Yam Slices Research Articles

Drying kinetics and evolution of the sample's core temperature and moisture distribution of yam slices (Dioscorea alata L.) during convective hot-air drying

ABSTRACTIn the present work, the drying kinetics and evolution of sample's core temperature and moisture distribution of yam slices during convective hot-air-drying were investigated. In terms of drying kinetics, the effect of drying temperature (50, 55, 60, 65, 70°C), relative humidity (20, 30, 40, 50%), and sample thickness (5, 7, 9 mm) on drying characteristics of yam slices were studied. Results indicated that all the three factors had significant influence on the drying kinetics, whereas drying temperature gave the most significant effect, followed by relative humidity and sample thickness. Moisture-effective diffusivity and activation energy were calculated, and it was found that the diffusivity was in the range of 5.5454 × 10−10–1.0804 × 10−9 m2/s and the activation energy was 29.528 kJ/mol. Heat and mass transfer models were developed based on the finite element method to calculate the core temperature and moisture distribution of yam slices during drying. Model validation exhibited good agreement between predicted and experimental data, which illustrated that the developed models could precisely predict the core temperature profile and moisture distribution of the sample. The current work provides further insights to understand the characteristics and mechanism of drying process of yam slices.

Drying characteristics and modeling of yam slices under different relative humidity conditions

ABSTRACTThe drying characteristics of yam slices under different constant relative humidity (RH) and step-down RH levels were studied. A mass transfer model was developed based on Bi-Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40%, temperature of 60°C, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample’s temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification from an initial RH of 40%. Using the developed Bi-Di correlation, the estimated Biot number, effective moisture diffusivity, and mass transfer coefficient ranged from 0.1024 to 0.1182, 1.1133 × 10−10 to 8.8144 × 10−9 m2/s, and 1.8992 × 10−9 to 1.7364 × 10−7 m/s, respectively. A rather high correlation coefficient of determination (R2 between 0.9871 and 0.9971) was determined between the experimental and predicted moisture contents. The present findings contribute to a better understanding of the effect of relative humidity on drying characteristics. The developed Bi-Di correlation provided a new method to determine the effective diffusivity of moisture in drying.

Studies on the Microstructure and Quality of Iron Yam Slices during Combined Freeze Drying and Microwave Vacuum Drying

The quality of freeze drying (FD)4.5 h-microwave vacuum drying (MVD) iron yam slices was almost the same with FD samples in terms of bulk density, color, etc., in the previous experiments. This work verified it by comparing the microstructure of the samples. In addition, the microstructure and quality of samples with different conversion point during combined FD and MVD were tested in order to illuminate the change mechanism for cell structure and the relationship between microstructure and corresponding quality. Moisture conversion point and melting significantly affected the microstructure of samples. The cell structure of samples dried by FD3.5 h-MVD with melting samples was maintained very well. However, obvious cell wall breakage and even collapse was observed in FD2 h-MVD samples. Quality indices such as bulk density and color are positively related to microstructure change. FD3.5 h-MVD with melting was better than FD4.5 h-MVD because of almost the same quality and shorter drying time. Practical Applications In most previous researches, the optimal moisture conversion point was usually the end of sublimation stage during freeze drying (FD)-microwave vacuum drying (MVD). There was no obvious ice crystal in sample at this point, which showed that the materials absorbed the microwave energy uniformly and caused high quality of final products. However, for vegetables and fruits materials that are rich in starch, such as iron yam, moisture conversion point was chosen in advance. Ice crystal in the samples melted slowly in low temperature and well-distributed moisture again after FD. The structure of the materials was maintained very well because of the support from raw starch granules. Therefore, high-quality dried products were also obtained under this technique conditions. In general, energy consumption for FD-MVD drying was decreased further while the quality of the materials was maintained well.

Mathematical modeling and effect of blanching pretreatment on the drying kinetics of Chinese yam (Dioscorea opposita)

The influences of blanching pretreatment on the drying kinetics of Chinese yam (Dioscorea opposita) slices were investigated. Drying experiments were carried out at 60, 70, 80 and 90?C. Six thin layer models were evaluated and the determination of coefficient (R2), chi-square (?2), root means square error (RMSE) were used to analysis the model performance for both raw and blanched samples. The Wang and Singh model gave best results with R2 of 0.9987 and RMSE of 0.0136 for raw yam slices, and R2 of 0.9989 and RMSE of 0.0119 for blanched samples. The effective moisture diffusivity coefficient Deff varied in the range of 0.7295?10-9 to 2.4087?10-9m2 s-1 for raw slices, and 1.3748?10-9 to 3.8524?10-9m2 s-1 for the blanche dones. The activation energy of yam slices drying were 41.149 and 33.499 kJ mol-1 for raw and blanched yam slices, respectively. Results show that blanching pretreatment can reduce the total drying time and improve the effective moisture diffusivity compared with the raw samples.

The application of superheated steam impingement blanching (SSIB) in agricultural products processing – A review

Blanching is an essential step before processing of agricultural products as it can inactivate enzymes that cause undesirable changes. However, the widely used traditional hot water blanching has many disadvantages such as nutrients loss during blanching and pollution caused by waste blanching water. Therefore to alleviate these problems, the traditional hot water blanching method should be replaced by new blanching technologies without using water as the thermal media. Superheated steam impingement blanching (SSIB) technology combines the advantages of superheated steam and impingement technology, resulting in a rapid, no waste water and efficient process. This paper reviews recent advances in SSIB. The equipment used for SSIB was first presented, then application of SSIB in processing agricultural products including yam slices, sweet potato, grape, sea cucumber, fresh-cut lettuce, and apple quarters were briefly outlined, and finally challenges and opportunities associated with this technology were discussed.

Development of Hot-Air Supplemented Solar Dryer for White Yam (Dioscorea Rotundata) Slices

In this study, a hot-air supplemented solar dryer was designed and fabricated for drying white yam slices Dioscorea rotundata. The capacity of the designed hot-air supplemented solar dryer was 14 kg. The equipment was tested in Federal University of Technology Akure (FUTA) using white yam Dioscorea rotundata to establish the effect of incorporating the hot-air section into the solar dryer. Drying experiments were conducted using a temperature of 60 °C for the hot-air supplemented solar drying process at a drying air velocity of 0.8 m/s. After the experiment, it was deduced that the total drying time used to reduce the moisture in the white yam slices to safe storage moisture content (SSMC) differs for the two different drying conditions giving a total drying time of 18 hours for solar dryer and 13 hours for hot-air supplemented solar dryer. The average dryer thermal efficiency for the solar dryer was 31.45 %, and the average dryer thermal efficiency is 42.10 % at solar/mechanical drying at 60 oC, and also the solar collector highest efficiency was calculated to be 83.28 % at solar radiation intensity of 1199.46 W/m 2 and lowest efficiency of the solar collector was 23.89 % at solar radiation intensity of 300.40 W/m 2 .

Development of Hot-Air Supplemented Solar Dryer for White Yam (Dioscorea Rotundata) Slices

Development of Hot-Air Supplemented Solar Dryer for White Yam (Dioscorea Rotundata) Slices

Development of frozen-fried yam slices: optimization of the processing conditions

The research performed on yam processing mainly concerns the production of crisps and flour. However, its transformation into deep-frozen French fries does not necessitate any other equipment than those used for potatoes. The industrial process of production of frozen French fries traditionally includes a pre-frying step. These steps contribute to the development of color and crispness, and the oil partially absorbed inhibits dehydration during the freezing step. The aim of this study was to optimize frying conditions of deep-frozen fried yam (Dioscorea cayenensis varKponan) slices. The effect of pre-frying time and temperature, final frying time and temperature on the oil uptake, texture, dry matter and colour of the fried yam slices has been studied. Frying conditions optimized with Box-Behnken experimental design were short pre-frying and frying conditions at high temperature characterized by prefrying temperature at 157-170°C during 5-9s and frying temperature at 181-188°C for 2min 15s-2min 30s; or long pre-frying and frying conditions at low temperature characterized by pre-frying temperature at 150-158°C during 10-15s and frying temperature 170-177°C for 3-3min 15s. An adiabatic system was also developed by means of an insulator in which the core temperature of fried yam slices can be maintained constant at about 55°C after 15min of cooling, facilitating texture measurements at constant temperature. The present results may help in choosing the yam slices frying condition to be applied in order to achieve the desirable fried yam slices quality, required for protection against certain diseases like obesity. These models may also provide guidance as to how to control these quality parameters by altering four key environmental factors, pre-frying temperature and time and, final frying temperature and time. This process can also be commercialized and does not necessitate any other cost for equipment than those used for potatoes French fries and might be an interesting way of added value processing for this highly perishable yam tuber.

Development of frozen-fried yam slices: optimization of the processing conditions

The research performed on yam processing mainly concerns the production of crisps and flour. However, its transformation into deep-frozen French fries does not necessitate any other equipment than those used for potatoes. The industrial process of production of frozen French fries traditionally includes a pre-frying step. These steps contribute to the development of color and crispness, and the oil partially absorbed inhibits dehydration during the freezing step. The aim of this study was to optimize frying conditions of deep-frozen fried yam (Dioscorea cayenensis varKponan) slices. The effect of pre-frying time and temperature, final frying time and temperature on the oil uptake, texture, dry matter and colour of the fried yam slices has been studied. Frying conditions optimized with Box-Behnken experimental design were short pre-frying and frying conditions at high temperature characterized by prefrying temperature at 157-170°C during 5-9s and frying temperature at 181-188°C for 2min 15s-2min 30s; or long pre-frying and frying conditions at low temperature characterized by pre-frying temperature at 150-158°C during 10-15s and frying temperature 170-177°C for 3-3min 15s. An adiabatic system was also developed by means of an insulator in which the core temperature of fried yam slices can be maintained constant at about 55°C after 15min of cooling, facilitating texture measurements at constant temperature. The present results may help in choosing the yam slices frying condition to be applied in order to achieve the desirable fried yam slices quality, required for protection against certain diseases like obesity. These models may also provide guidance as to how to control these quality parameters by altering four key environmental factors, pre-frying temperature and time and, final frying temperature and time. This process can also be commercialized and does not necessitate any other cost for equipment than those used for potatoes French fries and might be an interesting way of added value processing for this highly perishable yam tuber.

Development of frozen-fried yam slices: optimization of the processing conditions

The research performed on yam processing mainly concerns the production of crisps and flour. However, its transformation into deep-frozen French fries does not necessitate any other equipment than those used for potatoes. The industrial process of production of frozen French fries traditionally includes a pre-frying step. These steps contribute to the development of color and crispness, and the oil partially absorbed inhibits dehydration during the freezing step. The aim of this study was to optimize frying conditions of deep-frozen fried yam (Dioscorea cayenensis var Kponan) slices. The effect of pre-frying time and temperature, final frying time and temperature on the oil uptake, texture, dry matter and colour of the fried yam slices has been studied. Frying conditions optimized with Box-Behnken experimental design were short pre-frying and frying conditions at high temperature characterized by prefrying temperature at 157-170°C during 5-9s and frying temperature at 181-188°C for 2min 15s-2min 30s; or long pre-frying and frying conditions at low temperature characterized by pre-frying temperature at 150-158°C during 10-15s and frying temperature 170-177°C for 3-3min 15s. An adiabatic system was also developed by means of an insulator in which the core temperature of fried yam slices can be maintained constant at about 55°C after 15min of cooling, facilitating texture measurements at constant temperature. The present results may help in choosing the yam slices frying condition to be applied in order to achieve the desirable fried yam slices quality, required for protection against certain diseases like obesity. These models may also provide guidance as to how to control these quality parameters by altering four key environmental factors, pre-frying temperature and time and, final frying temperature and time. This process can also be commercialized and does not necessitate any other cost for equipment than those used for potatoes French fries and might be an interesting way of added value processing for this highly perishable yam tuber.

EFFECT OF SSB (SUPERHEATED STEAM BLANCHING) TIME AND DRYING TEMPERATURE ON HOT AIR IMPINGEMENT DRYING KINETICS AND QUALITY ATTRIBUTES OF YAM SLICES

ABSTRACTThe effects of SSB (superheated steam blanching) time (0, 3, 6, 9 and 12 min) and drying temperature (50, 60, 70 and 80C) on drying kinetics and quality of yam slices under air impingement drying were investigated in this paper. Results indicated that the positive effect of SSB on drying rate might be overshowed by the negative effect of starch gelatinization and appropriate SSB could accelerate drying rate, whereas the effect of excessive SSB is reversed. The moisture effective diffusivity ranged from 1.1540 × 10−9to 2.8431 × 10−9 m2/s, calculated using Fick's second law of diffusion. The activation energy determined from the slope of the Arrhenius plot, ln(Deff) versus 1/(T + 273.15), was 20.925 kJ/mol. Moderate increase in SSB time or decrease in drying temperature can improve the whiteness index of dried yam slices, whereas increase in SSB time and drying temperature can decrease the rehydration ratio of dried yam slices.PRACTICAL APPLICATIONSYam is a nutritional, economical and healthy plant, which has been widely used in traditional Chinese medicine. Fresh yams having relatively high‐moisture contents are very sensitive to microbial spoilage. Drying is one of the most common methods used for yam preservation. In China, the natural sun‐drying method is commonly used for drying yam. However, it requires a long drying time and the final product may be contaminated by dust and insects, especially toxic substances. Blanching is an essential step before the processing of vegetables and fruits, as it can accelerate the drying rate and prevent quality deterioration. Air impingement drying is an efficient drying technology, which has been successfully used in paper and textile industries. Understanding the effect of superheated steam blanching time and drying temperature on hot air impingement drying kinetics and quality of yam slices is helpful for improving the drying process efficiency and quality of dried yam slices.

The effect of frying conditions on moisture loss and oil uptake in yam slices (Dioscorea alata)

The effect of frying conditions on immersion on moisture loss and oil content were studied at different times in two yam varieties: Diamante 22 and Pico Botella. The experimental data adjusted itself to the first order exponential model for humidity transfer (r > 0.9) for both cultivars. The humidity diffusion coefficient for the two varieties of yam fried at 140°, 160° and 180°C for 1 to 9 minutes were 8.11±0.76, 8.2±0.81, and 8.62 ±0.71*10-9 m2/s for the Diamante 22 variety, respectively, and 8.32±0.82, 8.38±0.78, 8.54±0.84*10-9 m2/s for the Pico de Botella variety. Activation energy was 24.43 kJ/mol for the Diamante 22 variety and 21.27 kJ/mol for the Pico de Botella variety. The results showed that there was higher oil absorption after 180 s and at 140°C while taking into account 0.377 kg water / kg solid moisture loss for Diamond 22 and 0.410 kg water / kg solid Pico de Botella at 180°C for 540 s.

Effect of local preservatives on the sensory properties of dried yam slices 'Gbodo', flour and paste

Effect of local preservatives on the sensory properties of dried yam slices 'Gbodo', flour and paste

DEEP-FAT FRYING OF YAM SLICES: OPTIMIZATION OF PROCESSING CONDITIONS USING RESPONSE SURFACE METHODOLOGY

The deep-fat frying of yam slices was investigated with the aim of optimizing the processing conditions. During frying, frying temperature, initial dry matter and frying time have a significant effect on moisture loss and oil uptake. Response surface methodology central composite rotatable design was used to study the effects of the independent variables on quality attributes of yam chips. Breaking force, oil content, moisture content and color parameters were determined. Statistical analysis with response surface regression showed that breaking force, oil and moisture contents and color parameters (L*and a*) were significantly (P < 0.05) correlated with frying temperature, initial dry matter and frying time. The optimum conditions were a frying temperature of 175–180C, using tubers of initial dry matter of 0.179–0.214 kg/kg with a frying time of 4–5 min. It was suggested that the regression equation can be used to estimate the dependent variables for fried yam chips except b*(yellowness) parameter. PRACTICAL APPLICATIONS It is expected that the optimized processing conditions highlighted in this work will be useful in obtaining fried yam chips of acceptable quality attributes. This optimized condition would be a good prospect for commercialization in small-scale industries.

Optimization of Blanching Conditions Prior to Deep Fat Frying of Yam Slices

The effect of low-temperature blanching and frying time at a frying temperature of 170°C on moisture and oil contents, breaking force and colour of yam chips was investigated using response surface methodology to establish the optimum blanching conditions and frying time. A central composite rotatable design was used to study the effects of variation in levels of blanching temperature (60–80°C), blanching time (1–5 min) and frying time (2–6 min) on quality attributes of yam chips. The effect of blanching temperature and frying time was more significant than the time of blanching on the quality attributes. The response variables were fitted to predictive models applying multiple linear regressions. Statistical analysis with response surface regression showed that moisture content, oil content, breaking force and L* (lightness) parameter were significantly (P < 0.05) correlated with blanching temperature and time and frying time. However, the regression equation showed a poor fit for a* and b* respectively. The optimum conditions were a blanching temperature of 70–75°C, blanching time of 4–5 min while frying for about 5 min.

Convective hot air drying of blanched yam slices

SummaryIn this study, a laboratory convective hot air dryer was used for the thin layer drying of blanched yam slices and experimental moisture ratio was compared with Newton, Logarithmic, Henderson and Pabis, modified Henderson and Pabis, approximation of diffusion, modified page 1, two‐term exponential, Verma et al. and Wang and Singh models. Among all the models, the approximation of diffusion model was found to satisfactorily describe the kinetics of air‐drying of blanched yam slices. The increase in air temperature significantly reduced the drying time with no constant rate period but drying occurs in falling rate period. The effective diffusivity values varied between 7.62 × 10−8 to 9.06 × 10−8 m2 s−1 and increased with increase in temperature. An Arrhenius relation with an activation energy value of 8.831 kJ mol−1 showed the effect of temperature on moisture diffusivity.

Effects of Domestic Processing on Steroidal Saponins in Taiwanese Yam Cultivar (Dioscorea pseudojaponica Yamamoto)

The effects of domestic processing on steroidal saponins and furostanol and spirostanol glycosides in Taiwanese yam cultivar (Dioscorea pseudojaponica Yamamoto) were studied. The baking or frying of yam slices was conducted at 150, 180, and 200 degrees C for 3, 5, and 10 min. Yam slices were steamed or microwave cooked at 2450 MHz with an output power of 850 W for 3, 5, and 10 min. The various saponins were quantified by HPLC with an evaporative light scattering detector (ELSD). Results showed that the contents of saponins were decreased along with increasing cooking temperature and time except for the steaming treatment. None of the steamed yam slices significantly change their initial compositions or quantities of furostanol and spirostanol glycosides. Fried yam slices had the highest loss of saponins, especially at 200 degrees C for 10 min (93 and 97% reductions for total furostanol and spirostanol glycosides, respectively). After baking for 10 min at 200 degrees C, the total furostanol and spirostanol glycosides were reduced by 67 and 74%, respectively. There were 12, 44, and 84% decreases for total furostanol glycosides and 10, 35, and 75% reductions for total spirostanol glycosides in yam slices after microwave cooking for 3, 5, and 10 min, respectively. Diosgenin, the aglycone of these saponins, could be found in yams after microwave cooking and baking, but not in steamed and fried yams.

Effect of pretreatment and temperature on air-drying of Dioscorea alata and Dioscorea rotundata slices

Effects of pretreatment and drying conditions on yam varieties, namely Dioscorea alata and Dioscorea rotundata, in a fabricated laboratory scale hot air drier at temperature range of 50–80 °C and constant air velocity of 1.5 m 2/s were investigated. Mass transfer during air-drying of yam slices was described using Fick’s diffusion model. Drying took place entirely in the falling rate period. Temperature dependency of moisture on diffusivity was illustrated by the Arrhenius relationship. Over the range of temperature, moisture diffusivities varied from 9.92 × 10 −8 to 1.02 × 10 −7 and 0.829 × 10 −6 to 1.298 × 10 −5 m 2/s for D. alata and D. rotundata, respectively. Activation energy for drying of D. alata and D. rotundata varied from 25.25 to 46.46 and 41.75 to 72.47 kJ/mol, respectively.

Microbiological characterization of yam fermentation for 'Elubo' (yam flour) production

Yams (Dioscorea rotundata) were processed and fermented to the traditional West African dried yam flour ‘Elubo’. Yam slices were blanched at 60 °C for 10 min and then fermented at 30 °C for 24 h. The microbial population increased with fermentation time during the first 24 h of natural fermentation with a fall in the pH from 6.2 to 5.4. The dominant aerobic mesophilic bacteria consisted of Lactobacillus plantarum, Lactobacillus brevis, Lactobacillus delbrueckii and Bacillus subtilis. Significant contributions were also made by Lactococcus lactis, Klebsiella pneumoniae, Citrobacter freundii and the yeasts Pichia burtonii and Candida krusei. Accelerated natural lactic fermentation was achieved by repetitive use of the previous fermentation batch as an inoculum through back-slopping at a rate of 10% (w/v). After three consecutive fermentation cycles, the microflora was dominated by L. plantarum and L. brevis. Fermentation of blanched samples with pure cultures of the isolates showed L. plantarumL. brevis and B. subtilis to be the main species responsible for pH reduction and in changes in the browning levels of the reconstituted flour paste (amala). The typical colour of ‘amala’ is disliked by many individuals and it seems possible that amala with a lighter colour which will be more acceptable can be produced by fermentation.

Fermentation of yam: microbiology and sensory evaluation of cooked fermented yam tissues.

The microflora of white yam (Dioscorea rotundata L.) slices fermented anaerobically in 1.5 percent brine for five days at room temperature were studied. The hydrolysis of the carbohydrate and the subsequent conversion of sugars and minerals by the fermenting microbes contributed much to the increased microbial load especially within the first 72 hours of fermentation. The organisms implicated in the fermentation include the species of Pediococcus, Lactobacillus and Pseudomonas, Bacillus subtilis and two other Gram negative coccal cells, yet to be identified. The decrease in microbial counts at the latter stage of fermentation was attributed to the high total acidity of the medium, which was about 2.67 times the initial value of 0.027% lactic acid. Meanwhile, the lactic acid bacteria increased continuously throughout the period of fermentation.