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'Rocha' pear (Pyrus communis L.) was used as a model system to assess the effect of pH of dipping solutions on quality retention of fresh-cut fruit and its interaction with calcium additives. Pear slices were dipped for 60. s in a buffer solution at pH 3.0, 5.0 or 7.0 and stored at 4.5°C for 13 days. In other experiments, pear slices were dipped for 60. s in buffer solutions containing 250. mM of calcium ascorbate, lactate, chloride, and propionate, at pH 3.0 or 7.0, and stored at 4.5°C for 6 days. Browning and softening were more intense in slices dipped in a solution at pH 3.0 than at pH 5.0 or 7.0, but microbial growth was lower in slices treated at pH 3.0. The effect of calcium additives depended on the anion and significant interactions between the effects of calcium salt and pH were observed. Calcium ascorbate was very effective in preserving color and reducing microbial growth irrespective of pH, but enhanced pectin solubilization and tissue softening at pH 3.0. Slices treated with 250. mM calcium propionate or calcium lactate were softer and had higher electrolyte efflux when treated at pH 3.0 than at pH 7.0. Calcium lactate enhanced browning and reduced microbial growth at pH 3.0 but did not affect color or microbial counts at pH 7.0. All calcium treatments enhanced electrolyte leakage. pH of the dipping solution can affect, per se, the quality of fresh-cut fruit. The choice of calcium additives to prevent undesirable changes on visual and sensory quality of cut produce should involve pH ranges that provide the expected benefits.

Blackspot related pigments were partially purified from bruised tubers of two commercial potato cultivars (cv. Bildtstar and cv. Lady Rosetta). Chemical characterization showed that these pigments consisted of protein and a relatively small amount of covalently bound constituents. These polymeric structures absorbed light throughout the visible spectrum without any maximum. They did not contain eumelanin. Quinic acid was detectable in hydrolysates of the pigments from Bildtstar, but not in those of Lady Rosetta, which indicated that chlorogenic acid may take part in blackspot formation, but is not essential for the discoloration. The results support the hypothesis that blackspot pigments are products of non-regulated reactions between nucleophilic amino acid residues in proteins, and quinones, which are derived from endogenous substrates of polyphenol oxidase. This means that blackspot formation most probably takes place in disintegrated cells.

Partially purified blackspot pigments from potato tubers (Solanum tuberosum L.) of two commercial cultivars were subjected to a microassay for melanin, which consisted of specific chemical degradation and subsequent HPLC analysis. Permanganate oxidation yielded pyrrole-2,3,5-tricarboxylic acid, whereas hydrolysis in hydriodic acid liberated aminohydroxyphenylalanine isomers. These results indicate that the polymeric pigments, which have previously been found to contain a protein matrix, carry crosslinked 5,6-dihydroxyindole-2-carboxylic acid and benzothiazine units. This leads to the conclusion that free tyrosine and free cysteine are incorporated in the proteinaceous pigments via the polyphenol oxidase catalysed pathway of melanogenesis in the process of blackspot formation. The findings are in accordance with the hypothesis that the process of blackspot formation is a non-regulated cascade of reactions in disintegrated tuber cells, rather than a finely tuned biosynthesis.

In a study of six potato varieties (Solanum tuberosum L.), which were grown under two different fertilization regimes, the biochemical potential of tuber tissue to synthesize blackspot pigments was distinguished from the actual blackspot susceptibility exhibited after impact application. Blackspot susceptibility and biochemical potential for pigment synthesis were not correlated, which supports the hypothesis that the extent to which the blackspot potential is actually being used is mediated by the sensitivity to cell decompartmentation. Quantification of polyphenol oxidase (PPO), soluble protein, and endogenous PPO substrates demonstrated that the content of free tyrosine is the predominant determinant for the biochemical potential for blackspot synthesis. PPO was not a limiting factor for pigment synthesis, even if PPO content was relatively low. It was therefore concluded that manipulation of PPO activity may offer a solution to the problem of blackspot formation only if it leads to a virtually complet...