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Julio G. Loaiza-Velarde, Francisco A. Tomás-Barberá, and Mikal E. Saltveit

Wounding during minimal processing of lettuce (Lactuca sativa, L.) induces alterations in phenolic metabolism that promote browning and the loss of quality. The activity of phenylalanine ammonia-lyase (PAL; the first committed enzyme in phenylpropanoid metabolism) and the concentration of phenolic compounds (e.g., chlorogenic acid, dicaffeoyl tartaric acid, and isochlorogenic acid) increase in excised iceberg lettuce midrib segments after wounding. The effect of short heat-shock treatments on browning and phenolic metabolism in excised midrib segments of iceberg lettuce was studied. As the heat-shock temperature increased from 20 to 70 °C, there was a decrease in the subsequent increase in PAL activity and the accumulation of phenolic compounds in excised midrib segments. Treatments of 45 °C for 120 s, 50 °C for 60 s, or 55 °C for 30 s significantly reduced the increase in PAL activity and subsequent browning seen in control tissue after wounding. Exposure to 45 °C for 480 s, 50 °C for 60 s, or 55 °C for 45 s prevented PAL activity from rising above initial levels. Phenolic compounds remained at initial levels for 3 days in excised midribs exposed to 50 °C for 90 s or to 55 °C for 60 s. However, 55 °C damaged the tissue, as indicated by a* and L* Hunter color values. The synthesis of chlorogenic acid, dicaffeoyl tartaric acid, and isochlorogenic acid was greatly reduced by these heat-shock treatments. These treatments also decreased polyphenol oxidase activity and, to a lesser extent, peroxidase activity.

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M.S. Padda and D. H. Picha

Phenolic acids are one of several classes of naturally occurring antioxidant compounds found in sweetpotato. Simplified but reliable methodologies were developed to quantitate total and individual phenolic acids in sweetpotato roots. Total phenolic acid content was measured using both Folin-Denis and Folin-Ciocalteu reagents. The Folin-Ciocalteu reagent gave an overestimation of total phenolic acids due to the absorbance of interfering compounds (i.e., reducing sugars and ascorbic acid). The average total phenolic acid content in `Beauregard' sweetpotatoes was 60.9 mg/100 g fresh weight. Individual phenolic acids were separated with two reversed-phase C18 columns of different dimensions and particle size. The columns tested were a 7 × 53 mm, 3 μm, Alltima Rocket (Alltech Assoc.) and a 3.9 × 150mm, 4 μm, Nova-Pak (Waters Corp.). Different mobile phases were also evaluated. The Alltima C18 column using a mobile phase of 1% (v/v) formic acid aqueous solution: acetonitrile: 2-propanol, pH 2.5 (70:22:8) provided the best separation of individual phenolic acids. Total analysis time was less than 5 minutes. Chlorogenic acid was the major phenolic acid found in sweetpotato root tissue (15.8 mg/100 g fresh weight). In a comparison of different tissue preparation states (fresh, frozen, freeze-dried), fresh tissue gave the highest concentration of total and individual phenolic acids. Among the 3 extraction solvents tested (80% methanol, 80% ethanol, and 80% acetone), 80% methanol and 80% ethanol gave higher, but similar, phenolic acid extraction efficiency.

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Dariusz Swietlik

Sour orange seedlings were grown in water culture to which one of seven aromatic compounds, associated with allelopathic effects, was added to produce concentrations ranging from 0.5 to 2.0 mM. Leaf water potential (ψ1), leaf stomatal conductance (gs), and whole plant transpiration (T) were measured during a 7-day treatment period. At the end of that period, the total and average leaf surface area, shoot elongation, and fresh weight gain of seedlings were determined. Solutions of vanillic, coumaric, and ferulic acids of 2mM concentration reduced ψ1, gs, and T. Reductions of gs, and T but not (ψ1) occurred when vanillic acid of 1mM concentration was applied. Solutions of vanillic (0.5; 1.0; 2.0mM), coumaric (1; 2mM), cinnamic (1mM), or chlorogenic (1; 2mM) acids reduced fresh weight gain of seedlings. Only the coumaric and chlorogenic acids treatments of 2mM concentration reduced shoot elongation. No treatment affected total or individual leaf area. Gallic and caffeic acids had no effect on sour orange water relations and growth.

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M.S. Padda and D.H. Picha

Sweetpotatoes may be potentially high in concentration of certain phytochemical compounds, including phenolics. Low temperature stress-induced phenolic compounds may enhance the nutraceutical value of sweetpotatoes. However, extended exposure to low temperature results in chilling injury. Cured and non-cured roots of `Beauregard' sweetpotatoes were exposed to low temperature storage (5 °C) for up to 4 weeks. The total phenolics and individual phenolic acid contents were determined at weekly intervals using Folin-Denis reagent and reversed-phase HPLC, respectively. Total phenolics and individual phenolic acids increased with length of low temperature exposure. Non-cured roots had a higher phenolic content than cured roots after 4 weeks. A 3-day exposure period to room temperature (22 °C) following removal from low temperature storage typically resulted in increased phenolics. In a comparison of different tissue locations, the highest phenolic content was found in peel tissue and the lowest in the pith tissue. The major individual phenolic acid in all root tissues was chlorogenic acid.

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Lavanya Reddivari and J. Creighton Miller Jr.

Antioxidants have been widely reported to play an important role in disease prevention. In addition to preventing cancer, stroke, heart diseases, and inflammation, they are also involved in immune surveillance. Since the per capita consumption of potatoes in the U.S. is about 137 lb, even moderate levels of antioxidants in this most important vegetable crop probably have an important human health benefit. About 75% to 80% of antioxidant activity in specialty potatoes is due to phenolics and carotenoids. The objectives of this investigation were to evaluate antioxidant activity and total phenolic and carotenoid content of specialty potato selections from the Texas Potato Variety Development Program, and to identify candidate compounds for cancer cell culture investigations. Potato tubers were also used to identify and quantify individual phenolics and carotenoids. Some 320 specialty selections were screened for antioxidant activity (AA), total phenolic content (TP) and carotenoid content (CC) using DPPH (2,2-Diphenyl-1-picrylhydrazyl), FCR (Folin-Ciocalteu Reagent) and colorimetric assays, respectively. After the initial screening, the top 10% were used for analysis of individual phenolics and carotenoids using HPLC. Wide variability for antioxidant activity, phenolic content, and carotenoid content was found among specialty potato selections, providing evidence for genetic control of theses traits. The specialty selection CO112F2-2P/P (purple flesh, purple skin) had the highest AA (832 μg trolox equivalents/g fw), TP (1553 μg chlorogenic acid equivalents/g fw) and CC (590 μg lutein equivalents/100 g fw). Chlorogenic acid (55% to 60%), caffeic acid (≈5%), gallic acid (18% to 20%), and catechin (18% to 20%) were found to be the most prevalent phenolic acids, and lutein and zeaxanthin were the most prominent carotenoids contributing to antioxidant activity. Gallic acid was identified as the candidate compound for use in cancer cell culture investigations.

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Guiwen W. Cheng and Carlos H. Crisosto

The relationship of phenolic composition and polyphenoloxidase activity (PPO, E.C. 1.14.18.1) to browning potential (BP) was studied in buffer extracts of peach [Prunus persica L. Batsch) and nectarine [P. persica var. nectarine (L.) Batsch] fruit skin. The BP varied among cultivars with `Flavorcrest' having the highest value and `Maycrest' the lowest. On average, over 83 % of the browning measured at the end of the S-hour incubation occurred during the first hour. The total soluble phenolics (TSP), the total anthocyanin (TA), and glutathione content (GLU) varied among cultivars, but were not significantly correlated to the BP. Of the phenolics determined by HPLC, only chlorogenic acid had a significant positive correlation and epicatechin a significant negative correlation with BP by the first hour of incubation. The PPO activity, ranging from 4 to 11 optical density units per gram dry weight per minute among peaches and nectarines, was not significantly correlated with BP. However, no browning was detected if the buffer extract was previously boiled. These results indicated that browning in the buffer extracts of peach and nectarine skin tissue depends on the presence of PPO activity and chlorogenic acid, which are major contributors to enzymatic browning.

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Mustafa Ozgen, Artemio Z. Tulio Jr., A. Raymond Miller, R. Neil Reese, and Joseph C. Scheerens

In preliminary studies, we found that relative and absolute antioxidant (AO) levels varied within and among small fruit types. AO levels were affected by assay method used, time of reaction, volume of sample, and the ratio of reactants to total AO activity. To identify the physicochemical parameters that affect accuracy and reproducibility, a series of experiments were conducted to test the roles of AO assay, different AOs, and AO concentration on measured AO content and reaction kinetics. Three assays (DPPH, FRAP, ABTS) were used to evaluate AO capacity of seven fruit types (black and red raspberry, blackberry, strawberry, grape, elderberry, and cranberry) and nine purified AOs (ascorbic, caffeic, chlorogenic, gallic, and ellagic acids, α-tocopherol, trolox, cyanidin-3-glucoside, and quercetin). Ascorbic acid, trolox, caffeic acid, chlorogenic acid, and α-tocopherol exhibited simple reaction kinetics and reached endpoints quickly, regardless of assay. Gallic and ellagic acids, quercetin, cyanidin-3-glucoside, and all fruit extracts exhibited more complex kinetics and long reaction times (>70 min) to reach an endpoint. Moreover, the latter four AOs had the highest AO capacity among the compounds tested. We observed differences in reactivity between assays, compounds and fruit extracts, but relative AO activity was comparable, although the absolute values differed. Since AO capacity of fruit extracts is a composite of the individual AOs present, it is important that reactions progress to near steady state, assay reactants are in excess of (30–50×) the AO capacity being measured, more than one assay is used to describe the total AO activity of fruit samples. Thus, there may not be a single AO assay method that completely defines the AO activity of a given fruit.

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Md. Shahidul Islam, Makoto Yoshimoto, Koji Ishiguro, Shigenori Okuno, and Osamu Yamakawa

The phenolic content and the radical scavenging activity were compared in leaves of sweetpotato (Ipomoea batatas L.) cultivars Shimon-1, Kyushu-119 and Elegant Summer grown under different temperature and shading conditions. Compared to cultivar differences, there was less effect of temperature and shading on the total phenolic content in sweetpotato leaves, however certain polyphenolic components differed widely among the treatments. The positive correlation between the radical scavenging activity and the level of total phenolics (r = 0.62) suggests that phenolic compounds are important antioxidant components of sweetpotato leaves. All the reverse-phase high-performance liquid chromatography (RP-HPLC) profiles of the cultivars tested showed peaks at the same retention times but peak areas of individual phenolic compounds differed with respective temperature and shading treatments. The phenolic compounds identified in the sweetpotato leaf were caffeic acid, chlorogenic acid, 4,5-di-O-caffeoylquinic acid, 3,5-di-O-caffeoylquinic acid, 3,4-di-O-caffeoylquinic acid, and 3,4,5-tri-O-caffeoylquinic acid. Most of the phenolic compounds were highest in leaves from plants grown at 20 °C without shading except 4,5-di-O-caffeoylquinic acid. The results indicate that growing leaves under moderately high temperatures and in full sun enhances the accumulation of phenolic components. These phenolic components have possible value in enhancing human health.

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Joseph K. Peterson, Howard F. Harrison, and Maurice E. Snook

After removal of the periderm, cortex tissue of the sweetpotato cultivar Regal was collected. Polar extracts of this tissue strongly inhibited germination of proso-millet seed. C18 preparative, step-gradient chromatography (H2O → 100% methanol) gave some 50+ fractions, all of which were assayed for inhibitory properties. Analytical HPLC, using diode array detection and signal processing, showed the presence of chlorogenic, p-coumaric and caffeic acid, scopolin and some unknown phenolic acids. Most fractions were inhibitory to some degree; however, the least polar ones (in 90% and 100% methanol), containing unknown compounds, were most inhibitory. Semi-prep HPLC of these fractions produced eight major peaks (λmax at 210–213 nm, λ2 at 281–284 nm). In our bioassays, the compounds produced 50% inhibition of proso-millet seed germination at ≈60 ppm. It is likely that these compounds contribute significantly to the allelopathic properties of sweetpotato.

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Malkeet S. Padda and David H. Picha

Three different style cuts of minimally processed sweetpotatoes (shredded, French-fry, and sliced) were stored at 0 °C and 5 °C for 4 and 8 days. Total phenolic content, individual phenolic acids, and free radical scavenging activity were determined using Folin-Denis reagent, reversed-phase HPLC, and 1,1-diphenyl-2-picrylhydrazyl (DPPH) methods, respectively. Total phenolic content in sliced cut sweetpotatoes held at 5 °C was higher than in the shredded cut. Both sliced and French-fry cut sweetpotatoes held at 5 °C had significantly higher antioxidant activity than shredded cut sweetpotatoes. All treatments, except shredded sweetpotatoes held at 0 °C, had significantly higher total phenolic content and antioxidant activity after 4 and 8 days of storage. Minimally processed sweetpotatoes held at 5 °C accumulated more phenolic compounds and had a higher antioxidant activity than sweetpotatoes held at 0 °C. Chlorogenic acid followed by 3,5-dicaffeoylquinic acid were the predominant phenolic acids present in sweetpotatoes. The rate of increase in individual phenolic acid content with storage time was higher at 5 °C than at 0 °C. No tissue browning was observed in any of the cuts after 8 days of storage and the products were considered to be marketable.