A three-week algal meal supplementation of the cows' basal diet resulted in an increase in the firmness of milk fat crystallized isothermally at 5 °C for 24 h – the apparent elastic constant increased from 100 to 224 N/mm. This was accompanied by a decrease in solid fat content, from 47·7% to 44·4%. The crystallization behaviour of milk fat was also modified significantly. The rate constant of crystallization (Avrami constant) of the enriched milk fat at 19 °C was ∼20 times higher than that of control milk fat. A shorter induction time of nucleation was also observed in the temperature range [20, 27 °C]. These effects were attributed to a higher degree of supersaturation of the enriched milk fat. Enriched milk fat nucleated in a more stable β' polymorphic form at 5 °C, while control milk fat nucleated in the metastable α form, as determined by powder X-ray diffraction and differential scanning calorimetry. Changes in the microstructure of the material were observed by polarized light microscopy at 5 °C. The enriched milk fat displayed a greater amount of crystal clustering than the control. This effect was reflected in a decrease in the box-counting mass fractal dimension (Db) of the fat crystal network from 1·853 to 1·809. The decrease in Db closely predicted the observed 2·2-fold increase in the elastic constant of the fat. These changes in mechanical properties, crystallization behaviour and microstructure were driven by an increase in the 18[ratio ]1trans and a decrease in the 18[ratio ]1cis fatty acid content of the enriched milk fat.