The kinetics of the solanidine hydrolytic extraction and the yield of total extractive matters from potato haulm by solutions of 10% w/v HCl in 96% vol. ethanol and in 2% w/v acetic acid at various temperatures (room temperature, temperature of 40°C and 60°C and at water bath boiling temperature), during 5-120 minutes, were studied. The degree of the solanidine hydrolytic extraction, the yield of the total extractive matters and the yield of solanidine increased as the temperature rises. The kinetic curves show that two periods are observed during the hydrolytic extraction of solanidine: a fast and a slow period. The best degree of solanidine hydrolytic extraction was obtained by 10% w/v HCl in 96% vol. ethanol (98.0%) at water bath boiling temperature, in 30 minutes. The yield of total extractive matters then was 14.4% and the yield of solanidine was 1.20%.

Key words: potato, glycoalkaloids, kinetics, extraction, solanidine

Plants of the Solanum genus contain glycolakaloids (GA), naturally occurring, potentially toxic, secondary plant metabolites (Schreiber, 1968), also known as phytoalexins (Stanker et. al.,1996). GA are present in potato (Solanum tuberosum L.) haulm in range of 0.56-5.03% (Percival and Dixon, 1996). In potato haulm there are two major glycolakaloids, a-solanine and a-chaconine (Figure 1), which represent more than 95% of the total GA. The carbohydrate moiety (solatriose in a-solanine and chacotriose in a-chaconine) is attached to the 3-OH position of the aglycon (Fiesher and Fiesher, 1967).

Upon hydrolysis of GA, by action of the mineral acid, a carbohydrate moiety and aglycon solanidine are recovered (Figure 1). Solanidine is a very important precursor for the synthesis of hormones and some other pharmacologically active compounds (Pogorelova, 1968).

In order to find the optimal reaction conditions for acid hydrolysis of GA from haulm, in our previously published paper (Nikolić and Stanković, 1999) the concentration of hydrochloric, sulphuric and phosphoric acids (1-20 % m/v) in different solvents (water, 2 % w/v aqueous acetic acid, methanol and 96% ethanol) were investigated and the best results were obtained with 10% w/v HCl in 96% vol. ethanol and in 2% w/v acetic acid. In this paper the kinetics of the solanidine hydrolytic extraction, the yield of the total extractive matters and the yield of the solanidine from potato haulm at various temperatures by using these solvents were studied. The aim was to determine the optimal temperature for the solanidine hydrolytic extraction.

Figure 1. Hydrolysis of a-solanine and a-chaconine to solanidine

MATERIALS AND METHODS

Plant materials

The haulm of potato (Solanum tuberosum L.) cv. Désirée was harvested 75-100 days after planting, dried at 20-22°C during 21 days and milled to an average particle size of 0.17 mm (the specific particle surface was 49.6 cm²g^-1). The moisture content was about 9% and the ratio of a-solanine to a-chaconine was 1:1.15 w/w.

Kinetics of acid hydrolysis

The milled haulm (0.5 g) was treated by 10% w/v hydrochloric acid solutions in 96% vol. ethanol and in 2% w/v acetic acid at room temperature, temperature of 40°C and 60°C and at water bath boiling temperature, during 5-120 minutes and then filtered. In case of solutions of HCl in 2% w/v aqueous acetic acid, pH of filtrate was adjusted to 4.0 by adding aqueous solutions of sodium hydroxide (at first by 50 and then by 1% w/v). In case of solutions of HCl in ethanol, filtrate was evaporated to dryness first, the dry residue was dissolved in 10 cm³ of 2 % m/v aqueous acetic acid, at boiling temperature by using reflux condenser and then pH of the solution was adjusted to 4.0 as before. The spectrophotometric method (420 nm) with methyl orange as a colouring reagent (Tukalo and Carik, 1970) was used to determine the solanidine in extracts.

The kinetic curves of the solanidine hydrolytic extraction with 10% w/v HCl in 96% vol. ethanol and in 2% w/v acetic acid at room temperature, temperature of 40°C and 60°C and at water bath boiling temperature, during 5-120 minutes are shown in Figure 2.

The investigation results show that degree of solanidine hydrolytic extraction (DHE), the yield of the total extractive matters (YEM) and the yield of solanidine (YS) increased as the temperature rises. The best values of DHE obtained from the corresponding curve are given in Table 1.

The process of solanidine extraction from potato haulm consists of the extraction process of GA and the simultaneous hydrolysis process of GA to solanidine. The hydrolytic extraction is characterised by fast and slow phase. This type of kinetic curve can be shown by the following equation:

where DHE is the degree of the solanidine hydrolytic extraction, a is a part of GA converted to solanidine during the fast period of hydrolytic extraction, k is the rate constant of the slow period of hydrolytic extraction and t is time of the hydrolytic extraction.

The values of the coefficients a and k, obtained from corresponding curve are showed in Table 2. The best obtained values of YEM and YS are given in Table 3.

According to the results, the DEH increased as the temperature rises. The best DEH achieved by 10% w/v HCl in 96% vol. ethanol at water bath boiling temperature (98%) is bigger than those achieved by 10% w/v HCl in 2% w/v acetic acid (74.8%) and a shorter time was needed to reach them (30 minutes instead 120 minutes) (Figure 2, Table 1). The values ofcoefficients a are in range of 18% (achieved with 10% w/v HCl in 2% w/v acetic acid at room temperature) to 97% (achieved by 10% w/v HCl in 96% vol. ethanol at water bath boiling temperature) (Table 2). That means that the greatest part of GA (75-99%) from haulm is hydrolysed to solanidine in the fast period of extraction. The time of the fast period of extraction with 10% w/v HCl in 96% vol. ethanol is shorter than with 10% w/v HCl in 2% w/v acetic acid at all investigated temperatures (Table 2). The values of coefficient k are ranged from 3.2x10^-2 to 27.7x10^-2 (Table 2).

The YEM and YS also increased as the temperature rises. The values of YEM achieved with 10% w/v HCl in 2% w/v acetic acid are higher at all investigated temperatures than those achieved by 10% w/v HCl in 96% vol. ethanol, but YS is better by ethanolic solution. The best-achieved YS is 1.20% at the water bath boiling temperature (Table 3).

The degree of the solanidine hydrolytic extraction (DHE), the yield of total extractive matters (YEM) and the yield of solanidine (YS) increased as the temperature rises. The kinetic curves show that the two periods are observed during the hydrolytic extraction of solanidine: a fast and a slow period. The best DHE was obtained by 10% w/v HCl in 96% vol. ethanol (98.0%) at water bath boiling temperature, in 30 minutes. The YEM then was 14.4% and YS was 1.20%

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