Transfer of heat and moisture during microwave baking of potatoes

Microwave baking of potatoes comprised two phases. In the first phase the internal temperature of the potatoes rose to approximately 100 °C with little loss of water vapour. The uniformity of internal heating during this phase was assessed by identifying gelatinised starch. Starch gelatinisation began near the outer surface after 1.5 min of cooking and was complete by 2.5 min. During the second cooking phase the thermal energy absorbed was used to evaporate water. When immersion in boiling water was substituted for this second phase of microwave cooking, it took significantly longer for the internal texture to soften adequately for the potatoes to be considered cooked. This suggests that microwave cooking influences texture independently of the thermal profile of the cooking process. Damage by escaping steam is suggested as a mechanism. © 2002 Society of Chemical Industry     

Texture and Microstructure of Steam Cooked, Vacuum Packed Potatoes

Uniaxial compression tests combined with light microscopy and scanning electron microscopy were used to study textural properties of vacuum packed, steam cooked (VPS-cooked) potato tubers. Comparisons were made with conventionally water cooked tubers for cooking intervals of 0–60 min. In the VPS-cooked potatoes the gelatinized starch formed dense clusters. In contrast, water cooked cells were filled with gelatinized starch. The textural attributes declined with increased cooking time, faster for water cooked potatoes than for VPS-cooked. After >30 min cooking, firmness of VPS-cooked potatoes increased, while cell separation and disintegration of tissue made the water cooked tubers less firm. Textural properties and cell structure were directly affected by processing conditions.     

Effect of antiviral genetical modification on softening of potato tubers during cooking

An investigation into the influence of genetical modification on potato tubers should include also an estimation of their technological usability. Potato tubers of 15 clones of cultivar Irga transformed with viral genome sequences in order to improve their resistance to a necrotic strain of potato virus Y (PVY^N) were examined. The effect of cooking on tissue texture, expressed by fracture stress (F) and strain (D), and elastic modulus (E) from a compression test, was investigated. Although the influence of the modification type appeared to be statistically significant but irregular in particular years of cultivation, a detailed statistical analysis (variance and discrimination analyses) did not confirm any repeatable nor statistically significant differences in F, D, and E over the three-years period of examinations. While the effect of cultivation year was unmistakable, the order of F, D, and E values for particular clones was not constant in the successive years. The lack of stable tendencies in average values of the mechanical parameters for both groups and subgroups of clones representing the same modification model was also found. The statistical analysis of the variability of mechanical parameters of raw and heat-treated potatoes measured during the three successive years enabled classifying Irga and its genetically-modified clones as similar and did not allow to distinguish clones of the quickest softening.     

Changes in flour gelatinisation of trifoliate yam (Dioscorea dumetorum) tubers after harvest

BACKGROUND: Changes in the properties of starch during postharvest hardening of trifoliate yam (Dioscorea dumetorum) tubers need to be investigated. In this work the starch gelatinisation kinetics of D. dumetorum cv. Yellow stored under prevailing tropical ambient conditions (0.98–2.32 kg vapour per 100 m³ airflow) for 0, 2, 7, 14, 21 and 28 days was studied using a 3 × 5 factorial experiment with three cooking temperature (75, 85 and 95 °C) and five cooking times (0, 5, 10, 20 and 40 min). Samples were also evaluated for starch properties and gelatinisation temperature. RESULTS: Storage of D. dumetorum tubers caused starch damage. However, the starch gelatinisation temperature was between 70 and 75 °C irrespective of the tuber storage duration. The starch gelatinisation reaction followed first‐order kinetics defined by the relationship ln(1 ? α) = ? k_Gt. The gelatinisation rate was significantly influenced (P≤0.05) by cooking temperature and increased with tuber storage duration. The activation energy also increased with tuber storage duration. CONCLUSION: Although the gelatinisation temperature of D. dumetorum starch was not influenced by tuber storage, the starch gelatinisation process was affected by tuber storage duration, since tuber storage involved a greater need of energy for starch gelatinisation, which may be an effect of the postharvest hardening phenomenon. Copyright © 2008 Society of Chemical Industry     

Physicochemical properties of the starchy flour extracted from sweet potato tubers through lactic acid fermentation

The efficiency of starch extraction from sweet potato tubers was improved by lactic acid fermentation using a mixed culture inoculum. Study of the properties of the starchy flour showed that there was a significant reduction in the starch content and consequently the soluble and apparent amylose contents of fermented samples from all six varieties used. A fall in peak viscosity and viscosity breakdown was observed for fermented samples, while the pasting temperature was enhanced significantly. The fermented flour showed greater solubility and reduced swelling. Differential scanning calorimetry studies indicated a delay in gelatinisation and a fall in enthalpy of gelatinisation of the starch on fermentation. Unlike cassava, all the properties of the starch from sweet potato tubers, ie viscosity, swelling, solubility, gelatinisation temperature, amylose content and starch content, were affected by fermentation, and variation was also observed among cultivars. Copyright © 2005 Society of Chemical Industry     

Texture analysis of raw and cooked tubers of short‐duration lines of cassava by multivariate and fractional conversion techniques

BACKGROUND: Cassava tubers are mainly consumed after boiling in water. Their cooking quality depends on the variety, maturity, growing conditions and physicochemical and starch properties. In this study the textural and gelatinisation characteristics of ten short‐duration lines of cassava were assessed. Textural changes due to cooking were modelled by fractional conversion techniques and correlated with gelatinisation and physicochemical properties by principal component analysis. RESULTS: There was a significant variation (P < 0.05) in the textural properties of tubers and the thermal and pasting properties of flours among varieties. The degree of cooking and relative firmness of tubers were modelled by a linear equation. The thermal softening behaviour was modelled by a dual‐mechanism first‐order kinetic model with rate constant values ranging from 0.106 to 0.190 min^?1. About 69.21% of the total variation in texture profile, gelatinisation and physicochemical parameters could be explained by the first three principal components. CONCLUSION: The results indicated that the textural and gelatinisation properties varied considerably among different varieties. However, the interaction between the components of the tubers and the structural make‐up of the tuber tissues played a more important role than the physicochemical and functional properties. Copyright © 2008 Society of Chemical Industry     

Impact of food processing on the glycemic index (GI) of potato products

Potatoes are one of the most popular carbohydrate foods in industrialized and some developing countries. However, contradicting arguments and misconceptions on potatoes as a high glycemic index (GI) food is directly affecting potato consumption during the past years. Potato varieties, maturity level, starch structure, food processing techniques and composition of the meal contribute to the GI of potatoes. Domestic boiling, baking, microwave cooking, oven cooking, extrusion and frying result in different degrees of gelatinization, and the crystallinity of starch in potato. French fried potatoes contain more resistant starch whereas boiled and mashed potatoes contribute to significant digestible starch. Extrusion processing conditions could affect the starch physicochemical structure and resulting nutritional value. Extrusion cooking makes more gelatinized starch than conventional cooking methods. Cooling or storing after processing of potatoes significantly reduces the GI due to retrogradation of starch molecules. This review provides a brief idea about the glycemic index, glycemic load, and their importance to human diseases, and detail information on the effect of food cooking methods on the glycemic index of potatoes.     

Parenchyma cell microstructure and textural characteristics of raw and cooked potatoes

Cooking, microstructural and textural characteristics from four New Zealand potato cultivars (Agria, Nadine, Moonlight, and Red Rascal) were studied. Potatoes from the waxy cultivar, Nadine, showed lowest dry matter and starch content and also had highest cooking time compared to the other cultivars. The total colour difference, ΔE, indicative of browning and darkening during storage (for 1 h) of the peeled potatoes was observed to be highest for Red Rascal, followed by Nadine and was lowest for Agria. Light (LM) and confocal scanning electron (CSLM) micrographs revealed Moonlight and Red Rascal raw potato parenchyma cellular structure to be well integrated, showing compact hexagonal cells. Raw tubers from these cultivars also exhibited higher hardness and cohesiveness, as observed using texture profile analysis (TPA). Moonlight potato parenchyma retained cell wall outline after cooking and its cells were observed to be completely filled with gelatinised starch matrix, whereas the cellular structure of Nadine potato parenchyma was completely disintegrated after cooking. TPA results of cooked potatoes agreed well with these microstructural observations, with Moonlight cooked potatoes showing highest cohesiveness and hardness. The cell wall of parenchyma cells was observed to degrade partially upon cooking of potatoes, resulting in loosening of the cell wall microfibrils, as observed using transmission electron microscopy (TEM) of a selected potato cultivar. Also, the cell wall decreased in thickness after cooking. The remains of the primary cell wall along with some electron dense granular structures were observed floating in the cytoplasmic starchy matrix in potato tuber parenchyma cells.     

Effects of time/temperature treatments on potato (Solanum tuberosum) starch: a comparison of isolated starch and starch in situ

Previously, time/temperature treatments of starch have been performed mainly on starch/water systems. In this study the same time/temperature treatments were applied to starch/water systems and to potato starch in situ. Two potato varieties (Solanum tuberosum cultivars Asterix and Bintje) were used. The effect of time/temperature treatments on gelatinisation behaviour was evaluated using differential scanning calorimetry (DSC). A blanching process was simulated by heating samples to 74 °C and then cooling them to 6 °C. A DSC scan showed that starch was completely gelatinised after this treatment. Retrogradation of amylopectin increased during storage at 6 °C from 0 to 24 h after blanching. Annealing of starch, with the aim of altering cooking properties, was performed by heating samples to temperatures below the gelatinisation onset temperature. Treating samples at 50 °C for 24 h caused a shift in gelatinisation onset temperature of 11–12 °C for isolated starch and 7–11 °C for in situ samples. The extent of the annealing effect depended on the difference between onset and annealing temperatures, and prolonged treatment time increased the effect. Starch/water systems and tissue samples behaved similarly when exposed to time/temperature treatments. The most apparent difference was the shift of gelatinisation to higher temperatures in tissue samples. Copyright © 2003 Society of Chemical Industry     

Methods for the preparation of cell walls from potatoes

A group of new methods is described for preparing cell walls from potatoes and processed potato products. Starting from raw domestic potatoes, starch is degraded enzymatically after a very brief 100 °C gelatinisation step conducted after homogenisation to minimise the time required for heat transfer. Protein is removed by detergent and phenol extraction. This procedure (method 1) gave cell wall preparations containing <5% starch, with minimal degradation of wall polysaccharides. It did not, however, remove starch efficiently from industrial potatoes in which the starch content is much higher. A different procedure, method 2, was used in this case. In method 2 a 20 min starch gelatinisation step was used but the temperature was restricted to 70 °C and the pH to 4.0, with the aim of protecting pectins from depolymerisation. Method 2 and method 1A, which is a hybrid procedure involving the starch gelatinisation step from method 2 and other steps from method 1, gave low‐starch cell walls from industrial as well as domestic potatoes. These methods are suitable for a range of potato types and potato products and are either more efficient or more convenient than previous procedures for cell wall isolation. © 2002 Society of Chemical Industry     

Transfer of heat and moisture during oven baking of potatoes

Potatoes were baked for up to 60 min in a conventional fan‐assisted oven. Temperature profiles within the potato tubers were determined both by direct measurement and by following the inward progress of starch gelatinisation, which occurs at 65 °C. Temperature profiles with time were S‐shaped and about 30 min was needed for the centre to reach 100 °C. From the nature of the temperature profiles, long potato tubers will cook faster than round tubers of the same weight, and a varietal ‘shape factor’ was defined to quantify this effect. The slow temperature rise, compared to other forms of cooking, was due to evaporative cooling. Moisture loss was linear with time after an initial lag and reached 15–20% of the mass of the potato after 1 h of cooking. Approximately half of the moisture was lost from the outermost layer of the potato under the periderm, leaving behind a dried layer of flesh that appeared to restrict water transport and, unlike any other part of the potato, could exceed 100 °C. This has consequences for the thermal development of flavour compounds as well as for perceived texture. © 2002 Society of Chemical Industry     

Discerning intra‐tuber differences in textural properties in cooked Solanum tuberosum group Tuberosum and group Phureja tubers

BACKGROUND: The textural properties of potato tubers influence their acceptability and palatability and these properties differ between varieties, groups and progeny. The aim of this study was to compare the textural properties of cooked tubers of Solanum tuberosum group Phureja with those of group Tuberosum. RESULTS: To assess intra‐tuber differences, the textural properties of seven cubes from defined positions along the longitudinal axis of tubers of four Tuberosum group cultivars and three Phureja group lines were tested after cooking using an amended wedge fracture method. Tuberosum group tubers gave consistently higher peak force and work done values during fracture than the Phureja group tubers. Moreover, the values for cubes 1–6 from any tuber were not significantly different and only cube 7, from the stem end, gave higher values. Therefore, the use of any of cubes 1–6 is a valid measurement of the tuber as a whole but the central cube 4 may be most conveniently located. The dry matter content of the cubes did not influence the textural properties of the cubes, which suggested that starch swelling is not the main driving force for textural differences. Total pectin methyl esterase (PME) activity was consistently higher in cubes of the Tuberosum group cultivars over the Phureja group lines. CONCLUSION: The method developed is valid and consistent for assessing textural differences within potato germplasm. The relationship between PME activity and enhanced resistance to fracture suggests that PME may modulate pectin cohesiveness, perhaps through increasing Ca²⁺‐bridges, to provide greater resistance to fracture. Copyright © 2010 Society of Chemical Industry     

不同熟化方法对紫色马铃薯挥发性风味物质形成的影响

利用顶空固相微萃取和气相色谱质谱联用技术,对不同熟化方式的2种紫色马铃薯(杭引1号、紫罗兰)挥发性风味物质进行分析,结果表明:1)癸醛为蒸熟马铃薯主体挥发性风味化合物;癸醛,香叶基丙酮为煮熟马铃薯主体挥发性风味物质,微波烘烤的马铃薯主要挥发性风味物质有香叶基丙酮、2,2,4-三甲基-1,3-戊二醇二异丁酸酯;烘烤马铃薯主要挥发性风味物质有癸醛、香叶基丙酮、2,4-二叔丁基苯酚、1-吡咯烷-1-环己烯。2)熟化加热方式的不同,马铃薯产生的挥发性风味化合物的种类丰富程度也不同,烘烤熟化处理马铃薯产生的挥发性风味物质种类最丰富,分别为8种(杭引1号)、21种(紫罗兰);微波烘烤马铃薯检测出挥发性风味物质种类最单调,分别为2种(杭引1号)、4种(紫罗兰)。3)2个品种之间既有共有风味物质,又有独有的风味物质。     

Some physical and nutritional characteristics of genetically modified potatoes varying in amylose/amylopectin ratios

Transgenically modified potatoes with a large spread in amylose/amylopectin ratios were analysed both as tubers and in the form of isolated starch. Different microscopic techniques were used to study starch granules and tuber tissue. Starch gelatinisation properties and recrystallisation of amylopectin and amylose were studied by differential scanning calorimetry. Starch bioavailability and resistant starch (RS) were evaluated using enzymatic in vitro procedures. Glycaemic indices (GI) were predicted from low molecular weight carbohydrates (LMWC) contents and the in vitro hydrolysis rate of the starch moiety. For many of the examined parameters, differences of varying magnitude were found between the potato lines, especially for high amylose lines. High amylose starch granules had irregular shapes and showed only a limited swelling. Moreover, contents of RS and recrystallised amylose were elevated. GI’s for the starch moiety were reduced, though elevated contents of LMWC caused a high over-all predicted GI.     

Safety assessment of genetically engineered potatoes with designed soybean glycinin: compositional analyses of the potato tubers and digestibility of the newly expressed protein in transgenic potatoes

The compositions of transgenic potatoes with intact and modified genes for soybean glycinin were compared with those of control potatoes; non-transgenic or transgenic ones with a vector. The expression levels of glycinin proteins in the transgenic potato tubers were approximately 12–31 mg g⁻¹-total soluble protein. With reference to six major components, moisture, protein, lipid, fiber, ash and carbohydrate, there were no significant differences between transgenic and control potatoes. The levels of glycoalkaloids in transgenic potato tubers, as well as in transgenic controls, increased in comparison with those in non-transgenic ones, though the level was within the permitted limit. The modified glycinin expressed in the transgenic potato tuber was digested under the simulated gastric conditions. From these results, the transgenic potatoes with intact and modified glycinin genes are considered to be as safety utilization for food as non-transgenic potatoes. © 1999 Society of Chemical Industry     

Effect of processing on potato starch: in vitro availability and glycaemic index

The content of digestible starch (DS) and resistant starch (RS) in processed potatoes was assessed. In addition, the effect of domestic cooking on the in vitro digestibility of starch in this tuber, which may influence the glycaemic response, was studied. Resistant starch in raw potato is high, however different RS values were obtained when processed, ranging from 1.18% in boiled potatoes to 10.38% in retrograded flour. In general, cooked potatoes have high levels of DS. Starch digestibility is improved after processing and it is affected by the cooking methods. Boiled and mashed potatoes showed the highest rate of digestion, on the contrary raw potato was hardly digested. The estimated Glycaemic Index (GI) from the degree of starch hydrolysis within 90 min was in accordance with the reported GI values, for potatoes processed in the same way.     

Starch from Alberta potatoes: wet-isolation and some physicochemical properties

Starch content of six varieties of Alberta-grown potato tubers was more than 16% (wet basis). The tubers were wet fractionated in the laboratory and starch was recovered. The starch recovery ranged from 61 to 67%. The composition of the potato juice/fruit water (dry matter, protein, ash and mineral content) and starch isolations (moisture, starch, protein, lipid, ash, phosphorous and amylose content) was determined. The physicochemical characterization of the potato starch isolations were done in terms of granule size distribution, crystallinity (X-ray diffractometry), gelatinization (differential scanning calorimetry) properties, swelling factor and the changes in viscosity during heating and cooling of starch-water slurry (Brabender viscography). Substantial differences in the starch content of the tubers, fruit water composition and starch physicochemical properties were observed between potato varieties.     

Influence of heating conditions and starch on the storage modulus of Russet Burbank and Yukon Gold potatoes

The storage modulus of Russet Burbank (RB) and Yukon Gold (YG) potato discs (13.3 mm × 4.5 mm) was measured continuously during heating in water at temperatures ranging from 40 to 80 °C for 120 min using a dynamic mechanical analyser with a heating stage. The starch content of YG potatoes was higher than that of RB potatoes, but the raw starch granule size distribution of both varieties was similar. RB samples had the highest storage modulus values (25% higher than raw tissue) when heated in 60 °C water for 35 min. Image analysis revealed that the perimeters of starch granules in the potato samples increased by about 50% during heating for 30 min at 60 °C. Heating temperature and time and starch content influenced the storage modulus. © 2001 Society of Chemical Industry     

Effect of freezing/thawing conditions and long‐term frozen storage on the quality of mashed potatoes

The effects of freezing temperature (−80, −40 or −24 °C) and thawing mode (microwave or overnight at 4 °C) on quality parameters of mashed potatoes made from tubers (cv Kennebec) and from potato flakes were examined, as was the effect of long‐term frozen storage on the quality of mashed potatoes. Mashed potatoes were tested for texture profile analysis (TPA) and cone penetration, oscillatory and steady rheometry, colour, dry matter, Brix and sensory analyses. In natural mashed potatoes, TPA hardness and oscillatory parameters showed that processing resulted in a softer product than the fresh control. The parameters were lower in the samples thawed at 4 °C than in those thawed by microwave at all the freezing temperatures used, which may be ascribed to gelatinisation of the starch released from damaged cells. Differences from the freshly prepared product decreased when the samples were frozen at −80 °C and thawed by microwave. No difference was found in sensory acceptability between samples frozen at −80 and −40 °C, which probably reflects the panellists' mixed preferences for air‐thawed versus microwave‐thawed samples. Increasing the time in frozen storage led to a natural mash with a firmer texture, higher L*/b* value and Brix; nonetheless, panellists found the samples at 0, 3 and 12 months of frozen storage equally acceptable. In commercial mash, penetration and oscillatory parameters showed that processing made for a firmer product than the fresh control, probably owing to retrogradation of gelatinised starch. Thawing mode had a significant effect on parameters, which were lower in the samples thawed at 4 °C. The structure and quality of commercial mash was more detrimentally affected by freezing and, therefore, we would not recommend either freezing or frozen storage of this mashed potato in the used conditions. Natural mash made from Kennebec potatoes should be frozen quickly and thawed by microwave in the conditions described to obtain a product more similar to that freshly made. If the samples are frozen by air blasting at −40 °C, the product can withstand frozen storage for one year. Copyright © 2005 Society of Chemical Industry     

微波处理对‘新大坪’马铃薯贮藏品质的影响

为了提升马铃薯贮藏品质,分别使用不同功率微波对马铃薯块茎进行处理,并将处理后的马铃薯保存在4℃的冰箱里,贮藏期间每10 d测量马铃薯的失重率、Vc含量、可溶性固形物含量、淀粉含量、褐变指数、多酚氧化酶活性、过氧化物酶活性。结果表明,微波处理能够有效提升马铃薯的贮藏品质,但微波处理不足(115 W/1 min)对马铃薯贮藏品质的提升效果不明显,微波处理过量(220 W/3 min)则会对马铃薯生理和品质造成损伤。实验中以微波功率220 W处理2 min的保鲜效果最佳,能够有效避免马铃薯块茎质量及Vc的损失,保持可溶性固形物含量和淀粉含量,减缓褐变指数的升高,抑制多酚氧化酶和过氧化物酶活性。该研究为马铃薯提供了微波处理提升贮藏品质的方法。