1. RESULTS AND DISCUSSION
TEMPERING AND CRYSTALLIZATION EFFECTS ON MICROSTRUCTURE, MECHANICAL PROPERTIES AND APPEARANCE IN DARK CHOCOLATES DURING INDUSTRIAL MANUFACTURE
Emmanuel Ohene Afoakwa1*, Alistair Paterson2, Mark Fowler3 & Joselio Vieira3
1 Department of Nutrition & Food Science, University of Ghana, P. O. Box LG 134, Legon-Accra, Ghana
2Centre for Food Quality, University of Strathclyde, Glasgow; 3Nestle Product Technology Centre, York, UK
*Presenting author: / Tel: +233 (0)
ABSTRACT
UNIVERSITY OF GHANA
Tempering is a directed pre-crystallization that consists of shearing chocolate mass at controlled temperatures to promote cocoa butter crystallization in the most thermodynamically stable polymorphic form, to obtain products with good snap, colour, contraction, gloss and shelf life characteristics. This study investigated lipid crystallization behaviour during tempering of dark chocolates from different temper regimes and their effects on textural properties and appearance of finished dark chocolate products. Dark chocolates varying in particle size distribution (PSD) (D90 of 18, 25, 35 and 50 µm) were processed using standard industrial manufacturing processes and tempered using an Aasted Temperer. Tempering influences on products’ crystallisation were studied using TA.XT2 Texture analyzer, HunterLab MiniScan Colorimeter and Gloss meter to establish relationships with their mechanical properties and appearance. Particle size was inversely correlated with texture and colour, and the greatest effects were noted with hardness, stickiness and lightness at all temper regimes with significant interactions among factors. Over-tempering caused significant increases in product hardness and stickiness, reduced gloss and darkening of both surface and internal periphery of products. Under-tempering induced blooming in products with consequential quality defects on texture, and appearance. The blooming caused hardening, dullness and reduced gloss in the products. Attainment of optimal temper regime during pre-crystallisation of dark chocolate is necessary for the achievement of premium quality products with acceptable texture, colour and gloss. Over-tempering and under-tempering result in quality defects affecting mechanical properties and appearance of products.
INTRODUCTION
RESULTS AND DISCUSSION
Tempering is a directed pre-crystallization that consists of shearing chocolate mass at controlled temperatures to promote appropriate cocoa butter crystallization.
Temperature adjustments are utilized to promote formation of seed crystals in the correct polymorphic form to effect good product snap, contraction , gloss and shelf-life characteristics.
Though chocolate tempering is quite well understood, fat crystallization behaviour in differing temper regimes and associated effects on their structural properties and appearance still require in-depth investigation.
This study was aimed at investigating effects of tempering on microstructure, mechanical properties and appearance in dark chocolates from varying particle size distribution (PSD).
Tempered
Under-tempered
(bloomed)
Over-tempered
Fig 3. Pre-crystallization curves and their dark chocolates products during storage
MATERIALS AND METHODS
Materials
Cocoa liquor (Central West African Origin), sugar, cocoa butter and lecithin were obtained from Nestle Rowntree Factory, York, UK.
Products were formulated using dark chocolate recipe from Nestle PTC, York, UK.
Experimental Design
A 4 x 3 factorial experimental design was used, with principal factors;
i. PSD: 18, 25, 35 & 50 µm
ii.Temper regime: Optimal-temper, over-temper & Under-temper
Analytical methods
Textural properties (HD-Plus Texture Analyzer)
Gloss and colour (Gloss meter & HunterLab Colorimeter)
Scan Electron Microscope (Joel Systems Inc, Japan)
Statistical analysis
The data were statistically analyzed using Statgraphics (Graphics Software System, STCC, Inc. U.S.A). Comparisons between sample treatments and the indices were done using ANOVA, with a probability p<0.05.
Fig 4. Effects of temper regime and PSD on mechanical properties and appearance of dark
chocolates
Wide variations in mechanical properties and appearance were noted in products from different particle size and temper regimes.
Particle Diameter (µm)
Vol (%) 10 20 30 40 50 60 70 80 90
100
0.1
1.0
10.0
100.0
1000.0 a b c d
Fig 5. SEM micrographs of optimal, over and under-tempered dark chocolates
Particle size inversely correlated with texture and colour, with the greatest effects noted with hardness, stickiness and lightness at all temper regimes.
Over-tempering caused significant increases in product hardness and stickiness, reduced gloss and darkening of both surface and internal periphery of products. Under-tempering induced blooming in products with consequential quality defects on texture, whiteness and surface gloss.
Micrographs revealed clear variations in crystalline network structure and inter-particle interaction strengths among tempered, over-tempered and under-tempered (bloomed) samples.
Fig. 1. Particle size distribution with D90 of (a) 18 µm (b) 25 µm (c) 35 µm (d) 50 µm
CONCLUSION
PSD and tempering are central to the attainment of desired quality characteristics during dark chocolate manufacture as both over-tempering and under-tempering results in quality defects affecting mechanical properties, microstructure, appearance and gloss of products.
Fig 2. Micrographs of surface and internal structures of optimal, over & under-tempered products
REFERENCES
ACKNOWLEDGEMENT
Afoakwa EO, Paterson A, Fowler M (2007) Factors influencing rheological and textural qualities in chocolate – a review. Trends Food Sci. Technol. 18,
Beckett, S. T. (2008). The Science of Chocolate. Second Edition. London: Royal Society of Chemistry. Cambridge, U. K.
Many thanks to the Government of Ghana, Nestle UK and the African Women in Agricultural Development (AWARD) for financial and technical support.
Presented at the 16th World Congress of Food Science and Technology, Foz do Iguaçu, Parana State, Brazil, August 5 – 9, 2012