Pectins: structure, biosynthesis, and oligogalacturonide-related signaling.

Excessive softening is the main factor limiting fruit shelf life and storage. Transgenic plants modified in the expression of cell wall modifying proteins have been used to investigate the role of particular activities in fruit softening during ripening, and in the manufacture of processed fruit products. Transgenic experiments show that polygalacturonase (PG) activity is largely responsible for pectin depolymerization and solubilization, but that PG-mediated pectin depolymerization requires pectin to be de-methyl-esterified by pectin methylesterase (PME), and that the PG β-subunit protein plays a role in limiting pectin solubilization. Suppression of PG activity only slightly reduces fruit softening (but extends fruit shelf life), suppression of PME activity does not affect firmness during normal ripening, and suppression of β-subunit protein accumulation increases softening. All these pectin-modifying proteins affect the integrity of the middle lamella, which controls cell-to-cell adhesion and thus influences fruit texture. Diminished accumulation of either PG or PME activity considerably increases the viscosity of tomato juice or paste, which is correlated with reduced polyuronide depolymerization during processing. In contrast, suppression of β-galactosidase activity early in ripening significantly reduces fruit softening, suggesting that the removal of pectic galactan side-chains is an important factor in the cell wall changes leading to ripening-related firmness loss. Suppression or overexpression of endo-(1\to4)β-d-glucanase activity has no detectable effect on fruit softening or the depolymerization of matrix glycans, and neither the substrate nor the function for this enzyme has been determined. The role of xyloglucan endotransglycosylase activity in softening is also obscure, and the activity responsible for xyloglucan depolymerization during ripening, a major contributor to softening, has not yet been identified. However, ripening-related expansin protein abundance is directly correlated with fruit softening and has additional indirect effects on pectin depolymerization, showing that this protein is intimately involved in the softening process. Transgenic work has shown that the cell wall changes leading to fruit softening and textural changes are complex, and involve the coordinated and interdependent activities of a range of cell wall-modifying proteins. It is suggested that the cell wall changes caused early in ripening by the activities of some enzymes, notably β-galactosidase and ripening-related expansin, may restrict or control the activities of other ripening-related enzymes necessary for the fruit softening process.

Cell wall metabolism in fruit softening and quality and its manipulation in transgenic plants

Molecular Basis of Ca2+-Induced Gelation in Alginates and Pectins: The Egg-Box Model Revisited

The growing ecological and environmental consciousness has driven efforts for development of new innovative materials for various end-use applications. Polymers synthesized from natural resources, have gained considerable research interest in the recent years. This review paper is intended to provide a brief outline of work that covers in the area of biocomposites, major class of biodegradable polymers, natural fibres, as well as their manufacturing techniques and properties has been highlighted. Various surface modification methods were incorporated to improve the fibre–matrix adhesion resulting in the enhancement of mechanical properties of the biocomposites. Moreover, an economical impact and future direction of these materials has been critically reviewed. This review concludes that the biocomposites form one of the emerging areas in polymer science that gain attention for use in various applications ranging from automobile to the building industries.

A review of the recent developments in biocomposites based on natural fibres and their application perspectives

Pectin is a high value functional food ingredient widely used as a gelling agent and stabilizer. It is also an abundant, ubiquitous and multifunctional component of the cell walls of all land plants. Food scientists and plant scientists therefore share a common goal to better understand the structure and functionalities of pectic polymers at the molecular level. The basic properties of pectin have been known for nearly 200 years, but recently there has been tremendous progress in our understanding of the very complex fine structure of pectic polymers and pectinolytic enzymes. This has been made possible by synergies between plant and food research and by the application of a range of state-of-the-art techniques including enzymatic fingerprinting, mass spectrometry, NMR, molecular modelling, and monoclonal antibodies. With this increased knowledge comes the prospect of novel applications. Producers are beginning to develop a new generation of sophisticated designer pectins with specific functionalities. Moreover, the ability to manipulate pectin in planta would have a major impact on fruit and vegetable quality and processing, as well as on pectin production.

Pectin: new insights into an old polymer are starting to gel

Pectin is an important polysaccharide with applications in foods, Pharmaceuticals, and a number of other industries. Its importance in the food sector lies in its ability to form gel in the presence of Ca2+ ions or a solute at low pH. Although the exact mechanism of gel formation is not clear, significant progress has been made in this direction. Depending on the pectin, coordinate bonding with Ca2+ ions or hydrogen bonding and hydrophobic interactions are involved in gel formation. In low‐methoxyl pectin, gelation results from ionic linkage via calcium bridges between two carboxyl groups belonging to two different chains in close contact with each other. In high‐methoxyl pectin, the cross‐linking of pectin molecules involves a combination of hydrogen bonds and hydrophobic interactions between the molecules. A number of factors—pH, presence of other solutes, molecular size, degree of methoxylation, number and arrangement of side chains, and charge density on the molecule— influence the gelation o...

Chemistry and uses of pectin--a review.

Juice from transgenic tomato (Lycopersicon esculentum cv Rutgers) fruits with reduced levels of pectin methylesterase (PME) activity due to the expression of a PME antisense gene exhibited improvement in quality. The percentage increase in juice from transgenic fruits over that of juice from wild type Rutgers ranged between 5.1‐5.3 for total solids, 3.8‐6.1 for soluble solids, 70-80 for efflux viscosity, 180‐220 for serum viscosity and about 50 for precipitate weight ratio. Time of harvest had no effect on quality of juice. Ketchup prepared from transgenic fruit juice had a lower Bostwick value, reduced serum separation and high serum viscosity compared to ketchup from parental Rutgers.

Tomato Product Quality from Transgenic Fruits with Reduced Pectin Methylesterase

Pressurized homogenization of tomato juice at room temperature (28C) leads to an increased consistency and reduced serum separation in the juice. Percentage increase in consistency (as measured by Libby's tube) of hot processed juice from different cultivars ranged from 95 ± 3 to 282 ± 13 s. The magnitudes of change in consistency and serum separation of cold processed juice due to homogenization were smaller than those for hot processed juice. Increases in consistency of cold processed juice ranged from 42 ± 4 to 213 ± 10 s. Consistency in all the samples increased with increasing pressure of homogenization up to a pressure of 3000 psi, above which it became constant. Homogenization of juice at higher temperature resulted in a very small increase in consistency. However, the magnitude of increase in consistency decreased with increasing temperature. Serum separation slightly increased at higher pressure of homogenization, after reaching a minimum with increasing pressure initially. In general, the juice with higher initial efflux and serum viscosities showed a larger increase in consistency and greater reduction in serum separation.

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Effect of homogenization pressure on consistency of tomato juice

Pectins in tomato (Lycopersicon esculentum) juice processed from transgenic fruits with reduced levels of pectin methylesterase activity and from control fruits were characterized for total uronic acid, degree of methoxylation (DOM), and molecular mass. The molecular weights of pectins isolated from transgenic fruit juice are much higher than the molecular weights of pectins isolated from control fruit juice under all processing conditions. Depending on processing conditions, juice from transgenic fruits contained 30-50% higher amounts of total uronic acids and had 25-250% higher DOM compared to juice from control fruits.

Effect of an Antisense Pectin Methylesterase Gene on the Chemistry of Pectin in Tomato (Lycopersicon esculentum) Juice

Addition of soy proteins to tomato sauce resulted in an increased consistency and reduced serum separation in the sauce. Percentage increase varied from 1. 7 to 26.0% for Bostwick consistency, and 80.0 to 88.0% for serum viscosity of tomato sauce as protein concentration increased from 0.25 to 3.0%. The percentage decrease in serum separation in the sauce varied from 30. 0 to 96.0% with increasing concentration of added soy protein. Products containing soy protein up to 1 % were rated acceptable by a panel of trained judges on a nine point hedonic scale. Collectively, the results show that addition of soy protein causes significant improvement in quality attributes of tomato sauce including higher nutritional value.

B.R. Thakur

Papers

Chemistry and uses of pectin--a review.

Tomato Product Quality from Transgenic Fruits with Reduced Pectin Methylesterase

Effect of homogenization pressure on consistency of tomato juice

Effect of an Antisense Pectin Methylesterase Gene on the Chemistry of Pectin in Tomato (Lycopersicon esculentum) Juice

Effect of added soy protein on the quality of tomato sauce.