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In this study, the polyphenol oxidase (PPO) of artichoke (Cynara scolymus L.) was first purified by a combination of (NH(4))(2)SO(4) precipitation, dialysis, and a Sepharose 4B-L-tyrosine-p-aminobenzoic acid affinity column. At the end of purification, 43-fold purification was achieved. The purified enzyme migrated as a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polyacrylamide gel electrophoresis indicated that PPO had a 57 kDa molecular mass. Second, the contents of total phenolic and protein of artichoke head extracts were determined. The total phenolic content of artichoke head was determined spectrophotometrically according to the Folin-Ciocalteu procedure and was found to be 425 mg 100 g(-1) on a fresh weight basis. Protein content was determined according to Bradford method. Third, the effects of substrate specificity, pH, temperature, and heat inactivation were investigated on the activity of PPO purified from artichoke. The enzyme showed activity to 4-methylcatechol, pyrogallol, catechol, and L-dopa. No activity was detected toward L-tyrosine, resorsinol, and p-cresol. According to V(max)/K(m) values, 4-methylcatechol (1393 EU min(-1) mM(-1)) was the best substrate, followed by pyrogallol (1220 EU min(-1) mM(-1)), catechol (697 EU min(-1) mM(-1)), and L-dopa (102 EU min(-1) mM(-1)). The optimum pH values for PPO were 5.0, 8.0, and 7.0 using 4-methylcatechol, pyrogallol, and catechol as substrate, respectively. It was found that optimum temperatures were dependent on the substrates studied. The enzyme activity decreased due to heat denaturation of the enzyme with increasing temperature and inactivation time for 4-methylcatechol and pyrogallol substrates. However, all inactivation experiments for catechol showed that the activity of artichoke PPO increased with mild heating, reached a maximum, and then decreased with time. Finally, inhibition of artichoke PPO was investigated with inhibitors such as L-cysteine, EDTA, ascorbic acid, gallic acid, d,L-dithiothreitol, tropolone, glutathione, sodium azide, benzoic acid, salicylic acid, and 4-aminobenzoic acid using 4-methylcatechol, pyrogallol, and catechol as substrate. The presence of EDTA, 4-aminobenzoic acid, salicylic acid, gallic acid, and benzoic acid did not cause the inhibition of artichoke PPO. A competitive-type inhibition was obtained with sodium azide, L-cysteine, and d,L-dithiothreitol inhibitors using 4-methylcatechol as substrate; with L-cysteine, tropolone, d,L-dithiothreitol, ascorbic acid, and sodium azide inhibitors using pyrogallol as substrate; and with L-cysteine, tropolone, d,L-dithiotreitol, and ascorbic acid inhibitors using catechol as a substrate. A mixed-type inhibition was obtained with glutathione inhibitor using 4-methylcatechol as a substrate. A noncompetitive inhibition was obtained with tropolone and ascorbic acid inhibitors using 4-methylcatechol as substrate, with glutathione inhibitor using pyrogallol as substrate, and with glutathione and sodium azide inhibitors using catechol as substrate. From these results, it can be said that the most effective inhibitor for artichoke PPO is tropolone. Furthermore, it was found that the type of inhibition depended on the origin of the PPO studied and also on the substrate used.

Interactions between whey proteins in mixed systems (10% w/w total protein) containing α-lactalbumin (α-la) and β-lactoglobulin (β-lg) heated at 75 °C for different times were studied using gel fil...

Chickpeas, lentils, smooth peas, mung beans, and faba beans were milled into flours and fractionated to protein and starch fractions. Compositions of the seeds, cotyledons, and flours were compared for each legume and the weight and protein recovery of each fraction analyzed. Bean curds were prepared from the protein fractions through heat denaturation of protein milk, followed by coagulation with calcium sulfate or magnesium sulfate. The effect of chickpea protein concentration and coagulant dosage on the texture of bean curds was evaluated using a texture analyzer. Textural analysis indicated that curd prepared at 2.3-3.0% protein concentration and 1.5% CaSO(4) dosage had better yield and better texture than curds prepared under other conditions. Bean curds prepared from chickpeas and faba beans exhibited the second highest springiness and cohesiveness after those from soybeans. Curds of mung beans and smooth peas, on the other hand, had the highest yields and the highest moisture contents. The protein yield of the first and second soluble extracts used for curd preparation accounted for approximately 90% of the total protein of the seeds.

Lactate dehydrogenase (LDH) activity was measured in heated extracts of cooked bovine muscle tissues to evaluate the potential for developing a rapid, accurate assay to verify cooking endpoint temperatures in beef. Adductor and semimembranosus muscles from six Angus steers were assigned to one of four treatments: fresh, frozen, frozen-thawed, or aged. Each treatment was applied to beef rounds that did or did not receive electrical stimulation. LDH activity rates were monitored in heated and unheated samples following 3 weeks of vacuum-packaged storage at 4 degrees C. Treatments other than heating showed little effect on LDH activity. A major portion of activity was lost upon heating to 63 degrees C, and only marginal activity was detectable at 66 degrees C. Extractable protein decreased with increasing temperature. Decreases in LDH activity with increasing temperature may result from heat denaturation of the enzyme and/or from decreased amounts of extractable protein.

Mise en evidence, par dichroisme circulaire, de la formation de structure en feuilles au detriment de la structure helicoidale de la proteine native. Consequences sur les interactions intermoleculaires

Four organic thiolsulfonates and three organic ide; 2-aminoethanethiosulfuric acid; S-sulfocys- thiosulfates effectively inhibited cooked flavor teine; and S-sulfoglutathione. Their inhibitory when added to wbole milk, prior to heating, at a action is based on their ability to react with the level ranging between 0.003 to 0.05%. They are: sulfhydryl-containing compounds that form by 2-aminoethyl 2-aminoethanethiolsulfonate dihy- heat denaturation of P-lactoglobulin. The possi- drochloride; 5-aminopentyl 5-aminopentanethiol- ble implications of sulfhydryls removal with re- sulfonate dihydrochloride; 2-acetamidoethyl 2- gard to the stale flavor control are discussed. acetamidoethanethiolsulfonate; cystine S-diox