BETWEEN-SPECIES DIFFERENCES IN FRACTURABILITY LOSS: MICROSCOPIC AND CHEMICAL COMPARISON OF POTATO AND CHINESE WATERCHESTNUT1

Potato and waterchestnut tissue were used as experimental materials for comparing between-species differences in heat softening rate. Scanning electron microscope (SEM) techniques were employed to compare cellular and subcellular changes with corresponding fractur ability losses during cooking of potato and Chinese waterchestnut by four cooking methods. SEM observations strongly suggest that cell wall adhesion and its heat resistance play a major role in fracturability changes during heating. Intracellular substances such as starch and minerals may slightly alter the cell wall strength but do not substantially affect fracturability changes. In addition, differences in the gross chemical composition between species were inadequate for explaining fracturability changes of the two vegetable tissues during heating.     

ROLE OF TUBER DENSITY IN POTATO SLOUGHING

A new CPEM method was developed and used to study the cooking curves of two potato varieties showing differences in texture and of one variety cultivated in four different regimes. The two main cooking curve stages (i.e., cooking and disintegration) can be described by simple linear equations, with density as an independent parameter. The parameters in the equation describing the cooking stage were reformulated to express directly the role of starch. The two new parameters obtained were weakly correlated and expressed separately cooking time sensitivity to starch content and the basic potato characteristic in relation to sloughing. Data obtained indicate only a minor role of starch content during the first part of cooking that is determined primarily by basic potato characteristics probably related to the cell wall properties. In the second stage of disintegration, starch content plays a more important role.     

Cell wall modifications during cooking of potatoes and sweet potatoes

Sweet potato (Ipomoea batatas L) tissue, when cooked at 70 °C to activate β‐amylase and break down starch, takes on a distinctive firm, brittle texture and does not show the cell separation that occurs in, for example, cooked potato (Solanum tuberosum L). Similar cooking conditions increase firmness in other plants by activating pectin methyl esterase which de‐esterifies pectic polysaccharides and protects them from thermal depolymerisation. We therefore isolated cell walls from both potatoes and sweet potatoes cooked at 70 °C and 100 °C and determined the remaining degree of methyl esterification of their pectins. Pectins from both species were demethylated to a similar extent at 70 °C and 100 °C. Since cooking sweet potato at 100 °C induced cell separation and softening, it is concluded that β‐amylase is rapidly inactivated at that temperature and swollen starch distends and separates the cells, whereas the firm texture obtained by cooking that species at 70 °C is not the result of pectin demethylation but is caused by the breakdown of starch to oligomers that can escape from the cell. © 2000 Society of Chemical Industry     

Pectin changes in the pre-cooking step of dehydrated mashed potato production

Some in-vivo and in-vitro characteristics of pectin methylesterase (PME) from cell wall and whole potato tuber were determined and its role was elucidated in the pre-cooking and cooling steps of commercial dehydrated mashed potato production. The isolated cell walls contained 16.3% anhydrogalacturonic acid with a degree of esterification (DE) of 56, and 25% of tuber total PME. Cell wall pre-cooking in model systems resulted in a DE of 53.2. Native cell wall pectin solubilisation was much less than that of H-cell walls devoid of metal cations. Calcium but not Mg, supplied as Ca(Mg)-starches, retarded pectin solubilisation, while Ca-uptake by retained pectin was high when cooling followed pre-cooking. Texture measurements of potato slices pre-cooked, cooled and cooked suggested a firming effect which could be attributed to Ca-release from gelatinised starch during pre-cooking, and stabilisation of Cabridges with free COO^? groups of cell wall pectin duringcooling. The role ascribed to PME in causing firming is de-emphasised.     

PEELING AND THE PHYSICAL AND CHEMICAL PROPERTIES OF KIWI FRUIT

Hand peeling of kiwi fruit has some disadvantages such as difficulty during peeling, increase of loss in weight and nutritional value. Peeling of kiwi fruits with alkali (NaOH) was investigated. Some chemical (acidity, pectin, and chlorophyll) and physical properties (Hunter color value and weight loss) of the fruits were determined after selected alkali peeling methods were used. Peeling methods at 80, 90 or 100C temperatures; in 13, 18 or 23% of NaOH solutions and for 3, 4 or 5 min durations were tested and compared with hand peeling. Weight (fruit tissue) loss in hand peeling was higher than alkali peeling. Peeling with alkali was easier. Nutritional value (ascorbic acid content) of alkali peeled fruits was higher than hand peeled kiwi fruit. Because of less weight loss, better green color and high pectin content the method including 15% of NaOH solution at 95C for 4 min was selected as most advantageous peeling method.     

INFLUENCE OF CELL SIZE AND CELL WALL VOLUME FRACTION ON FAILURE PROPERTIES OF POTATO AND CARROT TISSUE

This article presents the influence of cell size and cell wall volume fraction on the failure parameters of potato tuber and carrot tissue. Confocal scanning laser microscope was used for obtaining images of the cell structure of the tissues. The mean cell face area and the cell wall volume fraction obtained from the images was compared with work to failure, failure stress, failure strain and secant modulus obtained in a compression test of potato and carrot tissue at two strain rates. Bigger cells and less amount of cell wall material weakened the tissue, which was visible as a linear decrease in the parameters: work to failure, failure stress and failure strain. There were differences between potato and carrot in the secant modulus. For carrot, the secant modulus changed with microstructural parameters, whereas for potato, the secant modulus did not depend on these values. The strain rate decreases all the failure properties for potato. For carrot, only the work to failure was affected by the strain rate.     

马铃薯果胶的提取、结构、活性及应用研究进展

果胶是植物特有的一种多糖,广泛存在于高等植物细胞壁中。在生产中,果胶常被制作成添加剂,在食品、医药、化工等产业中具有重要价值。目前国内果胶生产原料来源较为单一,以苹果渣、柑橘皮为主,且存在生产效率较低等问题。随着果胶新用途不断被开发,果胶的商业化生产已成为相关研究的热点之一。马铃薯渣是马铃薯淀粉加工产生的主要副产物,薯渣内果胶含量丰富,在果胶生产中具有较大发展潜力。该文重点介绍马铃薯果胶的提取方法、结构特征、生物活性和应用前景,探讨马铃薯果胶多糖的生理功能及作用机制。     

THE THERMAL FRACTURABILITY LOSS OF EDIBLE PLANT TISSUE: PATTERN AND WITHIN-SPECIES VARIATION1

The pattern of fracturability loss of 11 selected fruits and vegetables during thermal processing was studied. In general, the first-order kinetic model is adequate in describing such changes during heating of edible plant tissue before reaching equilibrium texture. However, textural changes are less predictable by the first-order kinetic model in tissues having thick cell walls (such as those derived from stems and roots) than in tissues having thin cell walls (such as those derived from fruits). For a given species (potato), the first-order model can be applied regardless of sample dimension, morphological origin of the sample, heating temperature, variety, prior storage conditions, maturity and minor agronomic variation.     

MODELLING THE MECHANICAL AND HISTOLOGICAL PROPERTIES OF CARROT TISSUE DURING COOKING IN RELATION TO TEXTURE AND CELL WALL CHANGES

A tensile test was used to measure four mechanical properties of carrot tissue cooked under various time-temperature conditions. A kinetic model describing the changes of these mechanical properties measured during cooking was developed. The histological properties of the rupture surfaces caused by the mechanical testing were investigated. The kinetic model was found capable of predicting the changes in the rupture mechanism of the cell walls. Determining the percentage of cell wall ruptures proved to be an accurate method to assess the textural state of carrot tissue during cooking as compared to the measurement of the mechanical properties.     

Effects of low‐temperature blanching on tissue firmness and cell wall strengthening during sweet potato flour processing

Free starch rate has been one of the most important criterions to evaluate the quality of sweet potato flour. Low‐temperature blanching (LTB) of sweet potatoes before steam cooking has shown significant increase in tissue firmness and cell wall strengthening. This research indicated that pectin methylesterase (PME) activity decreased by 87.8% after 30 min of blanching in water at 60 °C, while polygalacturonase (PG) and β‐amylase activity decreased 69.4% and 7.44%, respectively, under the same condition. Both PME and β‐amylase played important roles in tissue firmness. Further studies of tissue firmness and methyl esterification showed that the combination of LTB and Ca²⁺ could increase the activity of PME and significantly enhance the pectin gel hardness to strengthen the cell walls and decrease free starch rate from 12.83% to 7.28%.     

Influence of Temperature and Time on Cooking Kinetics of Potatoes

To study cooking kinetics, slices (6 mm thick, 30 mm diameter) of 3 potato varieties were treated in water at 90°, 100° and 110°C for varying times. Sensory, chemical and physical properties were investigated. Changes during cooking at constant temperature were mathematically described by a 0 order (texture, taste) and a 1st order (shear force, dry matter, pectin, soluble amylose, cell size) equation. The starch content remained constant. Temperature dependence of the rate constant could always be described by a first order equation. The z-values, which are necessary to calculate Cvalue for practical heat treatments, could be determined. Correlations established between kinetics of different properties showed that behavior of certain potato properties may be predicted by shear-force measurement.     

CHEMICAL, COOKING AND TEXTURAL PROPERTIES OF SEMIMEMBRANOSUS, SEMITENDINOSUS AND BICEPS FEMORIS MUSCLES OF PORK

The chemical, cooking and physical properties of semimembranosus, semitendinosus and biceps femoris of ham were determined. Significant differences in fat content of muscle and force required to penetrate the ham samples from the three muscles were found. No significant differences among three muscles in other properties were noted. A statistically significant correlation (r=-0.54) existed between the fat content of muscle and force required to penetrate the ham sample. Expressible juice from the muscles was significantly related to the cook loss and yield of hams. The moisture content of hams was also negatively related to the expressible juice. The possible mechanisms for these correlations are related to the ability of porcine muscles to chemically bind moisture during the cooking processes.     

THE CHEMISTRY OF TEXTURE IN FRUITS AND VEGETABLES

This review is concerned with the chemistry of the major primary cell wall components, pectins, hemicelluloses and cellulose, under conditions found during the normal handling of fruits and vegetables. These polymeric components are considered separately, then their combined changes during maturation, storage and processing are covered. The effects of tissue conditions, pH, enzymes and salt concentrations on the rate and degree of change are discussed. A large part of the review deals with the important texture-affecting reactions of pectic materials including glycosidic hydrolysis, β-elimination type depolymerization, demethoxylation, and complex formation.     

CELL WALL CHARACTERISTICS AND FIRMNESS OF FRESH PACK CUCUMBER PICKLES AFFECTED BY PASTEURIZATION AND CALCIUM CHLORIDE1

Changes in cell wall pectic substances, degree of pectin methylation, bound Ca⁺⁺, neutral sugar composition, and firmness were determined in mesocarp tissue of pasteurized and nonpasteurized fresh pack cucumber pickles. Large changes in solubility characteristics of pectic substances occurred in cell walls of nonpasteurized pickles that were attenuated by pasteurization. In particular, water and alkali soluble pectins declined, and nonextractable pectins increased during the first month of storage. The major changes in pectic substance solubility appeared to be related to reductions in the degree of pectin methylation with a minor influence of CaCl₂. Galactose in cell walls of nonpasteurized pickles was substantially reduced, and the reduction in galactose was hindered by CaCl₂ or pasteurization. The amount of bound Ca⁺⁺ appeared to be associated with tissue firmness after one month in storage, since firmer tissue had more cell wall bound Ca⁺⁺. While firmness was associated with the amount of bound Ca⁺⁺, the amount of bound Ca⁺⁺ was dependent on the supply of Ca⁺⁺ and the degree of pectin methylation.     

Low-Temperature Blanching of Sweetpotatoes to Improve Firmness Retention: Effect on Compositional and Textural Properties

ABSTRACT: Low-temperature blanching of sweetpotatoes (SP) prior to cooking has been shown to significantly increase firmness retention. This research investigated the effect of blanching on firmness, pectin methylesterase activity (PME), pectin methylation, and galacturonic acid and cell wall material concentrations in SP tissue subjected to blanching and cooking treatments. PME activity decreased 82% after 20 min of blanching in water at 62°C, while sample firmness continued to increase with blanching time (3.5 N for unblanched and 19.0 N for 90 min blanched, and cooked tissue), indicating that firming due to pectin demethylation explains part of the observed increased firmness retention caused by low-temperature blanching, but unknown factors also play a role.,     

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.     

ARTIFICIAL CELL STUDIES IN SIMULATED APPLE AND POTATO STARCH CELL COMPLEX DURING OSMOTIC DEHYDRATION

ABSTRACT

Visking tubing was employed as an artificial cell system to investigate the behavior of water in simulated apple and potato starch cell complex during osmotic dehydration. The study involved a factorial experimental design at three levels of temperature (32.2, 40 and 55C) and five levels of solute concentration solutions (four sucrose concentrations: 40, 50, 60 and 70%; one sodium chloride concentration: 0.5%). A time period of 0–3 h was implemented with 30‐min sampling period for every treatment. Three phases of a sharp decrease in the initial weight of the visking tubing in the first 30 min were observed for all temperatures and solute concentrations. In all the treatments, there was an incremental increase in water loss with increased temperature and sucrose and salt concentrations. Water loss was higher for simulated apple and 1% potato starch solutions compared with 15% potato starch solution. Starch in the visking tubing competed for the removal of water during osmotic dehydration. The amount of starch contributed to the differences in the rate of water loss in the visking tubing.

PRACTICAL APPLICATIONS

In order to better understand the behavior of water in simulated apple and potato starch cell complex during osmotic dehydration of plant materials, the Visking tubing was used as an artificial cell in simulating biological membranes. Simulated apple and potato starch was studied as representatives of cellulosic and starchy plant materials, respectively. Notably, there was antagonistic effect between starch and salt solutions, starch and sugar solutions similar to sugar and salt solutions as a result of differences in their molecular weights. This study is industrial applicable to achieve efficient osmotic dehydration processes for plant materials that are cellulosic or starchy in nature.     

RHEOLOGICAL AND TEXTURAL CHARACTERISTICS OF RAW AND PAR-COOKED TAEWA (MAORI POTATOES) OF NEW ZEALAND

The quality characteristics of raw and par-cooked (partially cooked) tubers from four different Taewa cultivars were evaluated and compared with a modern potato cultivar (Nadine). Significant differences in dry matter, starch content, color and specific gravity were observed among the raw tubers from the different cultivars. Moemoe and Tutaekuri Taewa cultivars showed higher dry matter content (21.97 and 21.57%, respectively) whereas the lowest specific gravity of 1.057 was observed for Nadine. Par-cooked chef-ready tubers were prepared from each cultivar and evaluated for quality during 21 days of storage at 4C. Tutaekuri raw as well as par-cooked tubers had higher fracturability and hardness whereas Nadine par-cooked tubers had higher adhesiveness than other cultivars. Rheological parameters such as storage modulus ( G' ), loss modulus and loss tangent (tan δ ) were dependent on the dry matter and starch contents of the tubers. The highest peak G' values of 24,930 Pa and 19,700 Pa were observed for Tutaekuri and Moemoe potato cultivars, respectively.

PRACTICAL APPLICATIONS

Ready-to-microwave par-cooked tubers with a refrigerated shelf life of 21 days were developed without using any chemical preservatives. The rheology of the tuber tissue was studied using a dynamic rheometer for the first time. The technique allows the continuous monitoring of the changes occurring in the potato tissue during the heating and cooling cycles. Significant correlations between the textural characteristics of raw and cooked tubers and the dynamic rheological characteristics of fresh tubers were observed. The technique may be helpful in predicting the viscoelastic characteristics of raw potatoes and the changes in the texture of cooked tubers during refrigerated storage.     

Cellular,biological, and physicochemical basis for the hard‐to‐cook defect in legume seeds

This review integrates current knowledge on the hard‐to‐cook (HTC) defect in legume seeds, with emphasis on the cellular and biological changes during storage and soaking, and the physicochemical changes during heating. Several postulated mechanisms, including the pectin‐cation‐phytate model, cell lignification, pectin β‐eliminative degradation, and protein denaturation in relation to starch gelatinization, are discussed in the context of current evidence. Subsequently, a developmental model of legume hardening is presented. It is held that the HTC defect develops during aging and soaking and is exhibited through cooking. During the process, there are many events involved. Free radical formation, lipid peroxidation, acid formation, membrane deterioration, protein denaturation, and leakage are events associated with aging and soaking, whereas pectin decomposition and solubilization, protein coagulation, and starch gelatinization are events that occur during cooking. Cooked HTC seeds are characterized by limited cell separation and restricted starch gelatinization. These defective features result from a restriction in pectin decomposition and solubilization as well as the protein coagulation that prevails over starch gelatinization during heating. This multichannel mechanism points to the direct involvement of two amphoteric colloids, cell wall pectin and storage protein, both of which are sensitive to pH and/or ion composition. The model also indicates the indirect involvement of cell membranes and starch granules. Except for events that occur during aging and soaking, it is likely that heat‐related textural problems in other plant tissues may proceed via a mechanism similar to legume hardening.