AbstractBACKGROUND: The size and strength of rapeseeds are factors likely to determine their suitability for storage and processing. The mechanical properties of whole seeds depend mainly on their coat composition. This study investigates variations in seed strength and colour between different‐sized rapeseeds. The paper also presents an explanation of the differences in seed mechanical properties through analyses of coat fibre composition.RESULTS: The strength properties of seeds were dependent on seed size and correlated with seed equivalent diameter. All measured strength indices showed that seeds of diameter above 2 mm were the most resistant. The mechanical properties of seeds were also correlated with the colour of seed surface, indicating that the most resistant seeds were those with values of H > 60°, S < 15% and I < 19% in the HSI colour space. The resistance of individual seeds was attributed to the surface density of soluble and insoluble dietary fibre, especially cellulose and lignin.CONCLUSION: The results suggest that the highest mechanical resistance shown by the largest and blackest rapeseeds can be associated with the high surface density of fibre components in the seed coat. Copyright © 2008 Society of Chemical Industry