Food Chemistry, Vol. 63, No. 3, pp. 319-324, 1998 0 1998 zyxwvutsrqponmlkjihgfedcbaZYXWV Elsevier Science Ltd. zyxwvutsrqponmlkjihgfe All rights reserved Printed in Great Britain PII: SO308-8146(98)00035-l 0308-8146198 $19.00+0.00 ELSEVIER Effect of amino acids on olive o-diphenols the chemical oxidation of in model systems Concepcibn Romero, Manuel Brenes, Pedro Garcia & Antonio Garrido” Food Biotechnology Department, Institute de la Grass (CSIC), Avda. Padre Garcia Tejero 4, 41012 SevilIa, Spain (Received 14 November 1997; revised version received 9 January 1998; accepted 9 January 1998) Addition of certain amino acids (L-tryptophan, L-histidine, L-cysteine and L- cystine) to model solutions of caffeic acid and hydroxytyrosol (the two most characteristic o-diphenols of ripe olives) improved the dark colour obtained after oxidation. A detailed study carried out with L-cysteine and caffeic acid showed that, after an initial inhibitory effect, increasing concentrations of amino acid led to darker solutions. The same effect was also found in hydroxytyrosol solutions with increasing amounts (2-8 IIIM) of cysteine without lag phase, except for 10 mM level. Cysteine also produced darker colour with oxidised storage brine of ripe olives. These results open the door to a possible use of this amino acid as additive or aid in processing to produce darker and homogeneous ripe olives. 0 1998 Elsevier Science Ltd. All rights reserved. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG INTRODUCTION Two types of phenolic browning reaction can take place in certain foods: enzymic oxidation when polyphenol oxidase is present or, if the enzyme is removed or inac- tived, non-enzymic autoxidation (Singleton, 1987). These reactions generally result in a loss of nutritional value and the appearance of undesirable brown colours. However, such colours are desirable in some processed foods, for example, in black tea, chocolate, and ripe olives. The quinones formed enzymatically or non-enzyma- tically under alkaline conditions are highly reactive substances and can polymerise with other quinones or combine with certain amino acids in food pro- teins, potentially producing a variety of different- coloured compounds. The reactions of chlor- ogenoquinone and caffeoquinone with peptides and proteins are likely to involve only the thiol, the terminal a-amino, and the lysine e-amino groups (Pierpoint, 1969). Pierpoint (1966, 1969) also studied the formation of coloured compounds in the reaction between o-qui- nones and free amino acids while the interaction between the sulphydryl groups of cysteine and o-qui- nones has been demonstrated by other researchers (Cilliers and Singleton, 1990; Zhang and Dryhurst, 1994; Shen and Dryhurst, 1996). The reaction of the amino acids serine (Cabanes et al., 1987), methionine *To whom correspondence should be addressed. Fax: + 34-5- 4691262; e-mail: garfer@cica.es 319 (Igarashi and Yasui, 1985), lysine (Pierpoint et al., 1977) and proline (Valero et al., 1988) with o-qui- nones has been studied. Hurrell and Finot (1984), employing a model aqueous system of casein and caffeic acid, have investigated the nutritional impli- cations of the reactions between proteins and oxidised phenolic acids. The processing of ripe olives consists of successive treatments of olives with sodium hydroxide solutions (lyes) on three consecutive days and, during the inter- vals between these treatments, fruits are suspended in water through which air is bubbled. Throughout this operation olives darken progressively due to the oxida- tion of o-diphenols, hydroxytyrosol (3,4-dihydrox- yphenylethanol) and caffeic acid (Brenes et al., 1992). Different iron salts are added to fix the colour formed (Brenes et al., 1995) and after 1 day at equilibrium, the product is canned and sterilised, as appropriate for a low-acid food (Fernindez et al., 1972). The whole pro- cess takes about 1 week. A shiny dark colour is not always obtained. The oxidation reactions of olive o-diphenols, caffeic acid and hydroxytyrosol have been reported in model solutions in the presence and absence of metal cations (Garcia et al., 1996). The manganese ion significantly accelerated those reactions and also gave darker solutions. The aim of this work was to determine the effect of different amino acids on the oxidation of olive o-diphe- nols in alkaline conditions, in order to accelerate these reactions and achieve darker solutions.