Browsing by Author "Vieira, J."

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Characterization of melting properties in dark chocolates from varying particle size distribution and composition using differential scanning calorimetry.
(2007) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Melting properties in dark chocolates processed from varying particle size distribution (PSD), fat and lecithin content were studied using differential scanning calorimetry (DSC). Compositional parameters were PSD (D90 (90% finer than this size) of 18, 25, 35 and 50 μm), fat (25%, 30% and 35%) and lecithin (0.3% and 0.5%) contents. Variations in PSD had no influence on crystallinity of products. Fat and lecithin content influenced the degree of crystallinity and melting properties (Tend, Tindex and DHmelt) of the products. Increasing fat content caused consistent increases in degree of crystallinity and crystal size distribution, thus effecting significant changes in Tend, Tindex and DHmelt of their derived products. Increasing lecithin content however reduced the crystal sizes in products. Particle size (PS) increases had limited effects on Tonset, Tpeak, and DHmelt independent of fat and lecithin content. Significant decreases in Tend and Tindex were noted with PS increases at all fat and lecithin contents. Similar increases in Tend and Tindex were noted with increases in fat content at all PS and lecithin levels. Contrary, increasing lecithin content in products resulted in significant decreases in Tend, Tindex and DHmelt. Thus, variations in fat and lecithin contents during dark chocolate manufacture influence the crystallinity of products, and with PSD, they all influence the melting index (duration) of their derived products
Comparison of rheological models for determining dark chocolate viscosity
(2009) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Parameters in chocolate rheology, namely shear viscosity and yield stress, are important in manufacture and directly influenced by product particle size distribution (PSD) and composition. The Casson model was the standard confectionery industry strategy to quantify rheological properties of molten chocolate until in 2000, the International Confectionery Association recommended the use of interpolation data to describe viscosity. The two strategies are compared and correlated in defining rheological properties of molten dark chocolates prepared using different PSD, fat and lecithin content. Rheological parameters were determined using a shear rate-controlled rheometer and data examined using correlation, regression and principal component analyses to establish their inter-relationships. Correlation and regression analyses showed high correlation (r = 0.89–1.00) and regression coefficients (R2 = 0.84–1.00). The newer International Confectionery Association technique gave higher correlation and regression coefficients than the Casson model, but multivariate principal component analysis showed that the two models were highly related and either could effectively quantify dark chocolate viscosity parameters.
Effects of tempering and fat crystallisation behaviour on microstructure, mechanical properties and appearance in dark chocolate systems
(2008) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Fat crystallisation behaviours in dark chocolates from varying particle size distribution (PSD) (D90 of 18, 25, 35 and 50 lm) was studied, yielding products from different temper regimes (optimal temper, over-temper and under-temper), and their effects on mechanical properties and appearance evaluated. Microstructures of derived products were determined using stereoscopic binocular microscopy. Wide variations in mechanical properties and appearance were noted in products from different particle size and temper regimes. Particle size (PS) was inversely related with texture and colour, with the greatest effects noted in hardness, stickiness and lightness at all temper regimes. Over-tempering caused significant increases in product hardness, stickiness with reduced gloss and darkening of product surfaces. Under-tempering induced fat bloom in products with consequential quality defects on texture, colour and surface gloss. Micrographs revealed variations in surface and internal crystal network structure and inter-particle interactions among tempered, over-tempered and under-tempered (bloomed) samples. Under-tempering caused whitening of both surface and internal periphery of products with effects on texture and appearance. Thus, attainment of optimal temper regime during pre-crystallisation of dark chocolate was central to the desired texture and appearance as both over-tempering and under-tempering resulted in quality defects affecting mechanical properties and appearance of products.
Fat bloom development and structure-appearance relationships during storage of under-tempered dark chocolates
(2009) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Fat bloom development and associated changes in microstructure, texture, appearance and melting properties were studied. Dark chocolates varying in particle size (PS) (D90of 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. Micro- structure 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, Tpeakand DHmelt) 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.
Influence of tempering and fat crystallization behaviours on microstructural and melting properties in dark chocolate systems
(2008) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Particle size distribution (PSD) and temper influences on dark chocolate fat crystallization were studied using differential scanning calorimetry (DSC) and microscopy to establish relationships with their melt- ing properties and microstructure. Variations in PSD had no influence on crystallinity of products at all temper regimes. Particle size (PS) increases had limited effects on Tonset, Tpeak, and DHmelt independent of temper regime but significant decreases in Tend and Tindex were noted. Contrary, varying temper regime influenced the crystallinity and melting properties (Tend, Tindex and DHmelt) of products. Under-tempered chocolate showed widened crystal size distribution (CSD) with significant changes in Tend, Tindex and DHmelt of products. Over-tempering caused moderate increases in CSD and melting properties, with significant effect on Tend, Tindex and DHmelt but no changes were noted in Tonset, Tpeak of products. Fat–sugar melting profiles showed similar levels in all products independent of temper regime, suggesting fat and sugar components are present in similar amounts in under-, over- and optimally-tempered products. Micrographs revealed clear crystalline network structure and well defined inter-crystal networks among tempered and over-tempered samples. Under-tempered products showed re-arrangement and re-crystallization of unstable fat crystals to smaller numbers of larger agglomerates with formation of solid bridges between the crystalline network structures. Attainment of optimal temper regime during pre- crystallization of dark chocolate is necessary for the achievement of premium quality products and avoidance of defects in structure and melting character.
Microstructure and mechanical properties related to particle size distribution and composition in dark chocolate
(2009) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Composition in dark chocolate was varied and the effects determined on microstructure, using light microscopy, and mechanical properties of molten and tempered chocolates, using a TA.HD Plus Texture Analyser. Compositional parameters were particle size distribution (PSD) (D90 of 18, 25, 35 and 50 lm), fat (25%, 30% and 35%) and lecithin (0.3% and 0.5%) contents. Micrographs revealed wide variations in sugar crystalline network structure and inter-particle interaction strengths related to PSD and fat level. Samples containing 25% fat had more crystal agglomerates, well flocculated with greater particle-to-particle interaction strengths than those with higher (30% and 35%) fat contents. Increasing the D90 to 35–50 lm caused broadening of the PSD, with particles becoming coarser, which were similar at all fat levels. Mechanical analysis showed that PSD, fat and lecithin content significantly influenced firmness of molten chocolate and hardness of solid (tempered) chocolate with significant interactions among factors. Particle size was inversely correlated with firmness (1235–173 g) and hardness (7062–5546 g). Greatest effect of PSD was with 25% fat and 0.3% lecithin. With higher fat and lecithin contents, the PSD influence was reduced. It was concluded that PSD, fat and lecithin contents and their interactions were central to mechanical properties of dark chocolates.
: Modelling tempering behaviour of dark chocolates from varying particle size distribution and fat content using response surface methodology
(2008) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Central Composite Rotatable Design (CCRD) for K=2 was used to study the combined effects of multi-stage heat exchangers for Stages 1 (14–30 °C) and 2 (12–28 °C) coolant temperatures at constant Stage 3 coolant and holding temperatures during tempering of dark chocolates using laboratory-scale mini-temperer. Quantitative data on chocolate temper index (slope) were obtained for products with varying particle size distribution (PSD) (D90 of 18, 25, 35 and 50 μm) and fat (30% and 35%) content. Regression models generated using stepwise regression analyses were used to plot response surface curves, to study the tempering behaviour of products. The results showed that both Stage 1 and Stage 2 coolant temperatures had significant linear and quadratic effects on the crystallization behaviour causing wide variations in chocolate temper index during tempering of products with variable PSD and fat content. Differences in fat content exerted the greatest variability in temperature settings of the different zones for attaining well-tempered products. At 35% fat content, changes in PSD caused only slight and insignificant effect on tempering behaviour. No unique set of conditions was found to achieve good temper in dark chocolate with a specified tempering unit. Thus, different combinations of temperatures could be employed between the multi-stage heat exchangers to induce nucleation and growth of stable fat crystal polymorphs during tempering. Variations in tempering outcomes of the dark chocolates were dependent more on the fat content than PSD.
Particle size distribution and compositional effects on textural properties and appearance of dark chocolates
(2007) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
Particle size distribution (PSD) and composition in dark chocolate were varied and their effects on textural properties of molten and tempered chocolates determined using a TA.HD Plus Texture Analyzer. Surface colour was evaluated in terms of CIELAB parameters L*, C* and h° using a HunterLab Miniscan Colorimeter. Compositional parameters for particle size distribution were [D90 (>90% finer) of 18 μm, 25 μm, 35 μm and 50 μm], fat (25%, 30% and 35%) and lecithin (0.3% and 0.5%) contents. Results showed that PSD, fat and lecithin content significantly (P 6 0.05) influenced the textural parameters with significant interactions among factors. Particle size was inversely correlated with firmness (1235–173 g), consistency (50,410–7029 g s), cohesiveness (1594–262 g), index of viscosity (5737– 1099 g s) and hardness (7062–5546 g) with chocolates containing 25% fat and 0.3% lecithin. With higher fat and lecithin contents, PSD influence was reduced. PSD and fat concentration inversely influenced all colour measurements (L*, C* and h°) of samples. High correlations (r = 0.71–1.00, P < 0.001) were observed between texture parameters and colour. It was concluded that PSD, fat and lecithin contents and their interactions were central to changes in textural properties and appearance during processing of dark chocolates.
Relationship between rheological, textural and melting properties of dark chocolate as influenced by particle size distribution and composition
(2008) Ohene Afoakwa, E.; Alistair, P.; Fowler, M.; Vieira, J.
In dark chocolate, rheological properties during processing are influenced by particle size distribution (PSD), fat and lecithin contents with consequential effects on finished texture and melting characteristics. Multivariate regression, correlation and principal component analyses (PCA) were used to explore their interrelationships. A 4 x 3 x 2 factorial experiment was conducted with varying PSD [D90 (90% finer than this size) of 18, 25, 35 and 50 μm], fat (25, 30 and 35%) and lecithin (0.3 and 0.5%). Rheological properties (yield stress and apparent viscosity), textural properties (firmness, index of viscosity and hardness) and melting index (duration) were respectively measured using shear rate-controlled rheometer, TA.HD Plus texture analyzer and differential scanning calorimetry. The PSD, fat and lecithin contents significantly influenced all rheological, textural properties and some melting characteristics. Increasing particles sizes reduced yield stress, apparent viscosity, firrmness, index of viscosity, hardness and melting index of products with greatest influence with 25% fat and 0.3% lecithin, reduced with increasing fat and lecithin contents. There were high correlation (r = 0.78–0.99) and regression coefficients (R2 = 0.59–0.99) among the rheological, textural and melting index indicating their high inter-relationships. In PCA, the rheological, textural and melting index accounted for >95% variance in the data