Phenolic substances occur primarily in fruits and vegetables and in the seeds of certain pigmented cultivars of sorghum, millets, and legumes. One of the major difficulties encountered in polyphenol research is the lack of a standard quantitative method for the analysis of phenolics that would be suitable for a wide range of seeds, forage crops, and food products and under a variety of experimental conditions. Some methods measure "total phenol", which may not be a true index of the nutritional quality of foods and thus does not distinguish polyphenols of nutritional concern from other low-molecular-weight phenols that also occur naturally in these products. Tannic acid (a hydrolyzable gallotannin) is commonly used as a "reference standard", but this may be a questionable practice since its biological properties differ from those of tannins of flavonoid origin. Polyphenols of cereals and legumes are predominantly of the latter type. Also, commercially available tannic acid has been shown to be a mixture of four phenolic compounds, the relative proportions of which vary with the samples. Thus, the choice of a suitable standard for tannin analysis is also important. The quantitative extraction of the condensed tannins from plant tissue is always difficult, since it may be complexed to a carbohydrate or protein matrix which could be quite insoluble due to a high degree of polymerization. The literature on tannin methodology is diverse and at times conflicting. Currently available methods for tannin analysis range from simple colorimetric, UV spectrophotometric, chromatographic, and enzymic to more sophisticated and expensive nuclear magnetic resonance (NMR) techniques. None of these methods of analyses is completely satisfactory nor can it be applied to different food products with the same degree of success. This review covers physical and chemical methods for tannin analysis of different food products, the problems in analysis and interpretation of data, and future research needs in this area.

Polyphenol oxidase in kiwifruit (Actinidia chinensis planch) was extracted and purified through (NH&S04 fractionation, dialysis and chromatography on DEAE-cellulose column. Polyacrylamide disc-gel electrophoresis showed eight bands with oxidase activity. The molecular weight of the dominant isozyme was 25,000 as determined by gel electrophoresis. The cresolase fraction appeared in the first peak (FAlP) and catecholase in the fourth peak (FA4P) when eluted from a DEAE-cellulose column. The optimum pH of the FA4P fraction was 7.3. The Km value was 50 mM (+) catechin for FAlP and 8.7 mM for FA4P. Ascorbic acid delayed enzymic browning in kiwi fruit. The activation energy with (+) catechin as substrate was 4.0 Kcal/mole for FAlP and 7.0 Kcal/mole for FA4P.

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SUMMARY Anthocyanin in freestone peaches was extracted, purified, and crystallized. The pigment was characterized by its absorption spectra in the visible and infrared region. Acid hydrolysis of the pigment yielded glucose and cyanidin, present in equal amount on a molar basis. No intermediate glycoside was found when the pigment was hydrolyzed under mild conditions, indicating a monoglycoside structure. Alkali degradation of the aglycone yielded phloroglucinol and protocatechuic acid, showing the presence of cyanidin in the pigment. The absorption peak of the pigment in a 0.01% methanolic HCl solution shifted from 525 to 568 mμ when aluminum chloride was added to form a chelate, indicating the presence of orthophenolic groups in the molecule. The peach anthocyanin is identified as a J‐mono‐glucoside of cyanidin. Variation in anthocyanin content between different varieties of California‐grown freestone peaches at various ripeness levels is presented. The importance of anthocyanin to discoloration in canned freestone peaches is discussed.

The polyphenol oxidase system in clingstone peach (Prunus persica)

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FirmnessK&fruit (Actinidiu chinensis Planch, Hayward cultivar) were ripened at 20°C under a stream of water-vapor saturated air containing 5 ppm ethylene gas. A remarkable rapid softening in the texture and rising of the soluble solids were observed in 24 hr. The total acidity, starch and amylose content decreased during ripening. Both fructose and glucose increased from 2.7% to 5.0% after 5 days of ripening. Sucrose content increased from 0.45% to 2.22% on the second day, then decreased to 1.19% after 5 days. No significant change in chlorophyll and total solids were observed throughout the experiment. There were stepwise decreases in the L and bL values during ripening. The ascorbic acid content decreased from 210 to 190 mg per 1OOg fresh fruit after 5 days of ripening.The fruits were peeled in a 10 x 10 mm area on both cheeks. The firmness of individual fruit was determined with a U.C. firmness tester with a 5116 inch (0.794 cm) plunger. The average of 12 readings at 20°C was reported. Sample preparationKiwifruit puree was prepared for analysis as follows unless described otherwise: The kiwifruits were hand peeled, followed by removal of seeds and cores. The products were homogenized at 1°C for 3 min in a Waring Blendor. The puree was frozen immediately in sealed jars at -26°C. The frozen puree was kept at -26°C in sealed glass jars.

The effects of germination, cooking and canning on the changes in total phosphorus, inorganic phosphorus and phytate retention in black-eyed, red kidney, mung, and pink beans were determined in this study. Soaking the dry beans in water for 12 hr at 24°C resulted in a slight decrease in phytate. After germination for 96 hr or longer there was a significant breakdown in phytic acid, and an increase in inorganic phosphorus. Cooking the dry beans at 100°C for 3 hr had little effect on phytate retention. Heat processing the dry beans at 1155°C for 3 hr in cans resulted in some increase in inorganic phosphorus and a reduction in phytate.