Effect of Blanching and Drying on Pectin Constitutents and Related Characteristics of Dehydrated Peaches

The effects of sulfuring, blanching, dehydration, and storage (of dehydrated fruits), on the pectic constitutents and other characteristics of peaches are reported. Adequate blanching stabilized pectins of the dehydrated peaches so higher rehydration capacity and lower rehydration losses were observed after 5 min blanching. Increase in degradation was observed in nonblanched or 1.5 min heat-treated fruits, which resulted in lower rehydration capacity. A significant correlation was found between the contents of total pectin and protopectin fractions and the firmness or rehydration ratio of the peaches. Thus, pectin, one of the major cell-wall and intercellular tissue components, plays a significant role in determining the textural-structural characteristics of dehydrated fruits.     

p-HYDROXYPHENYLPROPIONIC ACID (PHPPA) AND 3,4-DIHYDROXYPHENYLPROPIONIC ACID (3,4-DPPA) AS SUBSTRATES FOR MUSHROOM TYROSINASE

p-Hydroxyphenylpropionic acid (PHPPA) and 3,4- dihydroxyphenylpropionic acid (3,4-DPPA) serve as substrates for tyrosinase. The Km value of 3,4-DPPA for tyrosinase is 1.3 mM. The yellow o-quinone of 3,4-DPPA (4-carboxyethyl-o-benzoquinone) (λ_max= 400nm), is detected initially and it is then converted to a red product(s) (λ_max= 480±10 nm), the o-quinone of 6,7-dihydroxy 3-dihydrocumarin (dihydroesculetin). When the concentration of the latter is relatively high, it polymerizes to a final brown product(s), characterized by an ill-defined spectrum.
H₂O₂ shortens the lag period of PHPPA hydroxylation, hastens the conversion of the yellow o-quinone of 3,4-DPPA to the red o-quinone of dihydroesculetin, and prevents the polymerization of the latter to the final brown product(s).
The relatively unstable o-quinone of 3,4-DPPA interacts with amines such as hydroxylamine (NH₂OH), p-aminosalicylic acid (PASA) and p-aminobenzoic acid (PABA), forming relatively stable final product(s) characterized by different spectra from those formed in their absence.
Acetohydroxamic acid (AHA) and salicylhydroxamic acid (SHAM) each has an effect on the spectrum of product(s) obtained when 3,4-DPPA is oxidized by tyrosinase, indicating that these hydroxamic acids derivatives interact with the o-quinone of 3,4-DPPA. The spectrum of the final product(s) was also different when 3,4-DPPA was oxidized by tyrosinase in the presence of benzenesulfinic acid than in its absence, suggesting the formation of a stable phenylsulfonyl derivative.     

INTERFERENCE OF CARBOHYDRATES DURING PURIFICATION OF PEROXIDASE FROM PALM (EUTERPE EDULIS MART.)*

Palmito (Euterpe edulis Mart.) peroxidase was initially prepared as an acetone powder to eliminate phenols and lipids. A 30–80% (NH₄)₂SO₄ fraction was used to characterize some biochemical properties of this enzyme. The main interference during enzyme purification was found to be the interaction of acid carbohydrates with peroxidase. The optimum condition to eliminate this interference was found to be extraction at pH 4.0, which preferentially extracts most of the carbohydrates but not the enzyme. CaCl₂ was found to be a compound which selectively precipitates with acid carbohydrates and proteins and increased peroxidase activity. Gel isoelectrofocusing was used to study the interaction between the acid polymers and the proteins. The pH optimum of the enzyme was found to be around 4.0. The molecular weight of the enzyme was estimated to be 40,000. The Km of peroxidase with H₂O₂ was 2times10⁻³M and with guaiacol, 4.8 times 10⁻³M. NaCl, NaF and NaN₃ were found to inhibit peroxidase as a function of pH; as the pH decreased, the inhibition increased.     

EFFECT OF BENZOIC ACID AND SOME OF ITS DERIVATIVES ON THE RATE OF DL-DOPA OXIDATION BY MUSHROOM TYROSINASE1

Vanillic acid and salicylic acid inhibited the rate of dihydroxyphenylalanine (DL-DOPA) oxidation to dopachrome (γ_max=475 nm) by tyrosinase at all concentrations tested. Benzoic acid and p-hydroxybenzoic acid (PHBA), at relatively low concentrations, slightly stimulated the rate of DL-DOPA oxidation, whereas at higher concentrations each inhibited the reaction. p-Hydroxybenzoic acid methyl ester (PHBAME), at relatively low concentrations, had a pronounced synergistic effect on the reaction, whereas at relatively high concentrations it inhibited the rate of DL-DOPA oxidation. The synergistic effect of 1.6–6.6 mM PHBAME on the rate of DL-DOPA oxidation to dopachrome was found to be only an apparent effect due to the ability of PHBAME to be hydroxylated very slowly by tyrosinase to a yellow pigmented product(s) with DL-DOPA serving as a reductant (AH₂) for the hydroxylation reaction, thus hastening the conversion of PHBAME to pigmented product(s). Vanillic acid, salicylic acid, benzoic acid and PHBA could not be hydroxylated by tyrosinase.     

Hindrance of Hemicellulose and Cellulose Hydrolysis by Pectic Substances

Treating the tissue of the grapefruit segment membrane with pectinase and cellulase decreased the content of xylose and glucose significantly more than the sum of the separate effects. The synergistic effect obtained by the combination of pectinase and cellulase showed that the pectic substance sterically masked the hemicellulose and cellulose. Breaking down the barrier of the pectic substance with pectinase allowed a significant increase in the hydrolysis of the hemicellulose and cellulose. Preparation of alchol-insoluble solids from fresh tissue modified the structure of the pectin in the cell wall and prevented its steric hindrance to breakdown of hemicellulose and cellulase. Extraction of pectic substances from the tissue by NaOH greatly increased cellulose hydrolysis by cellulase.     

EFFECT OF HYDROXYLAMINE, p-AMINOBENZOIC ACID AND p-AMINOSALICYLIC ACID ON THE OXIDATION OF O-DIHYDROXY- AND TRIHYDROXYPHENOLS BY MUSHROOM TYROSINASE

The effect of hydroxylamine (NH₂OH), p-aminobenzoic acid (PABA) and p-aminosalicylic acid (PASA) on the spectrum of the final product (s) formed when o-dihydroxy- and trihydroxyphenols were oxidized by tyrosinase was examined. New pigmented product(s), probably oximes, were formed by the interaction of NH₂OH with the o-quinones of 4-methyl catechol, 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylpropionic acid (3,4-DPPA) but not with the o-quinones of catechol or protocatechuic acid. Interaction of PABA or PASA with the o-quinones of catechol, 4-methyl catechol, protocatechuic acid, DOPAC and 3,4-DPPA also yielded pigmented oximes. The interaction of the o-quinones of trihydroxyphenols with NH₂OH, PABA or PASA had little effect on the spectrum of the final product (s), suggesting that oximes are not formed in these reactions.     

EFFECT OF BENZENESULFINIC ACID ON THE OXIDATION OF o-DIHYDROXY - AND TRIHYDROXYPHENOLS BY MUSHROOM TYROSINASE

Benzenesulfinic acid inhibits the rate of oxidation of different o-dihydroxy-phenols and trihydroxyphenols by tyrosinase when assayed spectrophotometrical-ly but barely has an effect when assayed polarographically. Benzenesulfinic acid is a much more effective inhibitor of the rate of oxidation to pigmented product(s) by tyrosinase of o-dihydroxyphenols than of trihydroxyphenols. The spectrum of the final product(s) formed by the oxidation of most of the tested substrates was different in the absence versus presence of benzenesulfinic acid suggesting that the latter traps the o-quinones forming phenylsulfonyl derivatives in the reaction.     

Pectolytic Enzyme Studies for Peeling of Grapefruit Segment Membrane

In vitro studies of commercial pectolytic enzymes with grapefruit membranes and citrus pectin were conducted to determine optimum conditions for peeling membranes from grapefruit segments. Alcohol-insoluble solids residue (AIS) extracted from washed membrane consisted of ca. 46% pectic substances and 25% neutral sugars. The main neutral sugar of AIS was glucose, followed by arabinose and xylose. Optimum conditions for degradation of segment membranes by pectinase “C-80” were: pH between 4.0 and 5.0 at 55°C. Optimum temperature for degradation of commercial pectin was 50°C; this indicates that optimum conditions for enzymatic activation depend also on the substrate degraded. The enzyme was fully inactivated after 1 min at 80°C. This study provides additional basic information for industrial application of enzymatic peeling of citrus segments.     

Ascorbate Oxidase in Mature Orange Peel

Evidence of the occurrence of ascorbic acid oxidase in the peel of mature orange is presented. The enzyme was found to be insoluble and located in albedo only. It is specific for ascorbic acid but not for catechol and is oxygen-dependent, with pH optimal activity of 6.5. The ascorbic acid oxidase acitivity was inhibited by high temperature, a pH below 3.5, and copper-chelates such as sodium azide and sodium dithiocarbamate. Blanching and decreasing the pH of the peel to 3.5 are recommended to achieve maximum stability of ascorbic acid in orange peel.     

p-HYDROXYPHENYLACETIC ACID AND-3,4-DIHYDROXYPHENYLACETIC ACID AS SUBSTRATES FOR MUSHROOM TYROSINASE

The hydroxylation of p-hydroxyphenylacetic acid (PHPAC) and the oxidation of 3,4-dihydroxyphenylacetic acid (DOPAC) by mushroom tyrosinase are illustrated. DOPAC quinone (4-carboxy-methyl-o-benzoquinone (λ_max= 400±10 nm) is the initial pigmented product formed when DOPAC is oxidized by the enzyme. DOPAC quinone is very unstable, and, once formed, is converted rapidly to further oxidation product(s).
The relationships between the rate of p-dihydroxyphenylacetic acid hydroxylation and 3,4-dihydroxyphenylacetic acid oxidation as a function of various concentrations of each substrate and of mushroom tyrosinase, are described.
The effect of the addition of various chemicals that can potentially conjugate otherwise affect DOPAC quinone was studied The Km value of 3,4-dihydroxyphenylacetic acid for mushroom tyrosinase was estimated to be 4.0 mM.