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Featured researches published by Ivan Stewart.


Journal of Chromatography A | 1971

Continuous flow separation of carotenoids by liquid chromatography.

Ivan Stewart; T. A. Wheaton

Abstract A liquid chromatographic system has been developed that gives good separation of complex mixtures of carotenoids. Carotenes are separated on magnesium oxide and xanthophylls on zinc carbonate. The columns are regenerated after each sample which virtually eliminates repacking. Submicrogram quantities of carotenoids are readily detected and many cis-trans isomers are separated. An antioxidant is included to reduce on-column losses and isomerization of carotenoids. The method is quantitative, reproducible, sensitive, moderately rapid, and suitable for routine analysis. Use of the system for the study of isomer formation of carotenoids and for the identification of citrus peel pigments is included.


Analytical Biochemistry | 1965

Quantitative analysis of phenolic amines using ion-exchange chromatography

T. A. Wheaton; Ivan Stewart

Abstract A method has been developed for the quantitative analysis of mixtures of phenolie amines by separation on a column of a strong-acid ion-exchange resin and measurement of UV absorbance of the column effluent. Satisfactory results were obtained with samples containing 0.01 to 0.2 μmole of octopamine, synephrine, hordenine, tyramine, and N-methyltyramine. Little sample preparation is required for the citrus juices studied and a chromatogram may be completed in 2 hr.


Phytochemistry | 1973

Conversion of β-citraurin to reticulataxanthin and β-apo-8′-carotenal to citranaxanthin during the isolation of carotenoids from Citrus

Ivan Stewart; T. A. Wheaton

Abstract Reticulataxanthin and citranaxanthin were found to be artifacts formed from β-citraurin and β-apo-8′-carotenal during the saponification of citrus carotenoids in the presence of small amounts of acetone. The reaction takes place at room temperature under normal conditions of extraction. Based on these studies it was concluded that β-citraurin rather than reticulataxanthin is the main pigment contributing to the external reddish color of a number of citrus cultivars.


Phytochemistry | 1969

Biosynthesis of synephrine in citrus

T. A. Wheaton; Ivan Stewart

Abstract Synephrine is formed in citrus by a pathway involving tyramine and N-methyltyramine. Octopamine is probably not an important intermediate. 14 C-labeled tyramine was converted to N-methyltyramine, hordenine, octopamine, and synephrine in one of the citrus cultivars studied. Tyrosine was a less efficient precursor and phenylalanine, serine, ethanolamine, and epinephrine were ineffective.


Science | 1964

l-Octopamine in Citrus: Isolation and Identification

Ivan Stewart; T. A. Wheaton

l-Octopamine, [l-p-hydroxy-α-(aminomethyl)benzyl alcohol], has been isolated and identified from extracts of juice and leaves of the Meyer lemon. Identification was by chromatography, optical rotation, ultraviolet absorption curves, fluorescence spectra, and infrared spectroscopy. l-Octopamine has not previously been isolated and identified from plants.


Phytochemistry | 1967

Formation of diacetonamine and triacetonamine in plant extracts

Ivan Stewart; T. A. Wheaton

Abstract Diacetonamine and triacetonamine were isolated from extracts of tangerine leaves and shown to be artifacts formed during the isolation procedure.


Phytochemistry | 1968

Isolation of citrosamine from citrus leaves

Ivan Stewart; T. A. Wheaton

Abstract A nitrogen containing trisaccharide was found in citrus leaves, identified as glucosamido-glucuronido-inositol and given the trivial name citrosamine.


Journal of Agricultural and Food Chemistry | 1972

Carotenoids in citrus. Their accumulation induced by ethylene

Ivan Stewart; T. A. Wheaton


Journal of Agricultural and Food Chemistry | 1977

Provitamin A and carotenoid content of citrus juices.

Ivan Stewart


Journal of Agricultural and Food Chemistry | 1985

Enzyme immunoassay (EIA) of endogenous cytokinins in Citrus

Gary A. Barthe; Ivan Stewart

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