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Water Sorption Isotherms of Pistachio Nuts
Lebensm.-Wiss. u.-Technol., 29, 372–375 (1996)
Research Note
Water Sorption Isotherms of Pistachio Nuts S. Yanniotis* and I. Zarmboutis Agricultural University of Athens, Department of Food Science and Technology, Iera Odos 75, 118 55 Athens (Greece) (Received August 16, 1994; accepted August 16, 1995)
The adsorption and desorption isotherms of pistachio nuts (kernel and shell) were measured at 15, 25 and 40°C using a gravimetric method. Hysteresis was more pronounced in the shell than in the kernel and reduced as the temperature increased. The isosteric heat of desorption, calculated from the slope of a lnaw vs. 1/T plot, was higher than the isosteric heat of adsorption. The GAB equation gave satisfactory goodness of fit with average percent deviation of less than 5%. ©1996 Academic Press Limited
Introduction
Materials and Methods
Pistachia vera is cultivated in the Mediterranean region, in the United States and other countries. In 100 g of dried kernel of cultivar Aeginas, which is cultivated in Greece, there is about 27 g protein, 12 g carbohydrate and 53 g oil, the rest being water and minerals (1). This composition may vary with cultivar. The kernel constitutes about 50 to 55% of the nut weight and the shell the remaining 45 to 50%. Processing of the nuts includes dehulling, washing, drying to 60–80 g/kg moisture (related to whole kernel) in dryers at 65°C for 6 to 7 h or by sun-drying for 3 to 4 d, salting by immersion in 100 g/kg sodium chloride solution for 15 min, roasting at 150 to 180°C for 30 min and packaging. Studies on the water sorption characteristics of pistachio nuts are very limited. The water sorption isotherm of pistachio nuts at higher water activity (aw) values and at 20°C has been studied with particular regard to control of aflatoxin formation (2). From this investigation it was concluded that at water activity values of 0.87 to 0.88 growth of Aspergillus flavus occurred and was accompanied by aflatoxin production at 0.88 aw. At water activity values below 0.86, the growth of A. flavus and A. niger, and thus aflatoxin production, was prevented due to the competitive growth of more xerophilic fungi (i.e. Aspergillus amstelodami). The objective of the present work was to study the water sorption isotherms of pistachio nuts in more detail so that post-harvest handling and storage conditions could be established.
Materials Pistachio nuts cv. Aeginas were obtained from the Agricultural University in Spata, Attica. They were sundried and stored in bulk at room temperature for 3 to 6 months before the experiment. The water activity of the dried sample was 0.72 (measured with RotronicHygroskop DT) and the fat content 532 g/kg (3).
*To whom correspondence should be addressed.
0023-6438/96/040372 + 04$18.00/0
Equipment The gravimetric method developed by the European Cooperation in Scientific and Technological Research Project COST-90 (4) was used for this study. Saturated solutions of LiCl, MgCl2, Mg(NO3)2, NaBr, SrCl2, NaCl and KCl placed in 1L glass jars were used as sorbate source. The water activity of the salts was taken from Greenspan (5). Jars were placed in a bath with an insulated cover and the temperature maintained at the specified value (15, 25 and 40°C) within ± 0.2°C.
Procedure Initially, the kernels were separated from the shells. Both were ground in a laboratory mill (Newman equipped with 2.4 mm holes screen) to facilitate water vapour diffusion during the experiment. For the adsorption isotherm, 500 to 600 mg of ground sample were placed in each weighing bottle and dried in a vacuum oven for 6 h at 70°C. Dried samples were then placed in the sorbostat (five weighing bottles in each jar) and left for 8 to 12 d. By that time, equilibrium was reached in every jar, as determined by constant weight in three successive measurements on three consecutive days. As expected, the higher the temperature the shorter the equilibration time, and the higher the water activity in the jar the larger the equilibration time. For ©1996 Academic Press Limited
372
lwt/vol. 29 (1996) No. 4
Table 1 Experimental results for sorption at 15˚C Water content (g/100g of fat-free solids) Adsorption
Desorption
Kernel
Shell
Kernel
Shell
Water activity (aw)
x¯
sx
x¯
sx
x¯
sx
x¯
sx
0.113 0.333 0.559 0.607 0.741 0.756 0.859
3.95 7.72 12.81 13.97 21.04 21.41 32.79
0.10 0.21 0.13 0.07 0.11 0.21 0.10
3.19 6.37 9.99 10.92 15.20 16.13 21.10
0.04 0.03 0.02 0.09 0.04 0.02 0.03
5.81 9.95 n.d. 14.21 n.d. 23.14 35.37
0.30 0.33 n.d. 0.23 n.d. 0.14 0.86
5.37 10.01 n.d. 15.82 n.d. 21.04 23.51
0.05 0.06 n.d. 0.03 n.d. 0.06 0.09
Mean value (x¯) and standard deviation (sx) of five determinations. n.d.=not determined
Table 2 Experimental results for sorption at 25˚C Water content (g/100g of fat-free solids) Adsorption
Desorption
Kernel
Shell
Kernel
Shell
Water activity (aw)
x¯
sx
x¯
sx
x¯
sx
x¯
sx
0.113 0.328 0.529 0.576 0.709 0.753 0.843
3.42 7.30 11.33 11.72 17.02 20.86 29.30
0.12 0.10 0.15 0.32 0.13 0.21 0.34
2.56 5.80 8.60 10.02 12.92 14.32 18.17
0.03 0.07 0.06 0.06 0.05 0.04 0.12
5.35 8.02 11.07 12.21 18.30 20.86 29.56
0.06 0.05 0.17 0.15 0.13 0.45 0.26
3.89 8.01 11.30 13.15 17.09 17.15 20.41
0.07 0.06 0.04 0.04 0.06 0.08 0.15
Mean value (x¯) and standard deviation (sx) of five determinations.
Table 3 Experimental results for sorption at 40˚C Water content (g/100g of fat-free solids) Adsorption
Desorption
Kernel
Shell
Kernel
Shell
Water activity (aw)
x¯
sx
x¯
sx
x¯
sx
x¯
sx
0.112 0.316 0.484 0.532 0.747 0.823
3.37 6.51 9.86 10.00 19.86 26.77
0.16 0.04 0.09 0.10 0.16 0.34
2.27 5.16 7.52 8.16 14.90 17.40
0.05 0.02 0.08 0.17 0.11 0.20
4.33 6.58 10.00 10.23 20.02 27.07
0.16 0.10 0.12 0.14 0.22 0.50
3.16 6.84 9.77 10.49 16.94 19.03
0.02 0.05 0.07 0.12 0.07 0.06
Mean value (x¯) and standard deviation (sx) of five determinations.
Table 4 Parameters of the GAB equation for the adsorption and desorption isotherms of kernel and shell at 15, 25 and 40˚Ca Xm
Temperature (˚C)
Kernel
Adsorption
15 25 40
6.560 6.206 5.584
Desorption
15 25 40
6.656 5.597 5.198
aSee
K
Shell
c
Kernel
Shell
Kernel
5.795 5.715 5.381
0.938 0.940 0.970
0.862 0.835 0.874
9.747 8.457 9.609
10.404 8.885 7.683
0.941 0.969 0.990
0.695 0.721 0.775
39.989 48.778 21.000
Eqn [2] and Eqn [3].
373
E (%) Shell
Kernel
Shell
9.070 7.781 5.689
1.1 2.8 2.1
1.0 1.3 2.1
10.306 7.419 6.308
4.9 2.0 3.1
2.3 2.8 1.3
lwt/vol. 29 (1996) No. 4
Water content (g/100 g of fat-free solids)
40
deviation (D) between the mean and the individual values was determined by:
30
D =
100 5
5
|xi–¯x |
i=1
x¯
∑
Eqn [1]
where Xi is the individual value of moisture content of the five samples in each jar and x¯ is their arithmetic mean. The average D for all experimental determinations was 1.1 ± 0.9%. The highest deviation values were observed at the two ends of each sorption isotherm.
20
10
Results and Discussion 0
0.2
0.4 0.6 Water activity
0.8
1
Fig. 1 Hysteresis of kernel and shell at 15°C. (–––) = shell; (– – –) = kernel
the desorption isotherm determination, 500 to 600 mg of ground sample were placed in each weighing bottle and put in a desiccator which contained pure water at the bottom. Samples were left at room temperature to adsorb water vapour for 2 d. They were then weighed on each consecutive day for 3 to 4 d until no appreciable weight change was observed. The weighing bottles were then placed in the sorbostat (five bottles in each jar) and left for 6 to 10 d. Equilibration was determined as described for the adsorption isotherm determination. The dry weight of the samples was determined at the end of each experiment by drying in a vacuum oven for 6 h at 70°C.
Analysis of data The arithmetic mean of the water content in the five bottles of each jar was determined and plotted vs. water activity. For each experimental point, the relative %
The experimental data of the adsorption and desorption isotherms of the kernel and the shell at 15, 25 and 40°C are summarized in Tables 1–3. In both kernel and shell, as the temperature increased the moisture content decreased at each water activity value. At water activities higher than about 0.75, the isotherm of 40°C crossed the isotherm of 25°C. Such behaviour is known for high sugar foods (6), but in the present case where the sugar content of the material is low, such crossing was not expected. Similar behaviour has been observed in the isotherms of Pistachia terebinthus (7). In the shell, hysteresis was pronounced in the whole water activity range with almost constant separation between the two branches. The loop had the characteristic shape of cellulosic materials. In the kernel, the hysteresis was limited and mainly observed at the lower water activity range. The hysteresis of shell and kernel at 15°C is shown in Fig. 1. Similar plots for the 25 and 40°C isotherms show that hysteresis decreased as temperature increased. The parameters Xm, k and c of the GAB equation (Eqn [2]) were fitted to the experimental data using the parabolic form of the equation (Table 4): X Xm
Isosteric heat (kJ/mol)
40
c·k·aw (1–k·aw)(1–k·aw + c·k·aw)
Eqn [2]
The average of the relative % deviations between the experimental and predicted values was used as criteria to evaluate the goodness of fit. It was defined as:
30
E =
20
10
0
=
10
20
30
40
Water content (g/100 g of fat-free solids)
Fig. 2 Isosteric heat of adsorption and desorption for kernel and shell. (–––) = kernel adsorption; (………) = kernel desorption; (– – –) = shell adsorption; (–.–.–.–) = shell desorption
100 N
N
|Xe,i–Xp,i |
i=1
Xe,i
∑
Eqn [3]
where Xe,i is the experimental moisture content, Xp,i is the predicted moisture content by the GAB equation and N is the number of experimental points. As shown in Table 4, the deviation was always less than 5% and in most cases less than 2% which means that the GAB equation adequately describes the experimental results in the range of aw measured. The heat of sorption at constant water content values was calculated from the ln aw vs. 1/T plot assuming that it is constant over the temperature range 15°C to 40°C. Slopes were determined by linear regression analysis. The results, which are presented in Fig. 2, show that the
374
lwt/vol. 29 (1996) No. 4
isosteric heat of desorption was higher than the isosteric heat of adsorption for the kernel as well as for the shell, especially in the region of low moisture content. This is probably an indication of structural changes that occur in the product during dehydration which make the removal of water easier.
References 1 PONTIKIS, K. Study on the effect of the kind of pollen on the development of Pistachia vera L. nuts. Doctoral Thesis, Agricultural University of Athens (1975) 2 DENIZEL, T., ROLFE, E. AND JARVIS, B. Moisture–Equilibrium relative humidity relationships in Pistachio nuts with particular regard to control of aflatoxin formation. Journal of the Science of Food and Agriculture, 27, 1027–1034 (1976)
3 AOAC Official Methods of Analysis, 14th Edn. Method No. 27.006. (1984) 4 WOLF, W., SPIESS, W. E. L., JUNG, G., WEISSER, H., BIZOT, H. AND DUCKWORTH, R. B. The water-vapour sorption isotherms of microcrystalline cellulose and of purified potato starch. Results of a collaborative study. Journal of Food Engineering, 3, 51–73 (1984) 5 GREENSPAN, L. Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards. Physics and Chemistry, 81a, 89–96 (1977) 6 SARAVACOS, G. D., TSIOURAS, D. A. AND TSAMI, E. Effect of temperature on the water sorption isotherms of sultana raisins. Journal of Food Science, 51, 331–334 (1986) 7 AYRANCI, E. AND DALGIC, A. C. Moisture sorption isotherms of Pistachia terebinthus L. and its protein isolate. Lebensmittel-Wissenschaft und-Technologie, 25, 182–183 (1992)
375
Research Note
Water Sorption Isotherms of Pistachio Nuts S. Yanniotis* and I. Zarmboutis Agricultural University of Athens, Department of Food Science and Technology, Iera Odos 75, 118 55 Athens (Greece) (Received August 16, 1994; accepted August 16, 1995)
The adsorption and desorption isotherms of pistachio nuts (kernel and shell) were measured at 15, 25 and 40°C using a gravimetric method. Hysteresis was more pronounced in the shell than in the kernel and reduced as the temperature increased. The isosteric heat of desorption, calculated from the slope of a lnaw vs. 1/T plot, was higher than the isosteric heat of adsorption. The GAB equation gave satisfactory goodness of fit with average percent deviation of less than 5%. ©1996 Academic Press Limited
Introduction
Materials and Methods
Pistachia vera is cultivated in the Mediterranean region, in the United States and other countries. In 100 g of dried kernel of cultivar Aeginas, which is cultivated in Greece, there is about 27 g protein, 12 g carbohydrate and 53 g oil, the rest being water and minerals (1). This composition may vary with cultivar. The kernel constitutes about 50 to 55% of the nut weight and the shell the remaining 45 to 50%. Processing of the nuts includes dehulling, washing, drying to 60–80 g/kg moisture (related to whole kernel) in dryers at 65°C for 6 to 7 h or by sun-drying for 3 to 4 d, salting by immersion in 100 g/kg sodium chloride solution for 15 min, roasting at 150 to 180°C for 30 min and packaging. Studies on the water sorption characteristics of pistachio nuts are very limited. The water sorption isotherm of pistachio nuts at higher water activity (aw) values and at 20°C has been studied with particular regard to control of aflatoxin formation (2). From this investigation it was concluded that at water activity values of 0.87 to 0.88 growth of Aspergillus flavus occurred and was accompanied by aflatoxin production at 0.88 aw. At water activity values below 0.86, the growth of A. flavus and A. niger, and thus aflatoxin production, was prevented due to the competitive growth of more xerophilic fungi (i.e. Aspergillus amstelodami). The objective of the present work was to study the water sorption isotherms of pistachio nuts in more detail so that post-harvest handling and storage conditions could be established.
Materials Pistachio nuts cv. Aeginas were obtained from the Agricultural University in Spata, Attica. They were sundried and stored in bulk at room temperature for 3 to 6 months before the experiment. The water activity of the dried sample was 0.72 (measured with RotronicHygroskop DT) and the fat content 532 g/kg (3).
*To whom correspondence should be addressed.
0023-6438/96/040372 + 04$18.00/0
Equipment The gravimetric method developed by the European Cooperation in Scientific and Technological Research Project COST-90 (4) was used for this study. Saturated solutions of LiCl, MgCl2, Mg(NO3)2, NaBr, SrCl2, NaCl and KCl placed in 1L glass jars were used as sorbate source. The water activity of the salts was taken from Greenspan (5). Jars were placed in a bath with an insulated cover and the temperature maintained at the specified value (15, 25 and 40°C) within ± 0.2°C.
Procedure Initially, the kernels were separated from the shells. Both were ground in a laboratory mill (Newman equipped with 2.4 mm holes screen) to facilitate water vapour diffusion during the experiment. For the adsorption isotherm, 500 to 600 mg of ground sample were placed in each weighing bottle and dried in a vacuum oven for 6 h at 70°C. Dried samples were then placed in the sorbostat (five weighing bottles in each jar) and left for 8 to 12 d. By that time, equilibrium was reached in every jar, as determined by constant weight in three successive measurements on three consecutive days. As expected, the higher the temperature the shorter the equilibration time, and the higher the water activity in the jar the larger the equilibration time. For ©1996 Academic Press Limited
372
lwt/vol. 29 (1996) No. 4
Table 1 Experimental results for sorption at 15˚C Water content (g/100g of fat-free solids) Adsorption
Desorption
Kernel
Shell
Kernel
Shell
Water activity (aw)
x¯
sx
x¯
sx
x¯
sx
x¯
sx
0.113 0.333 0.559 0.607 0.741 0.756 0.859
3.95 7.72 12.81 13.97 21.04 21.41 32.79
0.10 0.21 0.13 0.07 0.11 0.21 0.10
3.19 6.37 9.99 10.92 15.20 16.13 21.10
0.04 0.03 0.02 0.09 0.04 0.02 0.03
5.81 9.95 n.d. 14.21 n.d. 23.14 35.37
0.30 0.33 n.d. 0.23 n.d. 0.14 0.86
5.37 10.01 n.d. 15.82 n.d. 21.04 23.51
0.05 0.06 n.d. 0.03 n.d. 0.06 0.09
Mean value (x¯) and standard deviation (sx) of five determinations. n.d.=not determined
Table 2 Experimental results for sorption at 25˚C Water content (g/100g of fat-free solids) Adsorption
Desorption
Kernel
Shell
Kernel
Shell
Water activity (aw)
x¯
sx
x¯
sx
x¯
sx
x¯
sx
0.113 0.328 0.529 0.576 0.709 0.753 0.843
3.42 7.30 11.33 11.72 17.02 20.86 29.30
0.12 0.10 0.15 0.32 0.13 0.21 0.34
2.56 5.80 8.60 10.02 12.92 14.32 18.17
0.03 0.07 0.06 0.06 0.05 0.04 0.12
5.35 8.02 11.07 12.21 18.30 20.86 29.56
0.06 0.05 0.17 0.15 0.13 0.45 0.26
3.89 8.01 11.30 13.15 17.09 17.15 20.41
0.07 0.06 0.04 0.04 0.06 0.08 0.15
Mean value (x¯) and standard deviation (sx) of five determinations.
Table 3 Experimental results for sorption at 40˚C Water content (g/100g of fat-free solids) Adsorption
Desorption
Kernel
Shell
Kernel
Shell
Water activity (aw)
x¯
sx
x¯
sx
x¯
sx
x¯
sx
0.112 0.316 0.484 0.532 0.747 0.823
3.37 6.51 9.86 10.00 19.86 26.77
0.16 0.04 0.09 0.10 0.16 0.34
2.27 5.16 7.52 8.16 14.90 17.40
0.05 0.02 0.08 0.17 0.11 0.20
4.33 6.58 10.00 10.23 20.02 27.07
0.16 0.10 0.12 0.14 0.22 0.50
3.16 6.84 9.77 10.49 16.94 19.03
0.02 0.05 0.07 0.12 0.07 0.06
Mean value (x¯) and standard deviation (sx) of five determinations.
Table 4 Parameters of the GAB equation for the adsorption and desorption isotherms of kernel and shell at 15, 25 and 40˚Ca Xm
Temperature (˚C)
Kernel
Adsorption
15 25 40
6.560 6.206 5.584
Desorption
15 25 40
6.656 5.597 5.198
aSee
K
Shell
c
Kernel
Shell
Kernel
5.795 5.715 5.381
0.938 0.940 0.970
0.862 0.835 0.874
9.747 8.457 9.609
10.404 8.885 7.683
0.941 0.969 0.990
0.695 0.721 0.775
39.989 48.778 21.000
Eqn [2] and Eqn [3].
373
E (%) Shell
Kernel
Shell
9.070 7.781 5.689
1.1 2.8 2.1
1.0 1.3 2.1
10.306 7.419 6.308
4.9 2.0 3.1
2.3 2.8 1.3
lwt/vol. 29 (1996) No. 4
Water content (g/100 g of fat-free solids)
40
deviation (D) between the mean and the individual values was determined by:
30
D =
100 5
5
|xi–¯x |
i=1
x¯
∑
Eqn [1]
where Xi is the individual value of moisture content of the five samples in each jar and x¯ is their arithmetic mean. The average D for all experimental determinations was 1.1 ± 0.9%. The highest deviation values were observed at the two ends of each sorption isotherm.
20
10
Results and Discussion 0
0.2
0.4 0.6 Water activity
0.8
1
Fig. 1 Hysteresis of kernel and shell at 15°C. (–––) = shell; (– – –) = kernel
the desorption isotherm determination, 500 to 600 mg of ground sample were placed in each weighing bottle and put in a desiccator which contained pure water at the bottom. Samples were left at room temperature to adsorb water vapour for 2 d. They were then weighed on each consecutive day for 3 to 4 d until no appreciable weight change was observed. The weighing bottles were then placed in the sorbostat (five bottles in each jar) and left for 6 to 10 d. Equilibration was determined as described for the adsorption isotherm determination. The dry weight of the samples was determined at the end of each experiment by drying in a vacuum oven for 6 h at 70°C.
Analysis of data The arithmetic mean of the water content in the five bottles of each jar was determined and plotted vs. water activity. For each experimental point, the relative %
The experimental data of the adsorption and desorption isotherms of the kernel and the shell at 15, 25 and 40°C are summarized in Tables 1–3. In both kernel and shell, as the temperature increased the moisture content decreased at each water activity value. At water activities higher than about 0.75, the isotherm of 40°C crossed the isotherm of 25°C. Such behaviour is known for high sugar foods (6), but in the present case where the sugar content of the material is low, such crossing was not expected. Similar behaviour has been observed in the isotherms of Pistachia terebinthus (7). In the shell, hysteresis was pronounced in the whole water activity range with almost constant separation between the two branches. The loop had the characteristic shape of cellulosic materials. In the kernel, the hysteresis was limited and mainly observed at the lower water activity range. The hysteresis of shell and kernel at 15°C is shown in Fig. 1. Similar plots for the 25 and 40°C isotherms show that hysteresis decreased as temperature increased. The parameters Xm, k and c of the GAB equation (Eqn [2]) were fitted to the experimental data using the parabolic form of the equation (Table 4): X Xm
Isosteric heat (kJ/mol)
40
c·k·aw (1–k·aw)(1–k·aw + c·k·aw)
Eqn [2]
The average of the relative % deviations between the experimental and predicted values was used as criteria to evaluate the goodness of fit. It was defined as:
30
E =
20
10
0
=
10
20
30
40
Water content (g/100 g of fat-free solids)
Fig. 2 Isosteric heat of adsorption and desorption for kernel and shell. (–––) = kernel adsorption; (………) = kernel desorption; (– – –) = shell adsorption; (–.–.–.–) = shell desorption
100 N
N
|Xe,i–Xp,i |
i=1
Xe,i
∑
Eqn [3]
where Xe,i is the experimental moisture content, Xp,i is the predicted moisture content by the GAB equation and N is the number of experimental points. As shown in Table 4, the deviation was always less than 5% and in most cases less than 2% which means that the GAB equation adequately describes the experimental results in the range of aw measured. The heat of sorption at constant water content values was calculated from the ln aw vs. 1/T plot assuming that it is constant over the temperature range 15°C to 40°C. Slopes were determined by linear regression analysis. The results, which are presented in Fig. 2, show that the
374
lwt/vol. 29 (1996) No. 4
isosteric heat of desorption was higher than the isosteric heat of adsorption for the kernel as well as for the shell, especially in the region of low moisture content. This is probably an indication of structural changes that occur in the product during dehydration which make the removal of water easier.
References 1 PONTIKIS, K. Study on the effect of the kind of pollen on the development of Pistachia vera L. nuts. Doctoral Thesis, Agricultural University of Athens (1975) 2 DENIZEL, T., ROLFE, E. AND JARVIS, B. Moisture–Equilibrium relative humidity relationships in Pistachio nuts with particular regard to control of aflatoxin formation. Journal of the Science of Food and Agriculture, 27, 1027–1034 (1976)
3 AOAC Official Methods of Analysis, 14th Edn. Method No. 27.006. (1984) 4 WOLF, W., SPIESS, W. E. L., JUNG, G., WEISSER, H., BIZOT, H. AND DUCKWORTH, R. B. The water-vapour sorption isotherms of microcrystalline cellulose and of purified potato starch. Results of a collaborative study. Journal of Food Engineering, 3, 51–73 (1984) 5 GREENSPAN, L. Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards. Physics and Chemistry, 81a, 89–96 (1977) 6 SARAVACOS, G. D., TSIOURAS, D. A. AND TSAMI, E. Effect of temperature on the water sorption isotherms of sultana raisins. Journal of Food Science, 51, 331–334 (1986) 7 AYRANCI, E. AND DALGIC, A. C. Moisture sorption isotherms of Pistachia terebinthus L. and its protein isolate. Lebensmittel-Wissenschaft und-Technologie, 25, 182–183 (1992)
375