PaPer Ital. J. Food Sci., vol. 28 - 2016 1 - Keywords: squash, thermal treatment, texture properties, convection steam oven - Changes in the texture of butternut squash following thermal treatment b. ślasKa-grzywna1, a. bliCharz-Kania1, a. sagan1*, r. nadulsKi2, z. hanusz3, d. andrejKo1 , m. szmigielsKi1 University of Life Sciences in Lublin, Poland 1 Department of Biological Bases of Food and Feed Technologies 2 Department of Food Process Engineering and Machines 3 Department of Applied Mathematics and Computer Science *Corresponding author: agnieszka.sagan@up.lublin.pl AbstrAct samples of butternut squash were heated in a convection steam oven at the temperature of 80°c and 100°c without any/or with addition of steam. the most significant changes of texture properties in the pulp were registered regarding its hardness and chewiness, while the alterations of its springiness and cohesiveness occurred within a smaller range. the decisive influence on changing the hardness and chewiness of butternut pulp was observed for the addition of steam, and, to a lower extent, for the time and temperature of treatment; in case of springiness the vital factor was the temperature of the process. mailto:agnieszka.sagan%40up.lublin.pl?subject= 2 Ital. J. Food Sci., vol. 28 - 2016 INtrODUctION Pumpkin belongs to the family of Cucurbita- ceae. Its edible part is the pulp of the fruit at different stages of ripeness, as well as its seeds (giant pumpkin and summer squash). the nu- tritional values of pumpkin fruit are high. this is determined primarily by a high content of ca- rotenoids (from 2 to 10 mg·100 g-1), which are characterized by antioxidant and anticancer- ous properties. Pumpkin fruit provides a good source of vitamins c, A and b, as well as min- erals, such as potassium, phosphorus, calci- um, magnesium, iron and selenium. they also contain organic acids (citric, malic and fumar- ic). studies have revealed that the polysaccha- rides extracted from pumpkin have hypolipidem- ic activity. Additionally, pumpkin has low con- tent of calories. Due to the presence of numer- ous, easily absorbed nutrients, it can be used as a component of slimming diets (cArVALHO et al., 2012; NAWIrsKA-OLsZAŃsKA et al., 2014; rAKAcJEVA et al., 2011; WOJDYLA et al., 2007; ZHAO et al., 2014). Pumpkin pulp may be a healthy and valuable component of many dishes and fruit products. It is used to manufacture juices, baby foods and canned foods. A disadvantage of pumpkin, which may contribute to its low consumption, is its bland flavor and specific cucumber-like smell. this problem may be resolved by mixing pump- kin with other materials (e.g. cornel berries or quinces) in order to obtain food products of bet- ter sensory properties and chemical composi- tion (NAWIrsKA-OLsZAŃsKA et al., 2012). Pump- kin seeds are used in bakery, oil manufacturing and in pharmaceutical industry. Oil from pump- kin seeds contains valuable bioactive elements: squalene, unsaturated fatty acids, tocopherols (ObIEDZIÑsKA and WAsZKIEWIcZ-rObAK, 2012). Prior to consumption, pumpkin fruits are sub- jected to different types of treatment, most often thermal processing, during which their proper- ties undergo changes (MAYOr et al., 2011; ŒLAs- KA-GrZYWNA et al., 2013), hence the aim of the study was to determine the changes in texture properties caused by thermal treatment in a con- vection steam oven with different parameters of the processes. MAtErIALs AND MEtHODs Raw material research material was provided by butternut squash (Cucurbita moschata Duch.) originating from Portugal and purchased in London chain supermarkets. butternut squash is an annual plant belonging to the gourd family (Cucurbita- ceae), from Latin America. 100 g of butternut squash contains (after cooking): 0.9 g of protein, 7.4 g of carbohydrates, including 3.9 g sugars, 0.1 g of fat, 1.4 g of fiber, trace amounts of salt, 15 mg of (19% of rDA) vitamin c. Its caloric val- ue is 156 kJ/37 kcal in 100 g. In the study we used ripe, healthy fruits, without any mechan- ical damage. Treatment the pumpkin was subjected to preliminary treatment: washing, peeling, removing the seeds. such material was used to cut out samples for analyses. the pumpkin was sliced into 1-centim- eter-thick slices. Next, cylinders of 2-centimeter diameter were cut out from the central part of the slices with the use of a calibrator. In this way cylinders were obtained of 1-centimeter height (h) and the diameter Φ = 2 cm. six representa- tive samples (cylinders) were selected for tests from each measurement series. treatment was conducted in a convection steam oven (HOUNӦ combislim cPE 2306 model, randers, Denmark) at the temperature of 80 °c and 100 °c; 0, 20, 40, 60, 80 and 100% of steam added in rela- tion to the initial humidity in the oven chamber; treatment time: 5, 10, 15, 20, 25 and 30 min. For the temperature of 100 °c only the 0, 20, 40, 60% of steam addition were conducted. In case of steam addition of 80 and 100% result- ed in structural changes going too far (over- cooking), which made it impossible to carry out strength tests. Strength tests Immediately after thermal treatment warm samples were subjected to strength tests. com- pression strength measurement for pumpkin samples was performed in the strength test machine, Zwick/roell Z.5. (Zwick roell Polska, Łódź, Poland). the material was subjected to double compression at the speed of head move- ment equal to 50 mm·min-1. the process of com- pression was carried out at a stable deformation of the plates equal to 50% of their height, while the interval between the series was 5 s. the mea- surements were performed in 6 replications. On the basis of the measurements obtained in the form of texturegrams in the arrangement of two coordinates of strength and time, the following texture parameters were determined: hardness, springiness, chewiness and cohesiveness. After the tests, the results of the measurement were subjected to a statistical analysis. Namely, a double variance analysis was performed with the interaction for each of the analyzed properties with six variations for the temperature of 80°c and four variations for the temperature of 100°c. Statistical analysis Detailed comparisons of the mean values in pairs were performed on the basis of tukey’s multiple confidence intervals. Also, a com- Ital. J. Food Sci., vol. 28 - 2016 3 bined analysis of the four variations was per- formed for the temperatures of 80°c and 100°c, with the use of triple cross classification with interactions. calculations were done in the sAs Enterprise Guide 5.1 software, adopting the significance level of 0.05 in all the statistical analyses. rEsULts the results regarding changes in the hard- ness of pumpkin pulp resulting from thermal treatment at the temperature of 80°c and 100°c during the time from 5 to 30 min at different levels of steam addition [%] are presented in Fig. 1(a,b). A significant impact of heating time and the amount of steam on the hardness of pump- kin hardness was observed. In case of pumpkin heated at the tempera- ture of 80ºc, the hardness of the pulp was decreasing along with increasing the amount of steam added for all the analyzed heating time spans (Fig. 1a). On the other hand, the hardness of pumpkin pulp heated at the tem- perature of 100ºc was decreasing along with the amount of steam added for all time spans adopted in the research program, except for the shortest period of 5 min (Fig. 1b). With this particular heating time and steam addition, an increase in pulp hardness was noted from 40 to 60%. regardless of the adopted heating temperature, the lowest hardness was recorded for pumpkin pulp after heating it for 30 min. to analyze the effect of heating time and the amount of steam added during heating we used double cross classification with interaction. the analysis conducted suggests that heating time, the amount of steam added and the interaction between the heating time and the amount of steam added significantly differentiate pumpkin hardness. the results of a detailed comparative analysis of mean pumpkin hardness, based on tukey›s multiple comparisons are presented in table 1 for butternut squash heated at the temperature of 80ºc, and 100ºc. the data in table 1 suggest that the aver- age hardness of pumpkin pulp heated at the temperature of 80ºc for 5, 10 and 15 min does not differ significantly, and it is significantly higher than the hardness of pumpkin pulp heated for 20, 25 and 30 min. Analyzing the amount of steam added, the significantly highest value was recorded when no steam was added, while the significantly lowest value was with a 100% addition of steam. the analysis of table 1 demonstrates that the average hardness of pumpkin pulp heated at the temperature of 100 ºc for 5 min was significantly higher than the hardness of squash heated for a longer time. the significantly lowest hardness of pumpkin pulp was obtained for the heating periods of 20, 25 and 30 min. the significantly highest hardness of squash was noted when no steam was add- ed. No significant differences in the mean hard- ness of pumpkin pulp were observed with add- ing 20, 40 and 60% of steam. the results of studies on the springiness of pumpkin pulp heated at the temperature of 80 ºc and 100ºc depending on the amount of steam added [%] and the time of heating [min] are pre- sented in Fig. 1(c,d). the springiness of pumpkin pulp heated at the temperature of 80ºc decreased when the amount of steam added increased from 0 to 40%, while a further increase in the amount of steam from 40% to 100% resulted in an increased val- ue of springiness for the majority of heating pe- riods. A different course of changes in springi- ness of squash heated during the shortest time of 5 min may be observed. the springiness of squash heated at the temperature of 100 ºc in- creased along with the amount of steam added for the analyzed heating periods, except for the time of 15 min (Fig. 1d). For this particular pe- riod springiness decreased with increasing the table 1 - the results of tukey’s studentized range test for mean hardness values of pumpkin pulp heated at the tempera- ture of 80ºc and 100ºc depending on heating time and the amount of steam added. Temperature 80ºC Time [min] 5 10 15 20 25 30 Mean values 205.92A 187.79A 185.51A 150.30B 129.10B 133.80B SD 27.07 25.19 26.02 24.20 19.01 25.58 Steam [%] 0 20 40 60 80 100 Mean values 442.17a 230.78b 118.33c 107.69c 68.39d 25.06e SD 12.05 14.06 7.38 8.82 3.82 1.09 100ºC Time [min] 5 10 15 20 25 30 Mean values 126.96A 86.85B 49.08BC 32.67CD 33.11D 67.10D SD 24.96 20.78 11.60 6.26 6.32 20.99 Steam [%] 0 20 40 60 - - Mean values 184.38a 33.74b 21.72b 23.99b - - SD 17.43 1.58 1.70 5.40 - - Means with the same letter are not significantly different at 0.05 significance level. 4 Ital. J. Food Sci., vol. 28 - 2016 Fig. 1 - Analyzed properties of pumpkin pulp depending on amount of steam added and heating time: hardness of pumpkin pulp heated at the temperature of 80ºc (a) and 100ºc (b); springiness of pumpkin pulp heated at the temperature of 80ºc (c) and 100ºc (d); chewiness of pumpkin pulp heated at the temperature of 80ºc (e) and 100ºc (f); cohesiveness of pumpkin pulp heated at the temperature of 80ºc (g) and 100ºc (h). Ital. J. Food Sci., vol. 28 - 2016 5 amount of steam from 40 to 60%. the highest springiness of pulp was recorded for squash heated for the shortest time. On the basis of double cross classification with interaction it can be concluded that the heating time, the amount of steam added and the interaction of heating time and the amount of steam added significantly differentiate the springiness of pumpkin pulp subjected to ther- mal treatment both at the temperature of 80°c and 100°c. the results of a detailed compara- tive analysis of mean springiness of pumpkin pulp on the basis of tukey’s studentized range test are presented in table 2. Analyzing the results presented in table 2, it should be noted that the average springiness of pumpkin pulp heated at the temperature of 80°c for 5 min was the highest, yet it was not significantly higher than the mean springiness of squash for the heating time of 20, 25 and 30 min. the lowest mean springiness was obtained for the heating time of 10 min. On the other hand, the highest mean springiness of pumpkin pulp was obtained after adding 100% of steam, while the significantly lowest mean values of springi- ness were recorded in the situation when 20, 40 and 60% of steam was added. table 2 suggests that the mean springiness of squash heated at the temperature of 100°c for the time of 5 min was significantly highest at the level of significance of 0.05. the significant- ly lowest springiness of squash was obtained for the heating periods of 25 and 30 min. Analyz- ing the amount of steam added, the significant- ly highest springiness of squash was recorded with adding 60% of steam. No significant differ- ences of mean squash springiness were noted when there was no steam addition or when 20% of steam was added. the results of studies on the chewiness of squash heated at the temperature of 80°c and at 100°c depending on heating time and the amount of steam added are presented in Fig. 1(e,f). the chewiness of squash heated at the temperature of 80°c was decreasing when the amount of steam added increased from 0 to 20 and 40%, and when the addition of steam con- tinued increasing no drop in chewiness was ob- served. the course of curves presented in Fig. 1f suggests that the chewiness of squash heat- ed at 100°c for 5 min differed from the chewi- ness of squash heated for longer periods includ- ed in the research program. In case of the peri- ods of 10, 15, 20, 25 and 30 min the chewiness of squash altered only slightly. Double cross classification with interaction used to analyze the effect of heating time and the amount of steam added during heating on changing chewiness of butternut squash re- vealed that the heating time, the amount of steam added and the interaction of the heating time and the amount of steam added do not dif- ferentiate significantly the chewiness of squash. the results of a detailed comparative analysis of average chewiness of pumpkin pulp based on tukey’s multiple comparisons are present- ed in table 3. While analyzing the results presented in ta- ble 3, it should be noted that the mean chewi- ness of squash heated at the temperature of 80ºc for 5, 10 and 15 min was the highest and it was significantly higher than the mean chewiness of squash for the heating time of 20, 25 and 30 min. On the other hand, the significantly highest chewiness of squash was obtained when there was no steam addition. No significant differentiation in the chewiness of squash was observed when 15, 20, 25 and 30% of steam was added. the analysis of table 3 demonstrates that the mean chewiness of squash heated at the temperature of 100ºc for 5 min was significantly highest at the significance level of 0.05. the significantly lowest chewiness of squash was observed for the periods of 15, 20, 25 and 30 min. Analyzing the amount of steam, the significantly highest chewiness of squash was obtained when no steam table 2 - the results of tukey’s studentized range test for mean springiness values of pumpkin pulp heated at the tempera- ture of 80ºc and 100ºc depending on heating time and the amount of steam added. Temperature 80ºC Time [min] 5 10 15 20 25 30 Mean values 0.390A 0.275C 0.325BC 0.350AB 0.339AB 0.362AB SD 0.030 0.023 0.030 0.027 0.023 0.027 Steam [%] 0 20 40 60 80 100 Mean values 0.299c 0.239d 0.237d 0.294cd 0.426b 0.545a SD 0.008 0.005 0.027 0.021 0.020 0.028 100ºC Time [min] 5 10 15 20 25 30 Mean values 0.568A 0.467B 0.297CD 0.338C 0.207DE 0.140E SD 0.044 0.041 0.028 0.031 0.020 0.009 Steam [%] 0 20 40 60 - - Mean values 0.268c 0.292bc 0.353b 0.431a - - SD 0.022 0.022 0.028 0.053 - - Means with the same letter are not significantly different at 0.05 significance level. 6 Ital. J. Food Sci., vol. 28 - 2016 was added. No significant differences of mean values of chewiness were noted when adding 20, 40 and 60% of steam. the results of studies on the cohesiveness of squash heated at the temperature of 80 oc and at 100°c depending on heating time [min] and the amount of steam added [%] are presented in Fig. 1(g,h). the cohesiveness of squash heated at the tem- perature of 80ºc was changing depending on the amount of steam added and the course varied depending on the heating time (Fig. 1g). the highest cohesiveness was observed for squash to which no steam was added and when 100% of steam was supplied. In case of treatment at the temperature of 100°c for the heating peri- ods of 10, 15, 20, 25 and 30 min the cohesive- ness of squash generally increased along with the amount of steam added and it reached its highest values for 30% of steam (Fig. 1h). After heating for 5 min the cohesiveness of squash decreased along with increasing the amount of steam added. the results of double cross classification with table 3 - the results of tukey’s studentized range test for mean chewiness values of pumpkin pulp heated at the tempera- ture of 80ºc and 100ºc depending on heating time and the amount of steam added. Temperature 80ºC Time [min] 5 10 15 20 25 30 Mean values 2.499A 2.005A 1.863AB 1.211BC 0.879C 1.249BC SD 0.557 0.492 0.420 0.228 0.114 0.281 Steam [%] 0 20 40 60 80 100 Mean values 5.7231a 1.5236b 0.7317c 0.5767c 0.6447c 0.5058c SD 0.5429 0.0946 0.1416 0.0233 0.0295 0.0372 100ºC Time [min] 5 10 15 20 25 30 Mean values 3.349A 1.023B 0.411C 0.356C 0.282C 0.363C SD 0.836 0.172 0.071 0.030 0.051 0.093 Steam [%] 0 20 40 60 - - Mean values 2.523a 0.408b 0.356b 0.569b - - SD 0.591 0.035 0.049 0.134 Means with the same letter are not significantly different at 0.05 significance level. interaction used to analyze the effect of heat- ing time and the amount of steam added during heating on changing cohesiveness of butter- nut squash revealed that the heating time, the amount of steam added and the interaction of the heating time and the amount of steam add- ed significantly differentiate the cohesiveness of squash. A detailed comparative analysis of mean values of squash cohesiveness on the ba- sis of tukey’s multiple comparisons revealed that the average cohesiveness of squash heated at the temperature of 80ºc for 5 and 10 min was significantly higher, as compared with the mean cohesiveness of squash heated for 20 and 25 min (table 4). the significantly highest cohesiveness of pumpkin pulp was recorded after heating with no steam addition and after adding 100% of steam. No significant differentiation of squash cohesiveness was observed after adding 15, 20, 25 and 30% of steam. An analysis of table 4 shows that the mean co- hesiveness of squash heated at the temperature of 100ºc for 20, 25 and 30 min was significantly table 4 - the results of tukey’s studentized range test for mean cohesiveness values of pumpkin pulp heated at the tempera- ture of 80ºc and 100ºc depending on heating time and the amount of steam added. Temperature 80ºC Time [min] 5 10 15 20 25 30 Mean values 0.0322A 0.0311A 0.0286AB 0.0264B 0.0261B 0.0292AB SD 0.0022 0.0025 0.0020 0.0014 0.0011 0.0016 Steam [%] 0 20 40 60 80 100 Mean values 0.0408a 0.0278b 0.0222c 0.0211c 0.0244bc 0.0372a SD 0.0029 0.0009 0.0008 0.0005 0.0011 0.0010 100ºC Time [min] 5 10 15 20 25 30 Mean values 0.0404C 0.0388C 0.0458B 0.0525A 0.0538A 0.0513A SD 0.0034 0.0023 0.0041 0.0040 0.0041 0.0026 Steam [%] 0 20 40 60 - - Mean values 0.0361c 0.0419b 0.0453b 0.0650a - - SD 0.0027 0.0011 0.0015 0.0035 - - Means with the same letter are not significantly different at 0.05 significance level. Ital. J. Food Sci., vol. 28 - 2016 7 highest at the significance level of 0.05. the sig- nificantly lowest mean cohesiveness of butter- nut squash was observed after 5 and 10 min of heating. Analyzing the amount of steam added, the significantly highest mean cohesiveness of squash was recorded after treatment with the addition of 100% of steam, while the significant- ly lowest values were noted when no steam was used. the results of three-agent variance analyses for the studied properties, namely hardness, springiness, chewiness and cohesiveness pro- vide a basis for claiming that all the agents an- alyzed in the present work, namely temperature, the amount of steam added, heating time and the interactions occurring between these agents significantly differentiate the studied qualities of butternut squash. the results of tukey’s multi- ple comparisons in pairs for the analyzed agents and for all the properties are presented in table 5. these results show that there were significant differences concerning the analyzed properties of squash at the temperature of 80ºc and 100ºc. For hardness and chewiness the mean values of the properties at the temperature of 80ºc were significantly higher, as compared with the mean values of the properties at the temperature of 100ºc. In case of springiness and cohesiveness, it may be claimed that the mean value at the tem- perature of 100ºc is significantly higher than the mean value at 80ºc. Analyzing the comparisons of mean values for the properties with the use of different amounts of steam, it may be noted that the significantly highest mean hardness and chewiness were observed when no steam was added. On the other hand, the significantly highest springiness and cohesiveness were observed for the biggest amount of steam added, amounting to 60%. the most varied mean values of the studied properties were observed for the periods of treatment studied in the work. Only in case of the mean cohesiveness there were no significant differences for the pairs of mean val- ues compared. the highest values of mean hard- ness, springiness and chewiness were noted for the shortest treatment time. structural and rheological properties deter- mine behavior of the squash pulp under com- pression (sHIrMOHAMMADI et al., 2014). recog- nizing the mechanical properties of squash en- ables improvement of processing its pulp (sOsIŃs- KA et al., 2012). the texture of the squash pulp exhibits characteristics of chewiness and springi- ness, which can be modified by thermal treat- ment. Earlier studies concerning thermal treat- ment of pumpkin pulp in a convection steam oven revealed significant modifications in its texture properties (ŚLAsKA-GrZYWNA et al., 2013). It was noted that the most significant effect on changing hardness, springiness and chewiness of squash resulted from the amount of steam added, and to a lower degree from the time and temperature of treatment. similarly, works by GONÇALVEs et al. (2007) suggest a significant decrease in the firmness of squash during thermal treatment at the temperature of 75-95oc for 50 min. Prior to thermal treatment the firmness of squash was ca. 60 N, while after the treatment it did not ex- ceed 10 N. changes in the texture of pumpkin pulp following thermal treatment during its stor- age were studied by rAtNAYAAKE et al. (2004). In their studies with the help of double-compression test they observed the most significant changes in the texture of pumpkin pulp in case of mea- suring its hardness and chewiness, while the changes were only slight in case of springiness and cohesiveness. the key factor affecting rheo- logical qualities of vegetables is turgor (LIN tA-tE and PItt, 1986). softening of tissues is related to the loss of turgor cells and their easier sepa- ration (GrEVE et al., 1994). Plant tissue is built of cells mutually linked by middle lamella. the cell wall is kept rigid due to hydrostatic pressure inside the cell, which normally amounts to 1-8 bar (0.1 - 0.8 MPa) (AGUILLErA et al. 1998). cel- lulose present in the cell wall affects the rigidi- table 5 - the results of tukey’s studentized range test for pairs of mean values for the three agents and analyzed properties. Analyzed agent Level of agent Analyzed property Hardness Springiness Chewiness Cohesiveness Temperature 80 224.74A 0.2674B 2.139A 0.0280B 100 65.96B 0.3360A 0.964B 0.0471A Steam 0 313.27A 0.2833B 4.123A 0.0385B 20 132.26B 0.2657B 0.966B 0.0349C 40 70.03C 0.2953B 0.544B 0.0338C 60 65.84C 0.3625A 0.573B 0.0431A Time 5 207.96A 0.4498A 3.3754A 0.035833A 10 167.42B 0.3527B 1.8921B 0.035208A 15 150.48B 0.2642CD 1.4421BC 0.037917A 20 117.53CD 0.2960C 0.9350CD 0.038958A 25 103.19D 0.2356DE 0.6769D 0.039167A 30 125.53C 0.2119E 0.9869CD 0.038125A Means with the same letter are not significantly different at 0.05 significance level. 8 Ital. J. Food Sci., vol. 28 - 2016 ty and strength of the plant tissue, while pectins and hemicellulose present in middle lamella are responsible for its plasticity (LEWIcKI and PAW- LAK, 2003). thermal treatment of vegetables re- sults in structure alterations, tissue disintegra- tion, enzyme inactivation, washing out soluble components, loss of firmness and, consequent- ly, their softening (crUZ et al. 2011; GALINDO et al., 2005). According to researchers, both raw and cooked squash pulp provides numerous health benefits and can be used in prevention and treat- ment of certain diseases (cAILI et al.; 2006, NIEW- cZAs et al.; 2005, stIrG, 1997). studies suggest a possibility of selecting adequate parameters of thermal treatment helping to maintain the tex- ture most required by consumers. Moreover, the research results will be useful for food produc- ers, allowing them to select the optimal parame- ters of thermal treatment of squash pulp. cONcLUsIONs thermal treatment in a convection steam oven results in statistically significant chang- es of all the studied parameters of squash tex- ture, namely its hardness, springiness, chewi- ness and cohesiveness. the range and dynamics of texture properties of butternut squash depends on the parame- ters of thermal treatment, such as temperature, amount of steam added and time. the most significant range of modifications concerning texture qualities of squash were reg- istered for its hardness and chewiness, while changes in its springiness and cohesiveness oc- curred to a smaller extent. the decisive influence on changing the hard- ness and chewiness of squash was exerted by the addition of steam, while treatment time and temperature were less significant. 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