The effect of industrial potato processing on the concentrations of glycoalkaloids and nitrates in potato granules

Food Control 28 (2012) 380e384

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The effect of industrial potato processing on the concentrations of glycoalkaloids and nitrates in potato granules * _ Rytel Elzbieta Department of Food Storage and Technology, Wrocław University of Environmental and Life Sciences, Norwida 25, 50-375 Wrocław, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 November 2011 Received in revised form 12 April 2012 Accepted 28 April 2012

The aim of this study was to determine the effects of the different stages of industrial processing of potatoes, on the content of glycoalkaloids (chaconine and solanine) and nitrates in the raw material, intermediates, finished granules and waste products. The material used for the study included samples of raw tubers, semi-finished products and finished products taken directly from 8 points in the technological line for the production of potato granules, and 3 points from the waste product line. The samples were collected three times within two years of research (in 2009 and 2010) from the following places: 1/unpeeled potato, 2/potato after peeling, 3/ potato after blanching, 4/potato after cooling, 5/steamed potato, 6/pneumatically dried potato, 7/product after fluidization drying, 8/granulated product, and waste products: 9/peels, 10/waste after air drying and 11/after fluidization drying. In the raw material, intermediates, finished granules and waste products, the concentrations of glycoalkaloids (a-chaconine and a-solanine) were determined using an HPLC method, and nitrates were determined colorimetrically using an RQflex analyser. It was found that the industrial processes of potato granules significantly decreased the concentration of glycoalkaloids (chaconine and solanine) and nitrates in intermediates and finished products when compared to raw material. The highest decrease in glycoalkaloids was caused by peeling (50%) and blanching (63%). The concentration of nitrates decreased the most after thermal processes e after blanching a decrease of 20% and after air drying e by 50%. The dehydrated potato granules contained on average 14% of the initial quantity of glycoalkaloids and 48% of nitrates. High content of toxic compounds was found in potato peels but dry wastes after pneumatic drying or after fluidization contained proportionally low contents of those compounds. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Potato Glycoalkaloids Nitrates Potato granules

1. Introduction Drying is one of the oldest methods of food preservation and an important factor in production technology (Bondaruk, Markowski, & B1aszczak, 2007; Vadivambal & Jayas, 2007). Perishable goods, such as raw vegetables or fruit, benefit from dehydration in many ways, primarily in substantially reducing losses resulting from raw material storage (up to 20%), lowering water activity of the material, slowing down many enzymatic reactions and microbial growth, which in effect permits much longer storage than fresh  ska & Leszczyn  ski, 1989, 391 pp.). material (Lisin Drying provides a whole range of different potato products, i.e.: dices, groats, granules, potato flakes or flour. These products should all have high nutritional value, low content of toxic compounds and good organoleptic characteristics. Dehydrated potato products and * Tel.: þ48 71 3205239; fax: þ48 71 3205221. E-mail address: [email protected]. 0956-7135/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2012.04.049

intermediates obtained during the processing of potatoes into granules contain the same compounds as the raw material but in different quantities and proportions. Processing into refined products, including those dehydrated, usually uses potato varieties with a low tendency to accumulate anti-nutritional or toxic compounds. Natural toxic compounds in potato include glycoalkaloids e achaconine and a-solanine. The synthesis of these compounds by the potatoes depends on genetic factors, environmental conditions (weather prevailing during the growing season, rainfall, sunshine),  ski, 2000; Lisin  ska and storage conditions (Donald, 2008; Leszczyn  ski, 1989, 391 pp.; Lisin  ska, Pe˛ ksa, Kita, Rytel, & Tajner& Leszczyn Czopek, 2009; Wünsch & Munzert, 1994; Zgórska, Czerko, &  ska, 2006). Potatoes should contain less than 10 mg per Grudzin 100 g
1, as a concentration as low as 11 mg per 100 g
1 results in an undesirable acrid aftertaste (Knuthsen, Jensen, Schmidt, & Larsen,  ski, 2000; Pe˛ ksa, Go1ubowska, Rytel, Lisin  ska, & 2009; Leszczyn Anio1owski, 2002; Speijers, 1998, pp.43e47). Natural anti-

R. Elz_ bieta / Food Control 28 (2012) 380e384

nutritional compounds in potato also include nitrates. Potato tubers
1 generally contain less than 200 mg NO
1 3 kg , a medium level compared to other vegetables that may accumulate more than 300 mg 100 g
1, such as lettuce (Hill, 1999; Murawa, Banaszkiewicz, Majewska, B1aszczyk, & Sulima, 2008). Their concentration in tubers depends on the cultivation of the potatoes, and mainly on the intensity and frequency of nitrogen fertilization, climatic conditions and type of storage (Cieslik, 1995; Rytel, Pe˛ ksa, Tajner ska, 2011). Czopek, Kita, & Lisin Processes utilized during the production of potato granules can have different effects on the content of anti-nutritional and toxic compounds, mainly due to the nature of these compounds and the distribution in the tuber. Initial processes used during the production of potato granules, including the washing and peeling of tubers, result mainly in a decrease in glycoalkaloids. As they are located mostly in or just beneath the skin, their content may be reduced by 50% on average (Friedman & McDonald, 1997; Pe˛ ksa,  ska, & Rytel, 2006; Rytel, Go1ubowska, Anio1owski, Lisin  ska, Pe˛ ksa, & Anio1owski, 2005; Tajner-Czopek, Go1ubowska, Lisin  ska, 2008; Valkonen, Keskitalo, Vasara, & Jarych-Szyszka, & Lisin Pietila, 1996). Thermal processes, such as blanching or drying, may contribute to leaching of compounds that dissolve well in water, mostly nitrates (Cieslik, 1995). According to Becka, Micka, and Vockal (1992) and Cieslik (1992) blanching as well as cooking influence on nitrates decreases on the level of about 20% when compared to raw material. Glycoalkaloids, due to their thermostable nature, are more resistant to high temperatures (Friedman, 2003, 2006). Many authors (Friedman, 2006; Friedman & Dao, 1992; Knuthsen et al., 2009; Valkonen et al., 1996) state that culinary processes, such as cutting, blanching, cooking and baking in a much lower degree in comparison to peeling, decrease TGA contents in potatoes. The production of dehydrated potato products results in many different waste products, some of which, such as waste from peeling or drying, contain ingredients that could be used as food additives, and not only in animal feed as has been the case so far. It seems necessary to thoroughly examine the various waste products of industrial potato processing, not only in terms of nutritional but also anti-nutritional compounds. The aim of this study was to determine the effects of the different stages of industrial processing of potatoes, on the content of glycoalkaloids (chaconine and solanine) and nitrates in the raw material, intermediates, finished granules and waste products. 2. Material and methods 2.1. Raw material The material used for the study included samples of raw tubers, semi-finished products and finished products taken directly from 8 points in the technological line for the production of potato granules, and 3 points from the waste product line. The samples were collected three times within two years of research (in 2009 and 2010) from the following places: 1/unpeeled potato, 2/potato after peeling, 3/potato after blanching, 4/potato after cooling, 5/steamed potato, 6/pneumatically dried potato, 7/product after fluidization drying, 8/granulated product, and waste products: 9/peels, 10/ waste after air drying and 11/after fluidization drying. There were prepared 1 kg samples from each stage of the technology line for glycoalkaloids (a-chaconine and a-solanine) and nitrates contents. 2.2. Potato sample preparation for analysis The samples from the technological line, from numbers 1 to 5, and peelings, were crushed and freeze-dried (apparatus of Edwards

381

firm). The dried samples were ground in an electric grinder, and together with samples 6, 7, 8, 9, 10, 11 were used as material for the determination of a-solanine, a-chaconine and nitrates. 2.3. The concentrations of a-solanine and a-chaconine 2.3.1. Apparatus A high-pressure liquid chromatograph HPLC (pro Star) was used (Varian, Walnut Creek, CA, USA). The HPLC was equipped with a UV detector e 310 type, Microsorb NH2 analytical column (25  46 cm LD) (Rainin Instrument, Woburn, Ma, USA), and a computer system for monitoring the chromatograph (Varian Chromatography System). 2.3.2. Conditions of glycoalkaloid separation A mixture of tetrahydrofuran (Merck, Germany), acetonitrile and water 50:20:30 þ KH2PO4 (1.02 g) per liter was used as an eluent. The process was carried out at a temperature of 35  C, with a speed of flow of 2 cm3 min
1 and pressure of 11.3 MPa, applying a light wavelength of 208 nm. 2.3.3. Sample preparation for chromatographic analysis The dried material (1 g) was homogenized with 4 cm3 of water and 30 cm3 of methanol (Labscan, Ireland) for 2 min, followed by filtration. The filtrate was brought to a final volume of 50 cm3 with methanol. A 5 cm3 aliquot of extract was cleaned up on the SPE column (Bond Elut C18; 500 mg; 6.0 cm3, Varian, USA). Glycoalkaloids were rinsed with methanol and evaporated to dryness in a vacuum at a temperature of 50  C. The resultant residue was dissolved in 1 cm3 of THF:ACN:H2O e 50:20:30. Before application into the column, the sample was cleaned using 0.45 mm filters. The volume of the injection was 10 ml. Standard solutions (1 mg/cm3) were prepared by dissolving 10 mg of a-solanine and a-chaconine (Sigma) in 10 cm3 of methanol. Standard solution was dissolved to obtain samples containing from 1 to 50 mg/cm3 of both a-solanine and a-chaconine. On the column 10 ml of solution was injected. Samples were prepared with the agreement of the methodology described by Pe˛ ksa et al. (2002) and Saito, Sanford, and Webb (1990). 2.4. The content of nitrates A reflectometric method with test strips was used, at
1 a measurement range from 5 to 225 mg NO
1 3 kg . According to the principle of reflectometry (remission photometry), the reflected light from the strip was measured. In classical photometry, the difference in the intensity of emitted and reflected light allows a quantitative determination of the concentration of specific analysis. 2.4.1. Sample preparation for nitrates analysis Nitrate concentrations were determined by reflectometry using a Rqflex analyser (Merck). Determinations were made in 20 g of distilled water solution containing 5 g of a dry sample. In the solution nitrate concentrations using test strips was measured. In agreement with methodology described by Rytel et al. (2005). 2.5. Analytical methods The nitrate content of the potato tubers was determined colorimetrically in the potato tubers, intermediates and finished products (Rytel et al., 2005). The quantities of a-solanine and achaconine were determined using the method of Pe˛ ksa et al. (2002) and Saito et al. (1990). All the analyses were carried out twice.

R. Elz_ bieta / Food Control 28 (2012) 380e384

382

2.6. Statistical analysis The results obtained in the experiment were subjected to statistical calculations of the Statistica 9.0 program. There was applied a multi-way analysis of variance and Duncan’s test (p  0.05) for the determination of the significance of differences between means. All experiments were performed in three technological replications from two years of investigation and the present results show the mean of all data combined.

3. Results and discussion In this research, production of potato granules concurred with significant changes in the concentrations of glycoalkaloids (solanine and chaconine) and nitrates. The material used contained on average 16 mg 100 g
1 TGA in dry weight (3.2 mg per 100 g
1 fresh weight) (Table 1). Usually potatoes used for consumption or in industrial food production contain below 10 mg per 100 g
1 glycoalkaloids in the fresh tuber mass e a content below 20 mg 100 g
1 fresh mass and 100 mg 100 g
1 dry mass is considered safe for humans (Friedman, 2006; Ginzberg, Tokuhisa, & Veilleux, 2009; Smith, Roddick, & Jones, 1996; Valkonen et al., 1996). Regardless the final product, potato processing in the food industry is associated with a series of similar technological stages. These include initial processes, such as washing, peeling, slicing and rinsing of tubers. Thusly prepared material is then subjected to blanching and processes characteristic for the given group of products, i.e. drying for the production of dehydrated potato products, or frying for the production of crisps and French fries (Pe˛ ksa & Rytel, 2008; Rytel et al., 2011). The initial processes, which includes peeling, may be conducted using various techniques: high-pressure steam, leaching or  ska & Leszczyn  ski, 1989, 391 pp.; Pe˛ ksa & mechanical peeling (Lisin Rytel, 2008; Rytel et al., 2011). Potatoes taken from the factory were peeled using steam in pressure containers with a temporary action, where steam pressure was 0.8 Mpa. Peeling process affected significantly glycoalkaloids (a-solanine and a-chaconine) content in potatoes in comparison to the initial content of those compounds in raw material (Table 1). After steam peeling, the TGA content in the processed potatoes decreased by 50% on average. A greater decrease was observed for a-solanine (55%) than for a-chaconine (46%). The manner of potato processing significantly influences the amount of remaining glycoalkaloids in the peels and the flesh of the peeled potatoes. In a research by Mäder, Fisher, Schnick, and Kroh (2009) steam peeling potatoes cultivar Karlena influences on a decrease of TGA by abort 25% when compared to tubers but

Table 1 Total glycoalkaloids (TGA) content (mg 100 g
1 d.m.) in potatoes during industrial processing of granules (mean of 2 years). Stage of the process

a-Solanine

a-Chaconine

TGA

Unpeeled potatoes Potatoes after peeling Potatoes after blanching Potatoes after cooling steaming potatoes product after pneumatic drying product after fluidization drying granulated product

5.94a 2.65b 2.08c 2.03c 1.36d 1.13de

10.06a 5.45b 3.88c 3.87c 2.45d 2.26e

16.0a 8.1b 5.96c 5.90c 3.81d 3.39e

     

0.36 0.04 0.02 0.04 0.06 0.02

     

0.17 0.09 0.07 0.06 0.06 0.05

     

0.40 0.20 0.08 0.05 0.03 0.08

1.05e  0.05

2.06f  0.04

3.11e  0.03

0.89e  0.18

1.77g  0.06

2.66f  0.04

SD standard deviation, n ¼ 6, a, b, c, d, e, f, g: differences superscripts within columns indicate significant differences.

according to Rytel et al. (2005), the amount of glycoalkaloids during the steam peeling phase of industrial production of French fries decreased by 40%, and during the production of crisps e where only mechanical peeling was used e only by 25% (Pe˛ ksa et al., 2006). Differences in the remaining amount of glycoalkaloids after steam peeling may be associated with the depth of peeling which depends on the quality of the potatoes. The lower the quality of the material, the longer the time of peeling e regardless of the economic context, this longer peeling is beneficial as more glycoalkaloids are being removed from the material. Depending on the size of the tubers, the peeling technique and the potato variety, the decrease in TGA may reach 80e96%, or 20e35% after incomplete skin removal (Mäder et al. 2009; Ostry, Ruprich, & Skarkova, 2010; Tajner et al. 2008). Glycoalkaloids in potatoes are located mainly in the skin and in the layer about 1.2e2 mm under the skin, while the content in the central pith and in the inner cortex is very low (Friedman & McDonald, 1997; Smith et al., 1996; Wünsch & Munzert, 1994). However, peeling alone may be insufficient in the processing of potatoes that accumulate high amounts of glycoalkaloids. Such potatoes after cooking may have a worse flavor (bitter), due to too high content of those compounds remaining (more than 10 mg per  ski, 2000; Lisin  ska & Leszczyn  ski, 1989, 391 pp.; 100 g
1) (Leszczyn Pe˛ ksa & Rytel, 2008). In this work, peeled potatoes were sliced (large tubers were sliced in half) and subjected to thermal processes. The sliced potatoes were blanched in water at 80  C for 15 min. In these potatoes after blanching there was stated significant decrease in TGA by 25% compared to their content in peeled potatoes was observed (Table 1). A decrease in a-chaconine was more pronounced than a-solanine (29% compared to 21%). According to Donald (2008) solanine is almost insoluble in water. Various authors (Bushway & Ponnampalam, 1981; Donald, 2008; Friedman & Levin, 2009) show that processes used in cooking at home, such as boiling, baking or frying, do no result in significant changes in TGA in potatoes. According to Takadi, Toyoda, Fujiyama, and Saito (1990), microwave cooking results in 15% lower TGA content in potatoes and Smith et al. (1996) suggest that tubers after cooking can contain still yet some glycoalkaloids in the range 27 to 42 mg 100 g
1. Processes used at home are significantly different from those used on an industrial scale. In a work by Rytel et al. (2005), a two-stage blanching of French fries in water at 80  C resulted in a TGA decrease of 40%. Cooling in water at 20  C for 15 min was a subsequent stage of industrial potato processing used in this study. The process did not result in significant changes in TGA content. The following steaming for 30 min decreased TGA by 35% on average, compared with cooled samples e the examined potatoes contained 1.36 mg per 100 g
1 of a-solanine and 2.45 mg per 100 g
1 of a-chaconine (Table 1). Drying was the last stage. The potato granules were dried in two stages e first in an air drier with an inlet temperature of 177  C and outlet temperature of 80  C and then in a fluidization drier, with alternating currents of hot and cold air. In the collected samples after air drying, a further TGA decline by 11% compared to samples after steaming, with greater changes in the content of a-solanine (17%) than a-chaconine, by 8% was observed (Table 1). Fluidization drying did not result in significant changes in the content of toxic compounds. TGA decreased from 3.39 mg per 100 g
1 (after air drying) to 3.11 mg
1 per 100 g
1 (after fluidization drying) (Table 1). Ready granules still contained 0.89 mg per 100 g
1 of a-solanine and 1.17 mg per 100 g
1 a-chaconine, 14% of the initial levels. According to the other authors (Friedman & Dao, 1992; Mäder et al., 2009; Smith et al., 1996) TGA contents in dried potato depend on the type of product as well as on the

R. Elz_ bieta / Food Control 28 (2012) 380e384

treatment during manufacture. Potato flakes can contain still yet 1.5e2.3 mg 100 g
1 glycoalkaloids and granules from 6.5 to 7.5 mg 100 g
1 of those compounds. During drying, despite the long duration and high temperature (more than 170  C), changes in the content of these compounds were not as high as during the frying of French fries or crisps. After frying of slices or potato strips, TGA decreased by about 40% (Friedman, 2003; Pe˛ ksa et al., 2006; Rytel et al., 2005). According to Friedman (2003), frying at 150  C does not result in significant changes in TGA, but after 10 min of frying at 210  C the declines in these compounds may even reach 40%. That author suggests that 170  C is a threshold temperature, above which TGA decrease significantly. These TGA compounds are thermostable, and hence their degradation requires a high temperature and hot oil which is a more effective factor in accelerating the drop of those compounds than high temperature alone. Nitrates were the next groups of examined compounds. The concentration of nitrates in potatoes used for the production of potato granules was not high e 144 mg NaNO3 kg
1 in dry mass (Table 2). Nitrates usually concentrate in and just below the skin, they are easily soluble in water and have little resistance to thermal  ska & Leszczyn  ski, 1989, 391 pp.). Hence in the processes (Lisin production of potato granules the nitrate drops occurred after peeling (by 7%) and thermal processes (Table 2) e after blanching a decrease reached 15% compared to samples after peeling and then by 12% after steaming. The successive thermal processes caused significant changes in the quantities of these compounds. The greatest decrease occurred after air drying e by 25% (Table 2). The finished product contained 48% of the initial nitrate content in the material. According to Cieslik (1995, 1998) initial processing of potatoes removed about 32% of these compounds and the thermal processes such as frying and boiling by 16%e71%, depending on the technique used. Also Pe˛ ksa et al. (2006) shows that the greatest nitrate loss occurs after peeling tubers (25e28%) and frying potato slices during chip production (85e89%). Dehydration does not result in high nitrate loss as frying, which can be due to the fact that during frying, fragmented potatoes (slices, strips) are exposed not only to a high temperature but also decomposition of nitrates in high temperature. During drying, potatoes are exposed only to a high temperature. The production of potato granules also results in a high amount of waste products, such as peels or potato parts removed from meshes after drying. These products are mainly used as feed for animals. In the food industry, potato skins are used as an additive enriching food with the dietary fiber. Potato skins contain relatively high concentrations of fiber (Friedman, 2006). In this work, the content of glycoalkaloids in the potato skin was determined, in which TGA was 38.18 mg in 100 g
1 of dry mass (Table 3). According to Friedman (2006), the skin contains from 39 to 80 mg 100 g
1 dry

Table 2 Nitrate content (mg NaNO3 kg
1 d.m.) in potatoes during industrial processing of granules (mean of 2 years). Stage of the process

Nitrate

Unpeeled potatoes Potatoes after peeling Potatoes after blanching Potatoes after cooling Steaming potatoes Product after pneumatic drying Product after fluidization drying Granulated product

144a 134a 114ab 109ab 96ab 72b 71b 69b

       

19.5 21.0 26.5 26.5 20.0 21.0 22.5 22.0

SD standard deviation, n ¼ 6, a, b: differences superscripts within columns indicate significant differences.

383

Table 3 Total glycoalkaloids (mg 100 g
1 d.m.) and nitrates (mg NaNO3 kg
1 d.m.) content in waste products. Waste products

Peels Waste after pneumatic drying Waste after fluidization drying LSD

Glycoalkaloids

Nitrate

a-Solanine

a-Chaconine

TGA

13.29a 1.18b 1.39b 0.57

24.89a 2.16b 2.31b 1.04

38.18a 3.34b 3.70b 1.61

234a 103b 107b 33.9

LSD least significant difference, n ¼ 6, a, b: differences superscripts within columns indicate significant differences.

mass. In the potato skin the chaconine/solanine ratio changes in favor of chaconine. In this work, this proportion was 2:1 in the potato skin, and 1.5:1 in the potato flesh (Table 3). The chaconine/solanine ratio is important due to the higher toxicity of chaconine compared to solanine. If the peeled potato skin is to be used as a food additive, one should try to decrease the proportion of TGA. In this work, potato peel also contained high nitrate concentrations e 234 mg NaNO3 kg
1 dry mass. The high content of both glycoalkaloids and nitrates in the potato peel shows the problematic nature of using this waste as a food additive. Much lower concentrations of toxic compounds were observed in waste collected after air and fluidization drying. Waste after air drying contained 3.34 mg TGA 100 g
1 dry mass (2.16 mg 100 g
1 chaconine and 1.18 mg 100 g
1 solanine), and 103 mg NaNO3 kg
1 dry mass (Table 3). Waste after drying and sifting remaining on the in coarse sieve the upper mesh, still contained some amounts of dehydrated peels, hence the amount of glycoalkaloids (3.70 mg 100 g
1 d.m.) and nitrates (107 mg NaNO3 kg
1 d.m.) is much higher than in finished products. Due to the high fiber values and nutrient contents of this waste they are the potential additive not only in the feed but also in food products processing. 4. Conclusions It was found that the industrial processes of potato granules significantly decreased the concentration of glycoalkaloids (chaconine and solanine) and nitrates in intermediates and finished products when compared to raw material. The highest decrease in glycoalkaloids was caused by peeling (50%) and blanching (63%). The concentration of nitrates decreased the most after thermal processes e after blanching a decrease of 20% and after air drying e by 50%. The dehydrated potato granules contained on average 14% of the initial quantity of glycoalkaloids and 48% of nitrates. High content of toxic compounds was found in potato peelings but dry wastes after pneumatic drying or after fluidization contained proportionally low contents of those compounds. This work shows that the quality control of materials, intermediates, finished products and waste should be performed constantly. Their toxic compounds may affect the quality of food that contain potato products: fried products (chipseltten), and noodles, potato cakes, croquettes, dumplings, etc., made from concentrates containing finished dehydrated potato products as the components mixtures. References Becka, J., Micka, B., & Vockal, B. (1992). Changes in the content of nitrate nitrogen in raw and boiled potatoes. Roslinna Vyroba, 28, 181e188. Bondaruk, J., Markowski, M., & B1aszczak, W. (2007). Effect of drying conditions on the quality of vacuum-microwave dried potato cubes. Journal of Food Engineering, 81, 306e312. Bushway, R. J., & Ponnampalam, R. (1981). a-Chaconine and a-solanine content of potato products and their stability during several modes of cooking. Journal of Agriculture and Food Chemistry, 29, 814e817.

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