Seiichi Homma

Quality of Cut Lettuce Treated by Heat Shock: Prevention of Enzymatic Browning, Repression of Phenylalanine Ammonia-lyase Activity, and Improvement on Sensory Evaluation during Storage

Stored cut lettuce gradually turns brown on the cut section after several days of storage, because cutting induces phenylalanine ammonia-lyase (PAL) activity, the biosynthesis of polyphenol is promoted, and the polyphenols are oxidized by polyphenol oxidase. Here, the effect of heat shock treatment at 50°C for 90 s on the quality of cut lettuce during cold storage was examined. The heat shock treatment significantly repressed the induction of PAL activity and phenolics accumulation in cut lettuce during storage, and prevented the browning of cut lettuce. Ascorbic acid content was not affected by the heat shock treatment. The sensory analysis showed that the organoleptic quality of cut lettuce treated by heat shock was significantly better than that of the control cut lettuce. These results show that heat shock treatment is useful for prolonging the shelf life of cut lettuce.

Browning of Model Orange Juice Solution: Factors Affecting the Formation of Decomposition Products

A model solution of orange juice was prepared and stored. Factors affecting browning and formation of such decomposition products as 3-hydroxy-2-pyrone (3OH2P), 5-hydroxymethylfurfural (HMF), furfural, 5-hydroxymaltol, and 2-furoic acid were examined. Ascorbic acid (AsA) was essential for browning, which was stimulated by amino acids and citric acid, and repressed by chelators and radical scavengers (RS). 3OH2P was derived from AsA. Its formation was stimulated by sugars and repressed by citric acid, chelating agents, and RS. HMF was derived from fructose. Furfural was derived from AsA, and its formation was stimulated by sugars and chelating agents and repressed by RS. 5-Hydroxymaltol and 2-furoic acid were derived from fructose and AsA respectively. We did not find any decomposition products showing the same formation pattern as the browning, but a furfural solution with added amino acids turned brown like the model orange juice solution. It might be an indicator for the browning of orange juice.

Browning and Decomposed Products of Model Orange Juice

A model solution of orange juice containing sugars, ascorbic acid, and citric acid was prepared and its browning during storage was examined. The solution gradually turned brown. Ascorbic acid (AsA) most contributed to the browning. Citric acid and such amino acids as Arg and Pro promoted the browning. DTPA, a strong chelator, inhibited the browning. 3-Hydroxy-2-pyrone (3OH2P), 5-hydroxymethylfurfural (HMF), furfural, 5-hydroxymaltol, and 2-furoic acid were identified as decomposed products in the stored solution. When 3OH2P was stored, the solution turned slightly brown. Furfural solution added with amino acids turned yellow. 3OH2P showed a positive relation with the browning of retail orange juice during storage.

Detection of changes in taste of japonica and indica brown and milled rice (Oryza sativa L.) during storage using physicochemical analyses and a taste sensing system.

Changes in the taste of japonica, hybrid, and indica brown and milled rice, stored for 10 months at low (5 degrees C, 65-70% relative humidity) and room temperatures were observed by physicochemical analyses and a novel method using a taste sensing system. During storage, some properties increased or decreased while others were fairly constant. The main taste components of cooked rice such as sweetness (sucrose) and umami tastes (glutamic acid and aspartic acid) were reduced during storage, whereas glucose and fructose increased. The increase of fat acidity and consequent decrease of the pH value of the cooking solution may contribute to the off-taste of cooked stored rice. A taste sensing system with 10 lipid membrane sensors was also used to classify new and old rice samples using principal component analysis. Fresh and room temperature stored japonica and indica rice could be clearly distinguished; however, it was not possible to differentiate the samples stored at low temperature.

Glycerol-3-phosphate Dehydrogenase Inhibitors, Anacardic Acids, from Ginkgo biloba

Four inhibitors of glycerol-3-phosphate dehydrogenase were isolated from Ginkgo biloba and identified as anacardic acids (6-tridecylsalicylic acid, 6-[(8Z)-pentadecenyl]salicylic acid, 6-[(9Z,12Z)-heptadecadienyl]salicylic acid, and 6-[(8Z)-heptadecenyl]salicylic acid) by instrumental analyses. Their 50% inhibitory concentrations against the enzyme were 1-3 μg/ml under the standard assay conditions. Anacardic acid inhibited the enzyme non-competitively. The sarcotesta contained most of anacardic acids, and nuts a little.

Immunological and molecular comparison of polyphenol oxidase in Rosaceae fruit trees

An antibody raised against apple polyphenol oxidase (PPO) cross-reacted with PPOs from Japanese pear (Pyrus pyrifolia), pear (Pyrus communis), peach (Prunus persica), Chinese quince (Pseudocydonia sinensis) and Japanese loquat (Eriobotrya japonica). Core fragments (681 bp) of the corresponding PPO genes were amplified and characterized. The deduced protein sequences showed identities of 85.3 to 97.5%. Chlorogenic acid oxidase activity of these PPOs showed higher activities when assayed at pH 4 than at pH 6. These results indicate that PPOs in Rosaceae plants are structurally and enzymatically similar.

Characterization of a Metal-Chelating Substance in Coffee

A metal-chelating substance in brewed coffee was separated and characterized by its chemical structure. This substance was a brown polymer. The contents of sugars, amino acids and phenolics in the substance were evaluated. This polymer contained small amounts of sugars and amino acids in its partial structure. After being decomposed by alkaline fusion, the decomposition products were identified by HPLC and GC–MS. Several phenolics were detected in the decomposed products. To characterize this substance, various types of model compounds were prepared by roasting chlorogenic acid, sucrose, and (or) protein with cellulose powder. Among these model compounds, the polymer-forming ability was highest in the model prepared from all four of materials, but the metal-chelating ability was the highest in the model prepared from chlorogenic acid and cellulose. These results suggest that this metal-chelating substance was a melanoidin-like polymer formed by the decomposition and polymerization of sugars, amino acids and phenolics.

Antioxidative Activity of a Zinc-Chelating Substance in Coffee

Coffee brew contains a brownish zinc-chelating polymer designated ApV. ApV was prepared from the precipitate formed in a solution of instant coffee by adding ZnCl2 and purified using ion-exchange and cellulose column chromatographies. The antioxidative activities of ApV and related compounds were evaluated in this study. The free-radical scavenging activity of ApV estimated by ABTS assay was at a similar level to that of instant coffee, while the O2− scavenging activity of ApV, which is superoxide dismutase-like activity, was lower than that of instant coffee. The hydroxyl-radical scavenging activity of ApV was higher than that of instant coffee, and the auto-oxidation of linoleic acid was more strongly inhibited by ApV than by caffeic acid.

Properties of Chlorogenic Acid Quinone: Relationship between Browning and the Formation of Hydrogen Peroxide from a Quinone Solution

Chlorogenic acid is the major polyphenol in foods derived from plants and is a good substrate for polyphenol oxidase. Chlorogenic acid quinone (CQA-Q), which is an oxidative product of chlorogenic acid by polyphenol oxidase, is an important intermediate compound in enzymatic browning. CQA-Q was prepared, and its properties and the relationship with browning were examined. The quinone solution was yellow or orange, and its molecular absorption coefficient was estimated to be 1.7×103 for 325 nm and 9.7×102 for 400 nm in an acidic aqueous solution. Chlorogenic acid and H2O2 were spontaneously generated in the CQA-Q solution as the yellowish color of the solution gradually faded. A pale colored polymer was the major product in the reaction solution. Amino acids such as lysine and arginine added to CQA-Q solution did not repress the fading of the yellowish color of the solution. We concluded from these results that CQA-Q itself and a mixture of CQA-Q and amino acids did not form intensive brown pigments in the acidic aqueous solution. H2O2 spontaneously formed in the CQA-Q solution, and other polyphenols might have played an important role in the formation of the brown color by enzymatic browning.

Browning of model orange juice solution and changes in the components

Abstract Model solution of orange juice was prepared and its browning during storage was examined. The solution gradually turned brown. AsA most contributed to the browning. Arg and Pro promoted the browning. 3-Hydroxy-2-pyrone (3OH2P), hydroxymethylfurfural (HMF), furfural, 5-hydroxymaltol and 2-furoic acid were identified as decomposed products. When 3OH2P was stored, the solution turned slightly brown. Furfural solution added with amino acids turned yellow.