Brown Pigment Formation Research Articles

Kinetics of non-enzymatic browning in amorphous solid systems: distinguishing the effects of water activity and the glass transition

Debates have emerged recently on whether water activity or the state of the system as dictated by the glass transition temperature ( T g) impacts the rates of chemical reactions in reduced-moisture solid systems. The objective of this study was to evaluate the kinetics of brown pigment formation in polyvinylpyrrolidone model systems of different molecular weights so that the effects of water activity and the glass transition could be distinguished. Browning rates at different T gs, but constant water activity, were significantly different except when all were in the glassy state. As the system changed from a glassy state to a rubbery state, the rate of browning increased 7-fold. The rate of browning also increased as water activity increased from 0.33 to 0.54, but then appeared to plateau with further increases in water activity. Thus, the rate of brown pigment formation is influenced significantly by the glass transition temperature of the system and less by the water activity. In addition, the concentration of reactants in the aqueous microenvironment had a significant impact on the rate of brown pigment formation.

Brown pigmentation of Xanthomonas campestris pv. phaseoli associated with homogentisic acid

The diffusible brown pigment produced by some strains of Xanthomonas campestris pv. phaseoli was found to be due to the secretion and subsequent oxidation of homogentisic acid (2, 5-dihydroxyphenylacetic acid) rather than tyrosinase activity as previously reported. Homogentisic acid is an intermediate in tyrosine catabolism for a number of bacteria. Brown-pigmented strains appeared to be disrupted in tyrosine catabolism and could not use tyrosine as a nutrient. Nonpigmented strains, however, could utilize tyrosine as a nutrient, and although they secreted homogentisic acid, the levels were approximately 1/100th that of brown-pigmented strains. Production of brown pigment was stimulated by tyrosine and repressed by glucose. Growth in glucose resulted in a drop in the pH of the media, and the greatest pigment formation was associated with a shift to alkaline conditions in the culture media. By buffering the media near pH 7.0, brown pigment formation was repressed, even though unoxidized homogentisic acid accumulated in the culture media. The disruption of tyrosine catabolism corresponds to other reports describing differences between brown-pigmented and nonpigmented strains, and provides further support for the retention of a special taxonomic status for the brown-pigmented strains.Key words: homogentisic acid, pigments, Xanthomonas, fuscans.

Cyst production and brown pigment formation in aging cultures of Azospirillum brasilense ATCC 29145

Encystation in Azospirillum brasilense ATCC 29145 was observed by using routine laboratory staining and phase-contrast and electron microscopy. Encystment occurred in liquid and in solid or semisolid media containing fructose (8 mM) and KNO3 (0.5 mM). The encysted forms consisted of a central body filled with poly-beta-hydroxybutyric acid granules, an electron-transparent intinelike region, and a thick outer layer. Enlarged giant encysted forms with multiple central bodies were also observed during the germination of a desiccated brown colony. Morphogenetically different forms in an aging culture could be resolved by sucrose density gradient centrifugation. The dense encysted forms along with numerous granules in a fibrillar network pelleted at 70% sucrose, while empty saclike envelopes along with vegetative cells and coccoid bodies pelleted at 55% sucrose. Different media induced various degrees of pigmentation in A. brasilense ATCC 29145 after aging. The pigment possessed several of the properties reported for microbial melanins, including insolubility in water and organic solvents, solubility in cold and hot alkali, and bleaching in hydrogen peroxide. The UV absorption maxima of the alkali extract were at 280 and 310 nm. Electron micrographs of the brown pigment showed that it occurred as aggregated granules surrounding the encysting cells as well as being excreted into the medium in an aging culture. It is concluded that A. brasilense ATCC 29145 produces compounds that form a brown pigment similar to melanin and are expressed under the influence of certain cultural conditions conducive for encystment.

A kinetic study of the non-enzymatic browning reaction between histamine and glucose.

The model system containing histamine, glucose and microcrystalline cellulose was used to investigate the non-enzymatic browning reaction. The kinetics of the brown pigment formation and loss of histamine were studied as a function of heating temperature, pH and water activity of the systems. The rates of the brown pigment formation and the histamine loss followed zero order after an initial induction period and first order kinetics of up to a loss of at least 50% level, respectively. These rates increased with increasing temperature and pH, and decreasing water activity. The activation energies of both reactions were in the range of 23-24 kcal/mol for the pigment production, 17-21 kcal/mol for the histamine loss. It was suggested that the content of histamine can be reduced by controlling the condition of the model systems before brown dis-coloration becomes appreciable.

Reactivity of sorbic acid and glycerol in non-enzymatic browning in liquid intermediate moisture model systems

The kinetics of brown pigment formation in a water—glycerol—sorbateglycine (WGSG) model system at pH 4 were studied as a function of water activity (0.55–0.90) and temperature (40–60°C). The rate of browning was found to follow zero-order kinetics and to increase with decreasing a w. There was no evidence of any browning rate maximum over the a w range studied. Activation energy values ranged from 15.5 to 22.2 kcal/mol, being higher at the lower a w levels. Glycerol was found to actively participate in the formation of brown pigments by reaction with sorbate or glycine, the rates also following zero-order kinetics and obeying the Arrhenius equation. Comparison was made of the rates of browning of WGSG, water—glycerol—sorbate (WGS), water—glycerol—glycine (WGG), and water—glycerol (WG) systems at the same a w level of 0.80 under steady state conditions of 40, 50 and 60°C. The rate of browning was found to increase in the order WGG < WGS < WGSG. The WG system did not exhibit any browning over the time span of the experiment even at 60°C.

A cytological and ultrastructural study of general resistance to the Late Blight fungusPhytophthora infestansin potato foliage

SummaryUsing both light and electron microscopy it has proved possible to evaluate the nature of general resistance to Late Blight in potato foliage. Evidence points to the existence of two distinct host responses operating in the cultivars Pimpernel and Shamrock, both possessing high general resistance to Phytophthora infestans. In Pimpernel resistance was primarily related to a marked reduction in epidermal cell penetration. However in Shamrock there was no reduction in cell penetration. Cytological and ultrastructural evidence indicated that here the primary event was a more rapid host response involving brown pigment formation and perhaps cell death. This usually resulted in the restriction of fungal growth to the epidermal cell layer, or the mesophyll cell layer immediately below it.

Studies on the action of the dichlorocyclopropanes on the host-parasite relationship in the rice blast disease

The effects of the systemic compound WL 28325 (2,2-dichloro-3,3-dimethyl cyclopropane carboxylic acid), active against rice blast, on some aspects of the phenol metabolism of the rice plant during infection by Piricularia oryzae Cav. have been studied. Treatment of the roots with WL 28325 led to an enhancement of the peroxidase activity of the leaves of between two and twelve times that of leaves of untreated plants. Infection of wounded leaves of untreated plants also enhanced peroxidase activity but only at the later stages of infection was peroxidase activity similar to that of treated plants. The protective activities of several dichlorocyclopropane derivatives correlated closely with the ability of these compounds to enhance the peroxidase activities of the leaves. The rate of formation, relative to fungal growth, and the distribution around the infection site of a brown pigment suggested that the WL 28325-induced resistance, as well as natural resistance, is associated with the production of this brown pigment. The brown material is presumed to be a melanin, a polymer containing oxidized phenols, the oxidation of which is mediated by peroxidase. Lignin formation is apparently not involved in the WL 28325-induced resistance. Possible ways in which brown pigment formation may be involved in resistance of rice to P. oryzae are discussed.

Brown-Pigment Formation in Tobacco Leaves Infected with Alternaria

Brown-Pigment Formation in Tobacco Leaves Infected with Alternaria

A MAIZE GENE CONTROLLING SILK BROWNING IN RESPONSE TO WOUNDING

ROWNING is a common degradative phenomenon of injured cIr senescent ' vegetable tissue (SZENT-GYORGYI 1969). It is due to enzymatic oxidation of polyphenolics resulting in quinones which form brown colored complexes with protein. When fresh corn silks (stigmas) were ground in an aqueous solution and allowed to stand, it was observed that the brei normally turned brown. Occasionally, ground silks from certain inbreds were found which did not turn brown hut remained yellow-green (i.e., no change in color). Later, it was learned that the browning or nonbrowning phenotypes could be determined on the plant by cutting back fresh silks, as in preparation for controlled pollination, and observing the cut ends after about 15 minutes. In this instance, cut ends become brown in the browning phenotype and remain unchanged in the ncnbrowning. The distinction between browning and nonbrowning types is also evident with senescent silks of mature ears. In this case, senescent silks from the browning type appear very dark brown while nonbrowning are tan in color. This study was conducted to determine the inheritance and chemical basis of the browning and nonbrowning response to wounding of corn silks. We will show that the browning response is controlled by a single dominant locus which permits, primarily, the synthesis of dihydroxyl flavoaes by the silks and that when the silks are injured these flavones are enzymatically oxidized resulting in brown pigment formation. The nonbrowning type will he shown to be due to a recessive allele which when homozygous blocks the synthesis of the dihydroxyl flavones. Absence of the flavone substrates precludes brown pigment formation in response to wounding. MATERIALS AND METHODS Inbred lines T61 and NC232, the browning type, were crossed with the nonbrowning inbreds, Kys and NC236, respectively. The resulting F,'s were backcrossed to each of the parents. Field grown backcross progenies were scored for the browning and nonbrowning phenotype by cutting back fresh silks and observing the cut silk ends approximately 15 min later.