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Effect of Time and Number of Cycles on Yield and Peeling Quality of Steam Peeled Potatoes and Asparagus
Lebensm.-Wiss. u.-Technol., 30, 448–451 (1997)
Effect of Time and Number of Cycles on Yield and Peeling Quality of Steam Peeled Potatoes and Asparagus Raul ´ L. Garrote, Enrique R. Silva, Ricardo A. Bertone and Adriana Avalle Instituto de Tecnolog´ıa de Alimentos (FIQ-UNL), Ciudad Universitaria, C.C. 266, (3000) Santa Fe (Argentina) (Received April 26, 1996; accepted July 23, 1996)
The effect of time and number of cycles used on yield and peeling quality of steam peeled potatoes (Ballenera variety) and asparagus (Argentuil variety) at 158 °C was studied. Heat penetration of potatoes and peroxidase residual activity of asparagus were also assessed. Best peeling quality, acceptable heat penetration and a yield of 90% were achieved for potatoes with a peeling time of 36 s and three cycles. For asparagus (two sizes considered), best peeling quality and an acceptable yield and peroxidase residual activity were obtained with a peeling time of 20 s and one cycle. Steam peeling of asparagus followed by an adiabatic holding time after steam exhausting and before water cooling might inactivate peroxidase sufficiently so that blanching prior to freezing of asparagus would not be necessary.
©1997 Academic Press Limited Keywords: steam peeling; potato; asparagus; yield; quality
Introduction Among modern methods of peeling one of the most popular is steam peeling. Its widespread application is due to its high automation, precise control of time, temperature and pressure by electronic devices to minimize peeling loss, and reduced environmental pollution as compared to chemical peeling (1). Steam peeling may be explained by a combination of two phenomena. First, building up of internal pressure due to high temperature which causes mechanical failure of the cell, and second, the effect of heat on the tissue which results in loss of rigidity and reduced turgor pressure due to biochemical changes, melting and breakdown of substances such as pectins and polysaccharides, and general disturbance and disorganization of the structure of the cell (2). It has been suggested that multi-stage steam peeling may improve efficiency and reduce peeling loss. During a multi-stage process, short consecutive heat treatments would provide the amount of heat needed to breakdown the first one or two layers of cells with minimum effect on the layers thereafter; the process of cell disruption would be greatly facilitated by the continuously changing chamber pressure, which rises and falls with the steam cycle (2). The objectives of the present investigation were: (a) to study the effect of time and number of cycles used on yield and quality of steam peeled potatoes and asparagus; and (b) to assess heat penetration of potatoes and peroxidase activity retention in asparagus, in order to
explore the feasibility, in the last case, of combining the steam peeling process with the blanching operation necessary before asparagus is frozen.
Materials and Methods Raw material Potatoes, Ballenera variety, weight 241.4 ± 10.7 g (n = 130), density 1.072 ± 0.031 g/cm3 (n = 6), skin surface area 186.1 ± 44.4 cm2 (n = 6); and asparagus, Argentuil variety, diameters 0.011, 0.012, 0.013, 0.014, 0.015 and 0.016 m, peroxidase activity 5.35 ± 1.35 AU (n = 4), were used (3).
Steam peeling apparatus A tumbling batch-type laboratory pilot-model steam peeler (inside diameter 0.17 m, inside height 0.44 m), was used (Fig. 1); steam pressure was 5.1 kg/cm2 (manometer pressure). Steam was fed through a manifold and entered two steam diffusers which distributed the steam along the length and on opposite sides of the peeler (4). Direct injection of tap water at ambient temperature into the peeling chamber through the steam diffuser was used for cooling. The peeler was preheated and then loaded; the peeling chamber was sealed and steam introduced, reaching the working pressure and temperature (158 °C) in approximately 7 s.
0023-6438/97/050448 + 04 $25.00/0/fs960191
448
©1997 Academic Press Limited
lwt/vol. 30 (1997) No. 5
Steam intake (∅ 19 mm)
Isolation
Water intake (∅ 19 mm)
Steam exhaust (∅ 19 mm)
Pipe steam diffusers (∅ 13 mm)
Fig. 1 Steam peeler apparatus
Experimental procedure Potatoes. Four potatoes were used in each treatment. Peeling times were 12, 24 and 36 s, applied in one, two or three peeling cycles of equal times; for example, if a peeling time of 36 s and two cycles were used, the total time was divided into two cycles of 18 s each; for three cycles, it was divided into three cycles of 12 s each. Steam temperature was followed using a thermocouple Ellab T and a Leeds and Northrup potentiometer. After the potatoes were cooled at 20 °C for 2 min and removed from the peeler chamber, one potato was used to determine heat penetration thickness and the other three were placed in a special basket under pressurized water for 1 min, drained and then yield, unpeeled skin surface area, heat ring thickness and peeling quality were determined, as described below (5). Peeling yield (%) was determined by weighing potatoes before and after peeling. The unpeeled skin surface area (%) remaining on the potato after peeling was measured (cm2/potato) using specially prepared transparent papers and referred to the average potato surface area. The peeling quality of potatoes was assessed by a four member panel on a scale of 1 to 7 developed by Miles Willard (Magnuson Engineers, Inc.), where optimum peeling was assigned grade 1, and with more than 90% of skin surface area unpeeled as grade 7. Heat ring thickness (mm) was determined from the central part of the potatoes. A thin slice (1 mm) was cut and placed over a mesh of known size, and the thickness of gelatinized starch was read using a magnifying glass. Heat penetration thickness in potatoes steam treated but without the pressurized water treatment was also measured. From the central part of one potato, slices of 12 3 12 3 3.5 mm (including the skin) were removed, fixed with formaldehyde–acetic acid–ethyl alcohol,
dehydrated with a series of ethanol solutions, embedded in paraffin, cut in sections of 16 µm with a Leitz Minot microtome, coloured with PAS method (Periodic acid-Schiff) and examined with an Olympus microscope, reading the thickness of gelatinized starch with a micrometer. Asparagus. Fresh asparagus, from a lot of 15 kg, were washed, cut to a length of 0.125 m and sorted into six diameters, as previously mentioned. Two groups of asparagus were formed: one including the sizes 0.011, 0.012 and 0.013 m base diameter, and the other 0.014, 0.015 and 0.016 m base diameter. Each treatment was separately applied to each group, consisting of six asparagus, two per size. The six asparagus were placed in a special basket of stainless steel mesh, and steam peeled for the following times: 10, 20 and 30 s in one or two cycles of equal times. After cooling at 20 °C for 1 min, the peel of asparagus was removed by hand under running water; asparagus were drained, and then yield, peeling quality and peroxidase activity were determined. Peeling yield (%) was determined by weighing asparagus before and after peeling. Peeling quality of peeled asparagus was assessed by a four member panel according to the following quality scale (6): 1, no peeling (asparagus is clean but with all its skin); 2, insufficient peeling (30% of the skin is retained); 3, fair peeling (10% of skin is retained); 4, acceptable peeling (minimum skin retained at bottom of spears, no tip deterioration); 5, optimum peeling (even peeling in all the asparagus surface, no tip deterioration); 6, good peeling (idem 5, but some loose fibres appear); 7, fair overpeeling (well peeled asparagus, loose fibres and some deterioration close to tip); 8, overpeeling (uneven peeling, loose fibres, some deterioration of tips); 9, excessive overpeeling (idem 8 but important deterioration of tips). Peroxidase activity was determined in fresh and steam peeled asparagus using o-phenylendiamine as a substrate and a Shimadzu QV-50 spectrophotometer (7,8). A peroxidase activity unit is defined as the absorbance change per gram of sample after 300 s of reaction at 25 °C (AU/g). Every steam peeling treatment was performed at least in triplicate. Results were evaluated using Fisher test and t-test, in order to compare means and determine statistical differences among treatments.
Results and Discussion Steam peeling of potatoes Table 1 shows the yield, unpeeled skin surface area, peeling quality, heat ring thickness and heat penetration thickness, as affected by peeling time and number of cycles used. Peeling time had a significant effect on almost all the responses at all the cycles considered. No statistical differences among cycles were found for yield and unpeeled skin at 24 and 36 s peeling times. There were statistical differences between one and three
449
lwt/vol. 30 (1997) No. 5
Table 1 Effect of peeling time and number of cycles on yield, peeling quality, unpeeled skin surface area, heat ring thickness and heat penetration thickness of steam peeled potatoes
Peeling time (s)
1
Yield (%)
Unpeeled skin (%)
Cycles
Cycles
2
100 a
3
– – 94.96db 95.38db 96.86db 88.89ec 92.17ec 89.85ec
12 24 36
1
2
3
Peeling quality
Heat ring thickness (mm)
Heat penetration thickness (mm)*
Cycles
Cycles
Cycles
1
100 a
7a 5db 2.4dc
– – b 23.0db 14.56eb 21.3d, e 0dc 0dc 0dc
2 – 5db 2.5cd, e
3
1
– 5db 1.5ce
1.93 a 2.37db 2.99dc
2
3
1
– – 2.27 2.69eb 2.69ec 3.00 c 2.65be 2.82d, e 3.65
2
3
– – 3.13 2.69 3.65 3.27
a–c Means d, e Means
with different superscripts in the same column for each response are significantly different (P < 0.05). with different subscripts in the same row for each response are significantly different (P < 0.05). * Means of only two values; no statistical evaluation performed.
Table 2 Effect of peeling time and number of cycles on yield, peeling quality and residual peroxidase activity of steam peeled asparagus of 0.011, 0.012 and 0.013 m base diameter Yield (%) Peeling time (s)
85.98a 78.89bd 69.77dc
10 20 30 a–c Means d, e Means
1 cycle
Peeling quality
2 cycles – 79.73bd 69.71dc
1 cycle 4a 4.75bd 8.67dc
Residual peroxidase activity (%)
2 cycles
1 cycle
2 cycles
– 5.5bd 9dc
47.82a 23.57bd 6.94dc
– 17.12bd 3.06ec
with different superscripts in the same column for each response are significantly different (P < 0.05). with different subscripts in the same row for each response are significantly different (P < 0.05).
Table 3 Effect of peeling time and number of cycles on yield, peeling quality and residual peroxidase activity of steam peeled asparagus of 0.014, 0.015 and 0.016 m base diameter Yield (%) Peeling time (s) 10 20 30 a–c Means d, e Means
Peeling quality
1 cycle
2 cycles
86.88a 81.44bd 77.50dc
– 78.09bd 73.10bd
1 cycle 4a 5bd 6.50dc
Residual peroxidase activity (%)
2 cycles
1 cycle
2 cycles
– 6.83be 8.33ce
49.18a 38.09bd 21.88dc
– 22.46be 10.35ec
with different superscripts in the same column for each response are significantly different (P < 0.05). with different subscripts in the same row for each response are significantly different (P < 0.05).
cycles for peeling quality at 36 s; and between 1–2 and 2–3 cycles at 24 s and 1–2 cycles at 36 s for heat ring thickness. It is seen that only at a peeling time of 36 s was a good peeling quality obtained at one or two cycles, while a very good quality was achieved with three cycles; at this last condition the yield was approximately 90%. With regard to heat penetration thickness, microscopically measured using a histological technique, it is also seen how the gelatinization front increases as a function of peeling time, a consequence of unsteady heat conduction. At 24 and 36 s and three cycles it is possible to get the minimum heat penetration thickness. So, the combination of three cycles at a total peeling time of 36 s produced the best peeling quality, acceptable heat penetration thickness and a very good peeling yield.
Steam peeling of asparagus Tables 2 and 3 show the yield, peeling quality and peroxidase residual activity of peeled asparagus, as affected by peeling time and number of cycles used, and
asparagus size. For 0.011, 0.012 and 0.013 m asparagus diameter (Table 2), peeling time had a significant effect on all the responses at both cycles considered. Keeping the peeling time constant, passing from one to two cycles had no significant effect on all the responses. It is also shown that as time increases, yield and peroxidase residual activity decrease, while the best rating for peeling quality is obtained at 20 s in one and two cycles; at 30 s overpeeling is severe. At 20 s peeling time in one or two cycles, a good yield, very good quality and quite good peroxidase inactivation are obtained. Additional enzyme inactivation may be achieved if the asparagus are adiabatically held in the steam peeler after the steam pressure has been exhausted at the end of the peeling operation and before cooling. A theoretical estimation of the equilibration-holding time was performed for the following conditions (9,10): asparagus average diameter (considered as an infinite cylinder) = 0.01105 m (corresponds to a base diameter of 0.012 m for an asparagus weight of 11.50 g and an asparagus density of 0.960 g/cm3); asparagus length = 0.125 m; steam temperature = 150 °C; asparagus initial temperature = 20 °C; asparagus average
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lwt/vol. 30 (1997) No. 5
thermal diffusivity = 1.505 3 10–7 m2/s (3); Biot number = ∞ ; peeling time = 15 s. After this peeling time, and assuming that the steam peeler instantly reaches the steam temperature, the mass average temperature of asparagus will be 89.55 °C; then the asparagus must be adiabatically held for 58 s so its centre temperature reaches a value of 88.75 °C, and almost all the peroxidase will be inactivated. In this way asparagus would be suitably blanched and the blanching operation prior to freezing would not be necessary after steam peeling. When asparagus diameters were 0.014, 0.015 and 0.016 m (Table 3), peeling time had a significant effect on all the responses except for yield at 20 and 30 s and two cycles. Keeping the peeling time constant, it is seen that two cycles produced a worse quality and less peroxidase activity (differences were significant), showing that at 20 and 30 s both treatments were more severe than at one cycle. Similar to Table 2, the best quality was achieved at a peeling time of 20 s, especially when one cycle was used. It is evident that in the case of asparagus, due to its delicate characteristics, steam peeling in one cycle is more suitable than in two cycles.
Acknowledgements The authors acknowledge the financial support of Universidad Nacional del Litoral (Programacion ´ CAID). They also gratefully acknowledge the assistance of Lic. Victor R. Coutaz.
References 1 BURKHARDT, G. J., MERKEL, J. A. AND SCOTT, L. E. A highpressure steam peeler for fruits and vegetables. ASAE Paper No. A-1660, presented at the Winter Meeting of the American Society of Agricultural Engineers, Chicago, IL (1970) 2 FLOROS, J. D. AND CHINNAN, M. S. Microstructural changes during steam peeling of fruits and vegetables. Journal of Food Science, 53, 849–853 (1988) 3 GARROTE, R. L., SILVA, E. R. AND BERTONE, R. A. Two stage water blanching of asparagus. International Journal of Refrigeration, 18, 148–152 (1995) 4 SMITH, D. A. Peeled yield and quality of ‘Delicious’ apples peeled by superheated steam, saturated steam, and caustic peeling. Journal of the American Society of Horticulture Science, 109, 364–367 (1984) 5 GARROTE, R. L., COUTAZ, V. R., LUNA, J. A., SILVA, E. R. AND BERTONE, R. A. Optimizing processing conditions for chemical peeling of potatoes using Response Surface Methodology. Journal of Food Science, 58, 821–826 (1993) 6 GARROTE, R. L., COUTAZ, V. E., SILVA, E. R. AND BERTONE, R. A. Determining process conditions for chemical peeling of asparagus. Lebensmittel-Wissenschaft und-Technology, 27, 19–22 (1994) 7 VETTER, J. L., STEINBERG, M. P. AND NELSON, A. J. Quantitative determination of peroxidase in sweet corn. Agriculture Food Chemistry, 6, 39–41 (1958) 8 GARROTE, R. L., SILVA, E. R. AND BERTONE, R. A. Distr´ıbution e inactivacion ´ termica ´ de las enzimas peroxidasa y lipoxigenas en el choclo (Zea mays). Revista de Agroqu´ımica y Tecnolog´ıa de Alimentos, 25, 373–383 (1985) 9 LUIKOV, A. V. Analytical heat diffusion theory. New York and London: Academic Press (1968) 10 LENZ, M. K. AND LUND, D. B. Temperature distributions during heat/hold processing of food. Journal of Food Science, 38, 630–632 (1973)
451
Effect of Time and Number of Cycles on Yield and Peeling Quality of Steam Peeled Potatoes and Asparagus Raul ´ L. Garrote, Enrique R. Silva, Ricardo A. Bertone and Adriana Avalle Instituto de Tecnolog´ıa de Alimentos (FIQ-UNL), Ciudad Universitaria, C.C. 266, (3000) Santa Fe (Argentina) (Received April 26, 1996; accepted July 23, 1996)
The effect of time and number of cycles used on yield and peeling quality of steam peeled potatoes (Ballenera variety) and asparagus (Argentuil variety) at 158 °C was studied. Heat penetration of potatoes and peroxidase residual activity of asparagus were also assessed. Best peeling quality, acceptable heat penetration and a yield of 90% were achieved for potatoes with a peeling time of 36 s and three cycles. For asparagus (two sizes considered), best peeling quality and an acceptable yield and peroxidase residual activity were obtained with a peeling time of 20 s and one cycle. Steam peeling of asparagus followed by an adiabatic holding time after steam exhausting and before water cooling might inactivate peroxidase sufficiently so that blanching prior to freezing of asparagus would not be necessary.
©1997 Academic Press Limited Keywords: steam peeling; potato; asparagus; yield; quality
Introduction Among modern methods of peeling one of the most popular is steam peeling. Its widespread application is due to its high automation, precise control of time, temperature and pressure by electronic devices to minimize peeling loss, and reduced environmental pollution as compared to chemical peeling (1). Steam peeling may be explained by a combination of two phenomena. First, building up of internal pressure due to high temperature which causes mechanical failure of the cell, and second, the effect of heat on the tissue which results in loss of rigidity and reduced turgor pressure due to biochemical changes, melting and breakdown of substances such as pectins and polysaccharides, and general disturbance and disorganization of the structure of the cell (2). It has been suggested that multi-stage steam peeling may improve efficiency and reduce peeling loss. During a multi-stage process, short consecutive heat treatments would provide the amount of heat needed to breakdown the first one or two layers of cells with minimum effect on the layers thereafter; the process of cell disruption would be greatly facilitated by the continuously changing chamber pressure, which rises and falls with the steam cycle (2). The objectives of the present investigation were: (a) to study the effect of time and number of cycles used on yield and quality of steam peeled potatoes and asparagus; and (b) to assess heat penetration of potatoes and peroxidase activity retention in asparagus, in order to
explore the feasibility, in the last case, of combining the steam peeling process with the blanching operation necessary before asparagus is frozen.
Materials and Methods Raw material Potatoes, Ballenera variety, weight 241.4 ± 10.7 g (n = 130), density 1.072 ± 0.031 g/cm3 (n = 6), skin surface area 186.1 ± 44.4 cm2 (n = 6); and asparagus, Argentuil variety, diameters 0.011, 0.012, 0.013, 0.014, 0.015 and 0.016 m, peroxidase activity 5.35 ± 1.35 AU (n = 4), were used (3).
Steam peeling apparatus A tumbling batch-type laboratory pilot-model steam peeler (inside diameter 0.17 m, inside height 0.44 m), was used (Fig. 1); steam pressure was 5.1 kg/cm2 (manometer pressure). Steam was fed through a manifold and entered two steam diffusers which distributed the steam along the length and on opposite sides of the peeler (4). Direct injection of tap water at ambient temperature into the peeling chamber through the steam diffuser was used for cooling. The peeler was preheated and then loaded; the peeling chamber was sealed and steam introduced, reaching the working pressure and temperature (158 °C) in approximately 7 s.
0023-6438/97/050448 + 04 $25.00/0/fs960191
448
©1997 Academic Press Limited
lwt/vol. 30 (1997) No. 5
Steam intake (∅ 19 mm)
Isolation
Water intake (∅ 19 mm)
Steam exhaust (∅ 19 mm)
Pipe steam diffusers (∅ 13 mm)
Fig. 1 Steam peeler apparatus
Experimental procedure Potatoes. Four potatoes were used in each treatment. Peeling times were 12, 24 and 36 s, applied in one, two or three peeling cycles of equal times; for example, if a peeling time of 36 s and two cycles were used, the total time was divided into two cycles of 18 s each; for three cycles, it was divided into three cycles of 12 s each. Steam temperature was followed using a thermocouple Ellab T and a Leeds and Northrup potentiometer. After the potatoes were cooled at 20 °C for 2 min and removed from the peeler chamber, one potato was used to determine heat penetration thickness and the other three were placed in a special basket under pressurized water for 1 min, drained and then yield, unpeeled skin surface area, heat ring thickness and peeling quality were determined, as described below (5). Peeling yield (%) was determined by weighing potatoes before and after peeling. The unpeeled skin surface area (%) remaining on the potato after peeling was measured (cm2/potato) using specially prepared transparent papers and referred to the average potato surface area. The peeling quality of potatoes was assessed by a four member panel on a scale of 1 to 7 developed by Miles Willard (Magnuson Engineers, Inc.), where optimum peeling was assigned grade 1, and with more than 90% of skin surface area unpeeled as grade 7. Heat ring thickness (mm) was determined from the central part of the potatoes. A thin slice (1 mm) was cut and placed over a mesh of known size, and the thickness of gelatinized starch was read using a magnifying glass. Heat penetration thickness in potatoes steam treated but without the pressurized water treatment was also measured. From the central part of one potato, slices of 12 3 12 3 3.5 mm (including the skin) were removed, fixed with formaldehyde–acetic acid–ethyl alcohol,
dehydrated with a series of ethanol solutions, embedded in paraffin, cut in sections of 16 µm with a Leitz Minot microtome, coloured with PAS method (Periodic acid-Schiff) and examined with an Olympus microscope, reading the thickness of gelatinized starch with a micrometer. Asparagus. Fresh asparagus, from a lot of 15 kg, were washed, cut to a length of 0.125 m and sorted into six diameters, as previously mentioned. Two groups of asparagus were formed: one including the sizes 0.011, 0.012 and 0.013 m base diameter, and the other 0.014, 0.015 and 0.016 m base diameter. Each treatment was separately applied to each group, consisting of six asparagus, two per size. The six asparagus were placed in a special basket of stainless steel mesh, and steam peeled for the following times: 10, 20 and 30 s in one or two cycles of equal times. After cooling at 20 °C for 1 min, the peel of asparagus was removed by hand under running water; asparagus were drained, and then yield, peeling quality and peroxidase activity were determined. Peeling yield (%) was determined by weighing asparagus before and after peeling. Peeling quality of peeled asparagus was assessed by a four member panel according to the following quality scale (6): 1, no peeling (asparagus is clean but with all its skin); 2, insufficient peeling (30% of the skin is retained); 3, fair peeling (10% of skin is retained); 4, acceptable peeling (minimum skin retained at bottom of spears, no tip deterioration); 5, optimum peeling (even peeling in all the asparagus surface, no tip deterioration); 6, good peeling (idem 5, but some loose fibres appear); 7, fair overpeeling (well peeled asparagus, loose fibres and some deterioration close to tip); 8, overpeeling (uneven peeling, loose fibres, some deterioration of tips); 9, excessive overpeeling (idem 8 but important deterioration of tips). Peroxidase activity was determined in fresh and steam peeled asparagus using o-phenylendiamine as a substrate and a Shimadzu QV-50 spectrophotometer (7,8). A peroxidase activity unit is defined as the absorbance change per gram of sample after 300 s of reaction at 25 °C (AU/g). Every steam peeling treatment was performed at least in triplicate. Results were evaluated using Fisher test and t-test, in order to compare means and determine statistical differences among treatments.
Results and Discussion Steam peeling of potatoes Table 1 shows the yield, unpeeled skin surface area, peeling quality, heat ring thickness and heat penetration thickness, as affected by peeling time and number of cycles used. Peeling time had a significant effect on almost all the responses at all the cycles considered. No statistical differences among cycles were found for yield and unpeeled skin at 24 and 36 s peeling times. There were statistical differences between one and three
449
lwt/vol. 30 (1997) No. 5
Table 1 Effect of peeling time and number of cycles on yield, peeling quality, unpeeled skin surface area, heat ring thickness and heat penetration thickness of steam peeled potatoes
Peeling time (s)
1
Yield (%)
Unpeeled skin (%)
Cycles
Cycles
2
100 a
3
– – 94.96db 95.38db 96.86db 88.89ec 92.17ec 89.85ec
12 24 36
1
2
3
Peeling quality
Heat ring thickness (mm)
Heat penetration thickness (mm)*
Cycles
Cycles
Cycles
1
100 a
7a 5db 2.4dc
– – b 23.0db 14.56eb 21.3d, e 0dc 0dc 0dc
2 – 5db 2.5cd, e
3
1
– 5db 1.5ce
1.93 a 2.37db 2.99dc
2
3
1
– – 2.27 2.69eb 2.69ec 3.00 c 2.65be 2.82d, e 3.65
2
3
– – 3.13 2.69 3.65 3.27
a–c Means d, e Means
with different superscripts in the same column for each response are significantly different (P < 0.05). with different subscripts in the same row for each response are significantly different (P < 0.05). * Means of only two values; no statistical evaluation performed.
Table 2 Effect of peeling time and number of cycles on yield, peeling quality and residual peroxidase activity of steam peeled asparagus of 0.011, 0.012 and 0.013 m base diameter Yield (%) Peeling time (s)
85.98a 78.89bd 69.77dc
10 20 30 a–c Means d, e Means
1 cycle
Peeling quality
2 cycles – 79.73bd 69.71dc
1 cycle 4a 4.75bd 8.67dc
Residual peroxidase activity (%)
2 cycles
1 cycle
2 cycles
– 5.5bd 9dc
47.82a 23.57bd 6.94dc
– 17.12bd 3.06ec
with different superscripts in the same column for each response are significantly different (P < 0.05). with different subscripts in the same row for each response are significantly different (P < 0.05).
Table 3 Effect of peeling time and number of cycles on yield, peeling quality and residual peroxidase activity of steam peeled asparagus of 0.014, 0.015 and 0.016 m base diameter Yield (%) Peeling time (s) 10 20 30 a–c Means d, e Means
Peeling quality
1 cycle
2 cycles
86.88a 81.44bd 77.50dc
– 78.09bd 73.10bd
1 cycle 4a 5bd 6.50dc
Residual peroxidase activity (%)
2 cycles
1 cycle
2 cycles
– 6.83be 8.33ce
49.18a 38.09bd 21.88dc
– 22.46be 10.35ec
with different superscripts in the same column for each response are significantly different (P < 0.05). with different subscripts in the same row for each response are significantly different (P < 0.05).
cycles for peeling quality at 36 s; and between 1–2 and 2–3 cycles at 24 s and 1–2 cycles at 36 s for heat ring thickness. It is seen that only at a peeling time of 36 s was a good peeling quality obtained at one or two cycles, while a very good quality was achieved with three cycles; at this last condition the yield was approximately 90%. With regard to heat penetration thickness, microscopically measured using a histological technique, it is also seen how the gelatinization front increases as a function of peeling time, a consequence of unsteady heat conduction. At 24 and 36 s and three cycles it is possible to get the minimum heat penetration thickness. So, the combination of three cycles at a total peeling time of 36 s produced the best peeling quality, acceptable heat penetration thickness and a very good peeling yield.
Steam peeling of asparagus Tables 2 and 3 show the yield, peeling quality and peroxidase residual activity of peeled asparagus, as affected by peeling time and number of cycles used, and
asparagus size. For 0.011, 0.012 and 0.013 m asparagus diameter (Table 2), peeling time had a significant effect on all the responses at both cycles considered. Keeping the peeling time constant, passing from one to two cycles had no significant effect on all the responses. It is also shown that as time increases, yield and peroxidase residual activity decrease, while the best rating for peeling quality is obtained at 20 s in one and two cycles; at 30 s overpeeling is severe. At 20 s peeling time in one or two cycles, a good yield, very good quality and quite good peroxidase inactivation are obtained. Additional enzyme inactivation may be achieved if the asparagus are adiabatically held in the steam peeler after the steam pressure has been exhausted at the end of the peeling operation and before cooling. A theoretical estimation of the equilibration-holding time was performed for the following conditions (9,10): asparagus average diameter (considered as an infinite cylinder) = 0.01105 m (corresponds to a base diameter of 0.012 m for an asparagus weight of 11.50 g and an asparagus density of 0.960 g/cm3); asparagus length = 0.125 m; steam temperature = 150 °C; asparagus initial temperature = 20 °C; asparagus average
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lwt/vol. 30 (1997) No. 5
thermal diffusivity = 1.505 3 10–7 m2/s (3); Biot number = ∞ ; peeling time = 15 s. After this peeling time, and assuming that the steam peeler instantly reaches the steam temperature, the mass average temperature of asparagus will be 89.55 °C; then the asparagus must be adiabatically held for 58 s so its centre temperature reaches a value of 88.75 °C, and almost all the peroxidase will be inactivated. In this way asparagus would be suitably blanched and the blanching operation prior to freezing would not be necessary after steam peeling. When asparagus diameters were 0.014, 0.015 and 0.016 m (Table 3), peeling time had a significant effect on all the responses except for yield at 20 and 30 s and two cycles. Keeping the peeling time constant, it is seen that two cycles produced a worse quality and less peroxidase activity (differences were significant), showing that at 20 and 30 s both treatments were more severe than at one cycle. Similar to Table 2, the best quality was achieved at a peeling time of 20 s, especially when one cycle was used. It is evident that in the case of asparagus, due to its delicate characteristics, steam peeling in one cycle is more suitable than in two cycles.
Acknowledgements The authors acknowledge the financial support of Universidad Nacional del Litoral (Programacion ´ CAID). They also gratefully acknowledge the assistance of Lic. Victor R. Coutaz.
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