Formation of internal browning in hazelnuts during roasting and effect of water activity on internal browning were studied. Water activity significantly affected color attributes of roasted hazelnuts. As the water activity increased internal browning also increased. Over the water activity range of study (0.29-0.83), water activity of 0.29 was found to be the least susceptible to internal browning. Roasted samples, having dark-coloured internals contained significantly more total sugar and sucrose than roasted samples, having light coloured internals. Brown centres contained more total sugar and sucrose than light-coloured outer layers. Inner layers of raw hazelnuts contained about 50% more total sugar and sucrose than light-coloured outer layers. Polyphenoloxidase activity could not be detected in the sample roasted at 130 degreesC for 16 and 30 min. These results indicated that internal browning during roasting was due to localised concentration of reactants and, subsequently, the higher non-enzymatic browning reaction rate in the centre of hazelnuts. (C) 2001 Published by Elsevier Science Ltd.