Textural quality assessment for fresh fruits and vegetables.

Academically, quantitative measurement of texture is essential for the study of the chemical and physiological mechanisms of texture. Commercially, quantitative measurement of texture is essential to ensure the quality of produce at packout. The diversity of tissues involved, the variety of attributes required to fully describe textural properties, and the changes in these attributes as the product ripens and senesces contribute to the complexity of texture measurement. Texture is a human assessment of the structural elements of a food. It is generally accepted that texture relates primarily to mechanical properties, so instrumental measurements relate mainly to mechanical properties. Fruits and vegetables exhibit viscoelastic behavior under mechanical loading, which means that the force, distance, and time involved in loading determine the value of any measurement. Because of their viscoelastic character, every effort should be made to hold the speed of the test constant in manual texture measurements and the rate of loading should be specified and controlled in mechanized measurements. There are many types of mechanical loading: puncture, compression, shearing, torsion (twisting), extrusion, crushing, tension, bending, vibration, and impact. The most widely used texture measurement for fruits and vegetables, after manual squeezing of course, is the Magness-Taylor fruit firmness test, which measures the maximum force to puncture the product in a specified way. The Kramer shear or shear-compression test is widely used in the processed foods industry, but is less commonly used by horticulturists. Nondestructive methods are highly desired both for sorting and for postharvest research. Compression tests of excised tissue pieces are frequently used in research. Nondestructive testing using impact, vibrational behavior, light scattering, and optical methods are being investigated but none has been widely accepted to date. Multiple instrumental measurements may be necessary to adequately the diversity of textural attributes sensed by the human consumer.

Down-regulation of tomato beta-galactosidase 4 results in decreased fruit softening.

Transcript abundance of the gene encoding beta-galactosidase II, a beta-galactosidase/exo-galactanase (EC 3.2.1.23) present during tomato (Lycopersicon esculentum) fruit ripening, was suppressed by expression of an antisense tomato beta-galactosidase 4 (TBG4) cDNA driven by the cauliflower mosaic virus 35S promoter. RNA gel-blot analysis was used to evaluate TBG4 mRNA levels in transgenic fruit. All of the antisense lines had attenuated TBG4 mRNA levels in turning stage fruit; however, TBG4 mRNA suppression was unstable, and mRNA levels varied in red-ripe fruit among the lines. Suppression of TBG4 mRNA levels in antisense fruit was correlated with a reduction in extractable exo-galactanase activity against a lupin galactan. All of the antisense lines had reduced free galactose levels at mature green stage 4, but levels comparable with controls during ripening. Total cell wall galactosyl contents in the antisense fruit were not significantly different from control fruit. Whole-fruit firmness was measured using a texture analyzer and the means of the peak force measurements for four of six antisense lines were significantly higher than control fruit. One antisense line had red-ripe fruit that were 40% firmer than controls. Fruit from this antisense line also had the lowest TBG4 mRNA and exo-galactanase levels and the highest wall galactosyl content during the early stages of ripening, implicating an involvement of this gene product in cell wall modification leading to fruit softening.