Fat bloom development and structure-appearance relationships during storage of under-tempered dark chocolates

Fat bloom development and associated changes in microstructure, texture, appearance and melting properties were studied. Dark chocolates varying in particle size (PS) (D90 of 18, 25, 35 and 50 μm) were processed and pre-crystallised to under-temper regime. Bloom was induced by storing products under ambient conditions (18 ± 2 °C, RH 50%) and changes in texture, surface whiteness, gloss and melting properties evaluated on cooling and after every 24 h in storage until reaching asymptotic values. Microstructure of products were characterised during blooming using stereoscopic binocular microscopy. Measurements on texture and surface whiteness showed initial rapid increases with consequential reductions in gloss within the first 96 h, followed by gradually decreasing gradient until reaching asymptotic levels. Storage influenced melting properties (Tonset, Tend, Tpeak and ΔHmelt) in products causing polymorphic transformation from βIV to βVI within 72 h. Micrographs showed similar surface crystalline network structure and inter-particle interactions among products from different PS after tempering, and bloom initiation occurred within 24 h in storage resulting in appearance of both liquid and unstable fat on the surface of products. The unstable fat then re-crystallised during storage into more stable polymorphs and crystal growth was promoted by Ostwald ripening (larger crystals growing at the expense of smaller ones), with the appearance of white crystalline structure which spread gradually throughout the chocolate mass after 96 h. Product containing the largest PS (50 μm) showed the fastest fat bloom rate, with the smallest PS (18 μm) the least, attributed mainly to hydrodynamic forces by capillary action. It was hypothesised that fat bloom development was initiated by capillarity, followed by growth of re-crystallised fat by diffusion across the entire chocolate mass until fully bloomed.