5^b ± 0.11	36.5^c ± 0.10	24.4^a ± 0.16
a	8.63^a ± 0.11	13.4^c ± 0.12	13.7^c ± 0.15	10.7^b ± 0.11
b	−0.29^a ± 0.20	−6.09^c ± 0.11	−5.41^b ± 0.21	−6.69^c ± 0.12

^a–dHomogenous group, indicate significant differences between potato variety (Duncan test, p ≤ 0.05).

± (SD), standard deviation; n = 6

The purple-fleshed potato varieties analysed in this study contained asparagine in the range from 187 mg/100 g f.m. (Vitelotte var.) to 340 mg/100 g f.m. (Blaue Sant Galler var.) (Table 2).The chemical composition of the potatoes has a significant effect on the quantity of acrylamide formed during thermal processes.^[11,18,31] According to the results presented by Shojaee-Aliabadi et al.,^[34] potatoes that are characterised by a higher content of asparagine and, at the same time, by a lower content of reducing sugars can yield products with lower acrylamide quantities. The high contents of reducing sugars and free asparagine in raw material affect, most of all, the colour of potato products.

Table 2. Free asparagine, total free amino acids, and relative amount of free asparagine in potatoes.

Potatoes varieties	Free asparagine (mg/100g)	Total free amino acids (mg/100g)	Asparagine/total (%)
Vitelotte	187^a ± 20	930^a ± 31	20.1
Blaue Annelise	294^b ± 17	1581^c ± 52	18.6
Salad Blue	318^c ± 15	1514^b ± 61	21.0
Blaue Sant Galler	340^d ± 19	1708^d ± 60	19.9

^a–dHomogenous group, indicate significant differences between potato variety

(Duncan test, p ≤ 0.05).

± (SD), standard deviation; n=6.

Some authors^[35–37] emphasised also that the amount of acrylamide formed in finished potato products is affected by higher contents of polyphenolic compounds in the raw material. Potatoes of coloured-flesh varieties are characterised by ca. 2–3 times higher content of total polyphenols compared with potatoes having the traditional yellow-coloured flesh.^[38] Table 1 summarises the results of colour measurements in raw potatoes. The analysed varieties of potatoes differed in the colour of their flesh. The darkest colour was found in potatoes of Blaue St Galler var. (L = 24.4) and Blaue Annelise var. (L = 28.5). Potatoes of these varieties were also characterised by a higher contribution of blue colour – from 6.09 to 6.69, which may indicate higher contents of polyphenolic compounds, anthocyanins in particular. According to Kalita et al.,^[36] a higher content of total polyphenols in potatoes of coloured-flesh varieties may have contributed to the lower contents of acrylamide in the finished products, which results from the strong antioxidative properties of these compounds. A similar correlation was demonstrated by Bassama et al.^[35] and Kotsiou et al.;^[37] however, as reported by these authors, the acrylamide content in the finished products depended not only on the content of polyphenolic compounds but also on the structure of these compounds. In this study, the potatoes with the darkest colour of the flesh – Blaue St. Galler and Blaue Annelise var – differed significantly between each other in acrylamide content (Table 1, Figs. 1 and 2). A higher content of this toxic compound was determined in dehydrated potatoes of Blaue St. Galler var.

Figure 1. Acrylamide content in semi-finished products pre-dried at different temperatures.

LSD – least significant differenceError bars represent the standard deviation (±SD), values are given as mean n = 6 (Duncan test, p ≤ 0.05).

Figure 2. Acrylamide content in finished products dried in different temperature.

LSD – least significant differenceError bars represent the standard deviation (±SD), values are given as mean n = 6 (Duncan test, p ≤ 0.05).

Effect of pre-drying and drying temperature on the colour of dried products

Maillard’s reaction proceeding during roasting or drying potatoes may cause colour deterioration by imparting the product a brown, burnt colour as well as taste deterioration once appropriate technological parameters of raw material pre-treatment are not provided. Table 3 presents the results of colour measurements of semi-finished products and finished products. The darkest colour was found for dried potatoes of Blaue St. Galler var. that had the highest contents of Maillard reaction precursors (reducing sugars and free asparagine). The colour of the dried potatoes was deteriorating along with increasing temperatures of the drying process. The value of the L colour component determined for the finished product dried at 120°C was at L = 65.8, whereas for the product dried at 160°C it was at L = 59.8 (Table 3). A correlation was demonstrated between the value of the L colour component and the content of acrylamide in dried products as affected by potato variety and the temperature of the drying process (Table 3 and Fig. 4). However, the strength of the correlation was significantly influenced by the analysed variety of potatoes. The highest correlation coefficient was determined in dried potatoes of Vitelotte and Blue St. Galler varieties. Potatoes of Vitelotte var. were characterised by the lowest content of Maillard reaction precursors among all the analysed varieties, whereas potatoes of Blaue St. Galler var. – by the highest content of these compounds (Tables 1, 2). It may only be concluded that the formation of acrylamide in dried products is probably affected by potato variety, as well as the composition and colour of the finished products.

Table 3. L value of semi-finished and finished products taken from laboratory dice processing.

L value	Potatoes variety				Means of potatoes varieties
	Vitelotte	Blaue Annelise	Salad Blue	Blaue Sant Galler
Potato dice after pre-drying in 120°C/2 h	74.8^e ± 0.10	68.9^e ± 0.10	71.6^e ± 2.10	61.3^d ± 1.82	69.1 ± 2.12
Potato dice after pre-drying 140°C/2 h	71.4^c ± 0.81	60.3^b ± 1.04	66.8^c ± 0.39	60.9^c ± 0.14	64.8 ± 0.99
Potato dice after pre-drying 160°C/2 h	66.6^b ± 0.39	61.5^c ± 1.10	67.4^d ± 1.10	58.9^b ± 1.00	63.6 ± 0.55
Potato dice after pre-drying in 120°C/2 h and dehydrated 50°C/8 h	72.3^d ± 1.14	62.7^d ± 3.10	67.2^d ± 1.13	61.2^d ± 2.10	65.8 ± 1.99
Potato dice pre-drying in 140°C/2 h and dehydrated 50°C/8 h	70.8^c ± 0.90	57.9^a ± 0.82	65.4^b ± 0.91	59.9^b ± 0.88	63.5 ± 0.90
Potato dice pre-drying in 160°C/2 h and dehydrated 50°C/8 h	62.9^a ± 1.09	58.6^b ± 2.08	60.0^a ± 2.10	57.9^a ± 0.90	59.8 ± 2.51

^a–eHomogenous group, indicate significant differences between potato variety (Duncan test, p ≤ 0.05).

± (SD), standard deviation; n=6.

Figure 3. Acrylamide content in pre-dried semi-product and dried finished products at different temperature (mean of potatoes variety).

LSD – least significant differenceError bars represent the standard deviation (±SD), values are given as mean n = 6 (Duncan test, p ≤ 0.05).

Effect of pre-drying temperature on the content of acrylamide in semi-finished products

Apart from the influence of potato tubers, the formation of acrylamide in potato products is affected by conditions of the technological process. Considering the results of Matthäus and Haase,^[4] there are four main factors that affect the quantity of acrylamide formed in products during the Maillard reaction: the content of reaction precursors, process temperature, content of water, and pH. Moreover, many researchers^[13,23,39] emphasise the high impact of temperature and frying time on the amount of acrylamide formed in potato products. Little research works are available concerning the effect of the production process of dried materials from cooked or raw potatoes on the formation of acrylamide in the finished products. The production process of dried potatoes differs significantly from that applied during potato processing into fried products (e.g. fries or chips). During potato drying, there is no factor that causes elution of the components of dm of tubers, e.g. reducing sugars (one of the precursors of Maillard reaction), whereas temperatures of the drying process are usually higher than during frying and range from 120°C to 160°C, and the process itself is long (it takes even 8–10 h). An additional problem is that dried potato materials are often semi-finished products used as additives to potato-based batters, which are subjected to another heat treatment (frying, baking) to obtain cakes, croquettes, and other products. According to Rytel et al.,^[11] acrylamide content in an industrially produced granulate reached ca. 200 μg/kg, whereas its content in chips produced from dough with the addition of this dried material and heat-treated was 10-fold higher. In addition, potatoes of coloured-flesh varieties are used for the production of potato chips, fries, or dried materials in countries of Western Europe or the USA, but research works are lacking on AA content in products made of them. Data is also missing on the effect of various temperatures of drying processes on acrylamide formation in the finished products obtained from coloured-flesh varieties of potato. In the conducted study, the diced potatoes were initially pre-dried for 2 h at 120°C, 140°C, or 160°C and then dried for 8 h at 50°C to obtain a final moisture content in the product of 8% or less. Such long drying times were needed given the high water content in the raw material – 80% on average. Additionally, shortening the drying time could necessitate the use of higher pre-drying and drying temperatures for diced potatoes and could possibly lead to excessive drying and burning. Moisture content in potato dice obtained after 2 h of pre-drying ranged from 22% (120°C) to 15% (160°C), and after drying it was 7% on average (Table 4). Although the raw material contained less than 0.5% of reducing sugars, the obtained semi-finished and finished products were characterised by different contents of acrylamide (Figs. 1 and 2). The dice obtained after the 2-h pre-drying process at 120°C contained 67–190 μg/kg dm of acrylamide, whereas raising the temperature by 20°C (140°C) increased its content by 5–6 times, and drying at 160°C by approximately 1.5–2 times again (Figs. 1, 2, and 3). The dice obtained from the Blaue Sant Galler potato variety was characterised by the highest content of acrylamide, from 190 μg/kg dm (at 120°C) to 1588 μg/kg dm (at 160°C) (Figs. 1 and 2), regardless of the pre-drying and drying temperatures. Furthermore, these potatoes had the highest content of reducing sugars (Table 1).

Table 4. Dry mass of semi-finished and finished products taken from laboratory dice processing.

Dry mass (%)	Potatoes variety				Means of potatoes varieties
	Vitelotte	Blaue Annelise	Salad Blue	Blaue Sant Galler
Potato dice after pre-drying in 120°C/2 h	82.9^c ± 0.10	77.6^b ± 0.10	70.4^a ± 0.10	84.5^d ± 0.14	78.8 ± 0.12
Potato dice after pre-drying 140°C/2 h	88.6^c ± 0.11	77.7^b ± 0.14	70.8^a ± 0.09	88.2^c ± 0.12	81.3 ± 0.10
Potato dice after pre-drying 160°C/2 h	96.0^d ± 0.09	84.4^b ± 0.12	71.6^a ± 0.10	90.0^c ± 0.13	85.5 ± 0.11
Potato dice after pre-drying in 120°C/2 h and dehydrated 50°C/8 h	92.1^c ± 0.14	92.0^a ± 0.10	92.5^c ± 0.13	91.9^b ± 0.10	92.1 ± 0.12
Potato dice pre-drying in 140°C/2 h and dehydrated 50°C/8 h	94.1^d ± 0.10	92.4^a ± 0.12	93.3^b ± 0.10	93.7^c ± 0.09	93.4 ± 0.10
Potato dice pre-drying in 160°C/2 h and dehydrated 50°C/8 h	96.2^d ± 0.09	94.4^b ± 0.08	93.4^a ± 0.10	95.7^c ± 0.10	95.0 ± 0.09

^a–dHomogenous group, indicate significant differences between potato variety (Duncan test, p ≤ 0.05).

± (SD), standard deviation; n=6.

Figure 4. Relationships between acrylamide content (μg/kg⁻¹) and “L” value in potatoes of different varieties: (A) Vitelotte, (B) Blaue Annelise, (C) Salad Blue, and (D) Blue Congo.

Effect of drying on the content of acrylamide in ready-to-eat products

Further drying of the dice was conducted at 50°C for approximately 8 h. Despite using a low temperature during the drying treatment, a significant increase (40% on average) was found in the acrylamide content of the ready-to-eat products (Figs. 2 and 4). The acrylamide content increased by approximately 80% in the diced potatoes pre-dried at 120°C and then dried at 50°C, whereas in the samples pre-dried at 140°C and dried at 50°C it increased by 30%, and in those samples pre-dried at 160°C and then dried at 50°C – by only 26% (Figs. 1 and 2). The large increase in the acrylamide content observed in the diced potatoes pre-dried at the lowest temperature could result from the greater dehydration of the raw material. In effect, the samples pre-dried at 120°C contained 79% of dm, and after the drying process, this content increased by approximately 16% (to 92.1%) (Table 4). The diced potatoes pre-dried at the highest temperature (160°C) were characterised by the lowest acrylamide content, which showed the lowest raw material moisture and the lowest moisture loss in the finished products (Figs. 1 and 2, Table 4). Figure 5 depicts the correlations between pre-drying temperature, moisture content, colour, and acrylamide formation in the finished products. The greatest effect in this toxic compound formation in dehydrated potato dice was evoked by the temperature of pre-drying (r = 0.936) and moisture content of the finished product (r = 0.789), and to a lesser extent – its colour. Additionally, after drying, the diced potatoes contained 236–1539 μg/kg dm of acrylamide (Figs. 1, 2, and 3). The lowest acrylamide content was found in dehydrated potato dice pre-dried at 120°C, i.e. by ca 2.5 times less than the product pre-dried at 140°C and almost six times less than the sample pre-dried at 160°C.

Figure 5. Relationships between acrylamide formation in dried potato dice and various parameters (A) pre-drying temperature on acrylamide content, (B) moisture of ready products on acrylamide content, (C) “a” value of dried products on acrylamide content, (D) “b” value of finished products on acrylamide content.

Conclusion

The analysed potato varieties can be recommended for the production of dehydrated potato dice due to their appropriate chemical composition. However, the content of toxic compounds should also be considered before marketing new varieties. The dice obtained from the Blaue St. Galler potato variety was characterised by the highest acrylamide content. The applied pre-drying and drying temperatures significantly influenced the acrylamide formation in finished products. The dice obtained after pre-drying at 120°C contained six times less acrylamide than the dice pre-dried at the higher temperature (160°C). Further drying at 50°C of the potato dice increased the acrylamide content in the finished products by 40% on average. After drying, the dice contained 236–1539 μg/kg dm of acrylamide.

Acknowledgements

This publication was supported by the Wroclaw Centre of Biotechnology programme at the Leading National Research Centre (KNOW) for the years 2014–2018