Effect of preheating on potato texture

Preheating potatoes at 50 to 80°C has a firming effect on the cooked potato tissue. This effect is particularly pronounced at a preheating temperature of 60 to 70°C followed by cooling. Several theories have been presented in the literature to explain this firming effect: retrogradation of starch, leaching of amylose, stabilization of the middle lamellae and cell walls by the activation of the pectin methylesterase (PME) enzyme, and by the release of calcium from gelatinized starch and the formation of calcium bridges between pectin molecules. Most probably, none of these theories alone can explain the phenomenon and more than one mechanism seems to be involved. Some of these mechanisms seem to be interdependent. As an example, calcium could be considered as a link all the way through release after starch gelatinization to cross‐linking pectin substances in the cell wall and the middle lamellae, which has been demethylated by the PME enzyme. More research and “clear cut” experiments are needed in order to elucidate the role of each mechanism, especially which of them is the main contributor to the process of firming. Most probably, the calcium‐pectin‐PME mechanism plays a secondary role, that is, it only retards the collapse of the tissue structure that would otherwise occur during the final heating without preheating, and it is not the main factor of firmness.     

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%.     

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.,     

Unravelling process-induced pectin changes in the tomato cell wall: An integrated approach

The activity of the pectin-modifying enzymes pectin-methylesterase (PME) and polygalacturonase (PG) in tomato fruit was tailored by processing. Tomatoes were either not pretreated, high-temperature blanched (inactivation of both PME and PG), or high-pressure pretreated (selective inactivation of PG). Subsequently, two types of mechanical disruption, blending or high-pressure homogenisation, were applied to create tomato tissue particle suspensions with varying degrees of tissue disintegration. Process-induced pectin changes and their role in cell-cell adhesion were investigated through in situ pectin visualisation using anti-pectin antibodies. Microscopic results were supported with a (limited) physicochemical analysis of fractionated walls and isolated polymers. It was revealed that in intact tomato fruit pectin de-esterification is endogenously regulated by physical restriction of PME activity in the cell wall matrix. In disintegrated tomato tissue on the other hand, intensive de-esterification of pectin by the activity of PME occurred throughout the entire cell wall. PG was selectively inactivated (i.e. in high-pressure pretreated tomatoes), with de-esterification of pectin by PME, which resulted in a high level of Ca²⁺-cross-linked pectin and a strong intercellular adhesion. In non-pretreated tomato suspensions on the other hand, combined PME and PG activity presumably led to pectin depolymerisation and, hence, reduced intercellular adhesion. However, because of the high amount of Ca²⁺-cross-linked pectin in these samples, cell-cell adhesion was still stronger than in the high-temperature blanched tomatoes, in which the absence of PME activity during suspension preparation implied few Ca²⁺-cross-linked pectic polymers and extensive cell separation upon tissue disruption.     

Firming of fruit tissues by vacuum-infusion of pectin methylesterase: Visualisation of enzyme action

Apple pieces were vacuum-impregnated with either a pectin methylesterase (PME) and calcium solution or with water prior to pasteurization. Pasteurized apple pieces impregnated with PME and calcium showed a significantly higher firmness. Moreover, solid state ¹³C NMR spectroscopy of apple cell wall residues revealed an increase of their molecular rigidity. Exogenous PME addition involved a decrease from 82% to 45% of apple pectin degree of methyl-esterification. Microscopic observations of apple slices immunolabelled with antibodies specific for pectins showed that (i) demethyl-esterification was more intense in the cell wall region lining intercellular spaces (demonstrating a key role for these intercellular channels in the enzyme penetration in the tissue during vacuum-infusion) and that (ii) the number of calcium-dimerized deesterified homogalacturonan chains increased. The results corroborate the hypothesis that vacuum-impregnated PME action liberates free carboxyl groups along pectin chains that could interact with calcium, increasing the rigidity of pectins and finally the mechanical rigidity of apple tissue.     

Cell wall break down of pitanga fruit (Eugenia uniflora L.) is associated with pectic solubilisation and softening

Pitanga fruit stands out for its exotic flavour and antioxidant properties. The objective of this work was to evaluate changes in the physical, chemical and biochemical variables of red pitanga variety, in four stages of development (green, yellow, orange and red) and associate them to the cell wall break down. Analysis of pH, titratable acidity, soluble solids, firmness, soluble pectin, pectinamethylesterase (PME), polygalacturonase (PG), cell wall swelling and microstructure were performed. pH and levels of soluble solids and soluble pectin increased, in addition to the cell wall swelling and reduction of firmness, throughout the development of pitanga. From the SEM, it was possible to observe the degradation of the fruit cell wall during ripening, suggesting mainly the degradation of pectic polysaccharides, verified by the solubilisation of these substances. PG and PME enzymes were not active, which suggests that other enzymes may be associated with the cell wall break down.     

Textural properties of gelling system of low-methoxy pectins produced by demethoxylating reaction of pectin methyl esterase

ABSTRACT:  After deesterification of commercial pectins with a pectin methyl esterase (PME), their gelling properties were characterized using instrumental texture analysis. The final degree of esterification (DE) of the high- and low-methoxy pectins reached approximately 6% after the PME treatment, while deesterification of low-methoxy amidated pectin stopped at 18% DE. Furthermore, DE of high-methoxy pectin was tailored to be 40%, which is equivalent to the DE of commercial low-methoxy pectin. As a result, significant changes in molecular weight (Mw) distribution were observed in the PME-treated pectins. The texture profile analysis showed that PME modification drastically increased hardness, gumminess, and chewiness, while decreasing cohesiveness and adhesiveness of the pectin gels ( P < 0.05). The pectin gel with relatively high peak molecular weight (Mp, 3.5 × 10⁵) and low DE (6), which was produced from high-methoxy pectin, exhibited the greatest hardness, gumminess, chewiness, and resilience. The hardness of low-methoxy amidated pectin increased over 300% after PME deesterification, suggesting that the effects of amide substitution could be reinforced when DE is even lower. The partial least square regression analysis indicated that the Mw and DE of the pectin molecule are the most crucial factors for hardness, chewiness, gumminess, and resilience of gel matrix.     

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     

Anti-homogalacturonan antibodies: A way to explore the effect of processing on pectin in fruits and vegetables?

The potential of the anti-homogalacturonan(HG) antibodies JIM5, JIM7, LM7, LM18, LM19, LM20 and PAM1 for investigating the effect of processing on pectin in fruit and vegetable tissues was screened in this study. In this respect, the specificity of the antibodies towards pectin and methoxylated polygalacturonic acid with defined degrees and patterns of methylesterification was elaborated, leading to a substantial extension of the information already available in literature. Based on the obtained specificities, the distribution of pectin methylesterification in carrot and broccoli tissue was mapped. It was established that pectin with a low degree of esterification (DE) and a more blockwise distribution of the methylesters is, both in carrot and in broccoli, preferentially located at the tricellular junctions between adjacent cells. This cell wall region, however, is also likely to contain other structural pectic domains. The inner face of the cell wall adjacent to the plasma membrane seems to contain pectin with a medium DE. To evaluate the potential of the anti-HG antibodies to detect changes in degree and pattern of methylesterification caused by processing, carrot and broccoli were subjected to a thermal treatment aimed to stimulate the endogenous pectinmethylesterase (PME) activity. It was revealed that process-induced de-esterification by endogenous PME mainly tends to take place at discrete regions of the inner face of the cell wall adjacent to the plasma membrane for carrot and in tricellular junctions and the middle lamella for broccoli.     

Towards a better understanding of the pectin structure–function relationship in broccoli during processing: Part II — Analyses with anti-pectin antibodies

To investigate the structure–function relationship of pectin during (pre)processing, broccoli samples (Brassica oleracea L. cultivar italica) were subjected to one of the following pretreatments: (i) low-temperature blanching (LTB), (ii) LTB in combination with Ca²⁺ infusion, (iii) high-pressure pretreatment (HP), (iv) HP in combination with Ca²⁺ infusion, or (v) no pretreatment (control sample), whether or not in combination with a thermal treatment of 15 min at 90 °C. Anti-homogalacturonan antibodies were used to perform in situ (microscopy) and ex situ (immuno-dot assays) analyses on broccoli pectin which resulted in information concerning the localisation of defined pectic domains in broccoli cell walls and pectin's structure. Water-soluble pectin appears to contain unbranched, high-esterified pectin and some pectic polymers with abundant side chains that are less esterified. Ionically cross-linked pectin, on the other hand, contains low-esterified pectin with either highly branched or unbranched domains. The in situ visualisation of pectin in broccoli suggested that de-esterification of pectin by PME during LTB as well as during HP mainly takes place in the tricellular junctions of adjacent cells in broccoli tissue. Ca²⁺-cross-linked pectin could be found in cell walls lining intercellular spaces and was particularly abundant at the corners of intercellular spaces, indicating its important role in cell–cell adhesion. Both LTB and HP created pectin–Ca²⁺-cross-links in parts of the cell wall where these cross-links were originally absent. The influence of thermal processing and the effect of pressurisation on the pectic components in the cell wall could also be visualised using the antibodies.     

Enzyme infusion and thermal processing of strawberries: Pectin conversions related to firmness evolution

Strawberries were infused with fungal pectinmethylesterase (PME) and/or calcium chloride with the aim of minimising tissue damage during subsequent thermal processing (95 °C). Firmness measurements and micrographs provided information on the extent of tissue damage. These observations were linked to the chemical structure of pectin. When PME was infused in absence of Ca²⁺, the degree of methoxylation of pectin was lowered, but chains remained water soluble, indicating that they were not crosslinked. Thermal processing of PME-infused strawberries resulted in pectin solubilisation and depolymerisation which was reflected in pronounced firmness decrease and tissue damage, comparable to non-infused processed strawberries. On the other hand, when a combination of both PME and Ca²⁺ was infused, an important decrease in processing-related tissue damage was perceived. This can be explained by increased crosslinking of pectin chains with low degree of methoxylation, rendering them insoluble and less susceptible to thermal depolymerisation.     

Effect of Cooking and Pre-Cooking on Cell-Wall Chemistry in Relation to Firmness of Carrot Tissues

The aim of this work was to investigate heat-induced changes in cell wall polysaccharides of carrot in relation to texture. Discs of fresh carrot (Daucus carota cv Amstrong) tissue were subjected to cooking (100°C, 20 min), with or without a pre-cooking treatment (50°C, 30 min). Alcohol-insoluble residues were prepared from the tissues and were extracted sequentially with water, NaCl, CDTA, Na₂CO₃ and 0·5 M KOH to leave a residue. These were analysed for their carbohydrate compositions, their degree of methyl esterification and the molecular size of selected soluble polysaccharides. Cooking caused tissues to soften. This involved cell separation, an increase in water- and salt-soluble, high-molecular-weight pectic polysaccharides and a concomitant decrease in the pectic polymers in all wall extracts and the residue. Pre-cooking prior to cooking enhanced cell–cell adhesion and reduced the extent of softening. This was accompanied by a general reduction in the degree of methylesterification of cell-wall pectic polymers, and a decrease in the cooking-induced modification to all pectic fractions. The firming effect of pre-cooking could be reversed by extracting the precooked+cooked tissue with CDTA, a chelating agent. The role of Ca²⁺ cross-linked polymers and pre-cooking in the enhancement of firmness are discussed. © 1997 SCI.     

预处理对增加红富士苹果细胞壁物质降解和出汁率的影响

为了通过简便易行的预处理增强果实细胞壁物质降解,提高苹果果实出汁率。文中测定了500mg/L乙烯利(E_1)和100mg/L乙烯利配合60℃热水(E_2)处理,对红富士果实堆放期细胞壁酶活性、细胞壁物质含量和出汁率的影响。结果表明:E_1使其果胶甲酯酶(PME)、纤维素酶(CS)活性高峰分别比对照提前6、3d,多聚半乳糖醛酸酶(PG)活性在前15d持续高于对照,木聚糖酶(Xyl)活性基本不变;处理12d后,果实细胞壁多糖中果胶类多糖、半纤维素类多糖和细胞壁残渣多糖含量分别下降到对照的80.1%、70.4%和75.7%;处理后9～18d,红富士苹果的出汁率较对照提高2.3%～4.0%;E_2抑制了果实PME活性,却普遍增强了0～6d内PG、Xyl、CS活性,亦促进了各类细胞壁物质的降解,0～9d内果实出汁率较对照高3.2%～7.1%。     

Effect of steam cooking on the residual enzymatic activity of potatoes cv. Agria

BACKGROUND: The aim of this work was to study the influence of steam cooking on pectin methylesterase (PME) and endogenous α‐ and β‐amylase activities in different tissues (cortex and pith) of raw and heat‐treated potatoes cv. Agria. Three different cooking temperatures were chosen (55, 70 and 85 °C). For each cooking trial, time–temperature profiles were recorded and the degree of cooking was expressed in terms of cooking factor. RESULTS: Steam cooking contributed to significantly activate PME at 55 °C and to reduce its activity at the final processing temperature (85 °C), with the highest amount in the cortex (0.3745 ± 0.0007 µmol galacturonic acid (GA) g^?1 fresh weight (FW) min^?1) compared with the pith (0.2617 ± 0.0012 µmol GA g^?1 FW min^?1). The presence of heat‐labile and heat‐stable isoforms of PME in the considered potato tissues was also assumed. Heat treatment by steam resulted in a significant decrease in endogenous α‐ and β‐amylase activities in both tissues compared with the raw potato, though without complete deactivation. Starch‐degrading enzymes were also found to be differently distributed in the raw tuber. CONCLUSION: Steam cooking affected in different ways the assessed residual enzymatic activity in the considered tissues of potatoes cv. Agria. Further research is needed to confirm the results obtained. Copyright © 2011 Society of Chemical Industry     

Texture improvement of fermented minced pepper under vacuum impregnation with pectin methylesterase and CaCl2 during fermentation

Fermented minced pepper (FMP) usually suffers from the deterioration of texture quality during fermentation, which can affect sensory and consumer acceptance. In this study, vacuum impregnation (VI) with CaCl₂, pectin methylesterase (PME) and CaCl₂ and PME (PME + CaCl₂ + VI) were compared to improve the texture quality of FMP. FMP treated with PME + CaCl₂ + VI showed the relatively intact cells structure after fermentation. In that case, its firmness maintained high level, while water-soluble pectin (WSP) content was minimum after fermentation. Meanwhile, the molar ratio of most monosaccharides of WSP in PME + CaCl₂ + VI treated FMP decreased, while rhamnose (Rha) molar ratio significantly (p < 0.05) increased after fermentation. The high Rha content represents the stability of rhamnogalacturonan-I linear skeleton of WSP. The negative effect on molecular weight of WSP was delayed by PME + CaCl₂ + VI treatment, and its peak area and value increased after fermentation. Atomic force microscope images indicated that PME + CaCl₂ + VI treatment could retain the long chain and branch structures, and inhibit the degradation of WSP net-like structure at some extent. Hence, PME + CaCl₂ + VI treatment was effective to improve the texture of FMP and inhibit the solubilisation of WSP via the formation of cross-linked pectin chains between Ca²⁺ and demethylesterified pectin.     

Texture and distribution of pectic substances of mango as affected by infusion of pectinmethylesterase and calcium

Fresh cut mangos were infused with pectinesterase (PME) and calcium chloride, and the effect on textural properties, distribution of pectic substance and degree of esterification was determined. Temperature gradient infusion with PME and/or calcium chloride increased gumminess and chewiness, but had no impact on hardness and adhesiveness. The distribution of pectic substances, as protopectin or alkaline soluble pectin, was approximately twice that of water‐ or chelator‐soluble pectin. The degree of esterification of water‐ and chelator‐soluble pectic substances was near 50–60%, and less than 10%, respectively. Heat‐sensitive PME inhibitor in mango was detected. The initial hardness of Kent mango was variable, and differences in distribution of pectic substances were observed. Texture of Kent mango is most likely moderated by changes in the solubility of insoluble pectin or by non‐pectin components in the cell wall. Copyright © 2004 Society of Chemical Industry     

不同成熟度橄榄果实冷藏期间细胞壁代谢对采后冷害的响应特性

研究冷藏期间橄榄果实细胞壁代谢的变化,探讨不同成熟度橄榄果实冷害发生与细胞壁组分含量、细胞 壁降解酶活性的关系。以白露、寒露、立冬、大雪节气时采摘的‘檀香’橄榄果实为材料,在温度（2±1）℃、 相对湿度85%～90%冷库内贮藏,定期测定橄榄果实冷害指数、果肉细胞壁组分含量和细胞壁降解酶活力的变化。 结果表明,不同成熟度橄榄果实冷藏期间冷害发生与其细胞壁组分降解密切相关,冷害指数与离子结合型果胶 （ionic-soluble pectin,ISP）、共价结合型果胶（covalent-soluble pectin,CSP）、半纤维素和纤维素含量呈负相 关;且果胶甲酯酶（pectin methylesterase,PME）、多聚半乳糖醛酸酶（polygalacturonase,PG）、β-半乳糖苷酶 （β-galactosidase,β-Gal）和纤维素酶（cellulase,CEL）等细胞壁降解酶的活力变化不平衡或活力提高是导致冷藏 橄榄果实细胞壁结构解体、细胞壁代谢异常、冷害发生的主要原因。同时,与成熟度Ⅰ、Ⅲ和Ⅶ的橄榄果实相比, 成熟度Ⅴ保持较低的果实冷害指数及冷藏中后期果肉PME、PG、β-Gal和CEL活力,延缓冷藏中后期果肉水溶性果 胶、ISP、CSP、半纤维素和纤维素含量降低。因此认为,成熟度Ⅴ的橄榄果实可较好维持细胞壁结构的完整性, 有效减轻冷害发生。     

Characterization of Cell Wall Enzyme Activities, Pectin Composition, and Technological Criteria of Strawberry Cultivars (Fragaria×ananassa Duch)

ABSTRACT: The effects of physical characteristics and cell wall enzymatic activities of several strawberry culti-vars were investigated for possible industrial use. The enzymes study showed that the softest varieties had the highest pectin methylesterase (PME) and polygalacturonase (PG) activities. Differences in alcohol-insoluble pectin, water-soluble pectin, and parietal residue compositions were observed between Darsanga ("firm fruit") and Senga sengana ("soft fruit"). Finally, the study of pectin composition of Darsanga and Senga sengana indicated that the softest fruit had the highest water-soluble pectin content. The measurement of fruit PME activity permitted a preliminary screening of fruit maturity characteristics.     

Pectin methyl esterase treatment on high‐methoxy pectin for making fruit jam with reduced sugar content

BACKGROUND: Pectin methyl esterase (PME) has been postulated to catalyse the transacylation reaction between pectin molecules. The present study aimed to prove the occurrence of this reaction. The feasibility of applying PME‐catalysed transacylation between high‐methoxy pectin molecules in making fruit jam with reduced sugar content was also investigated. RESULTS: PME treatment increased the turbidity and particle size in pectin solution and the molecular weight of pectin, while it decreased the number of methoxy ester linkages and the intensity of the CH₃ absorption peak in the Fourier transform infrared spectrum without changes in the number of total ester linkages in pectin molecules. These findings support the occurrence of PME‐catalysed transacylation between pectin molecules. Higher values of hardness, gumminess and chewiness were found in a jam containing PME‐treated citrus pectin (10 g L^?1) and sugar (350 g L^?1) as compared with either a jam containing untreated citrus pectin (10 g L^?1) and sugar (350 g L^?1) or strawberry jam containing pectin (10 g L^?1) from the fruit and sugar (650 g L^?1). CONCLUSION: The demand for sugar in jam making can be greatly reduced by the use of PME‐treated high‐methoxy pectin. Copyright © 2012 Society of Chemical Industry     

Aspects of material science in food processing: changes in plant cell walls of fruits and vegetables

 Foods can be regarded as complex, dispersed systems which are normally metastable. Food processing causes state transitions (second-order transitions) when raw materials, food components or food systems are subjected to external stresses. The state transitions occurring during processing are detectable as changes in structure and properties of the investigated systems. The processing of fruits and vegetables is often connected with changes in cell walls. Cell wall materials in dispersed fruit and vegetable systems can be regarded as a model substrate of the dispersed phase. During processing, cell walls undergo modifications in terms of their physical state, macrostructure, microstructure, and composition, as well as structure-dependent changes in their functional and material properties. The interactions and connections (dependencies) between state transitions, and various changes in structure and properties, are very complex and multivariate and are not well understood as yet. For the evaluation of changing material properties during processing, examination of hydration, rheological (external mechanical stress) and thermal (external thermal stress) characteristics is important. The changes occurring during the processing of fruits and vegetables are determined by external factors (especially various mechanical and thermal stresses) and by internal factors. External stress in many cases causes solubilisation of the cell wall, loss of firmness and favours cell separation. Thermal processing increases pectin degradation by β-elimination. Internal factors such as pH and modified ionic strength, e.g. by applying soak solutions, can have an important influence on the changes in the cell wall during processing. So, calcium ions on the one hand can favour cell wall degradation by β-elimination and, on the other hand, after low temperature blanching and de-esterification of the pectin by activated pectin methyl esterase, can contribute to stabilisation of the texture by formation of a calcium-pectin complex. Knowledge about cell wall degradation mechanisms can be markedly improved by studies using model substrates such as pectin, or cell wall materials like the alcohol-insoluble residue and materials with cellular structure. This knowledge has been used to improve the technology used to process fruits and vegetables and to produce products with better properties. Moreover, testing and applying cell wall materials as ingredients for the production of textured foods and potential health-related foods is suggested.