The authors, who published their findings in the journal Food Chemistry, said the method, based on flame atomic absorption spectrometry (FAAS), enables the evaluation of the essential mineral status of chocolate with minimum sample manipulation, and, moreover, is reproducible and cost effective.
Considering that chocolate is an extremely rich source of many essential minerals, it can contribute to a healthy diet, said the team from the Federal University of Santa Maria in Brazil.
Nevertheless, they point out that the evaluation of nutrient ingestion is a very complex task, with accurate determination of metals in chocolate still an analytical challenge, due to difficulties arising from matrix characteristics.
“The available nutritional data are frequently old and incomplete and in many cases unreliable due to lack of description of the analytical procedures,” note the team.
The literature reports methods for chocolate sample treatment involving microwave digestion, wet digestion, and dry ashing. Acids and peroxides are usually added to improve sample decomposition.
“All these procedures result in additional steps, which may lead to inconveniences such as contamination and losses during handling,” claim the Brazilian researchers.
And the goal of their study, they continued, was to investigate the use of emulsions as sample preparation for routine determination of sodium, potassium, calcium, magnesium, iron and zinc in chocolate using the FAAS method.
But while the “advantages of FAAS include well-characterized interferences, low operator skill required for operation, and comparatively low cost of instrumentation and maintenance,” the authors note that direct chocolate analysis by FAAS is not possible and the determination of metals in this type of matrix necessarily involves sample digestion, considering that it contains a high content of organic compounds.
The chocolate samples analyzed in the research project were purchased in local supermarkets, explained the authors, and the percentage of cocoa varied. The chocolate samples included five well know white chocolate brands, as well as five popular milk chocolate and seven dark chocolate brands.
In this study, oil-in-water formulations were optimized to determine sodium, potassium, calcium, magnesium, zinc, and iron in emulsified chocolate samples using FAAS. “This method is simpler and requires fewer reagents when compared with other sample pre-treatment procedures and allows the calibration to be carried out using aqueous standards,” report the team.
Octyl stearate was used as oily phase, while Tween 80 and Triton X100 were tested as surfactants. Furthermore, the authors said that the optimum type and proportion of formulations were determined and their choice depended on the element studied.
The emulsion preparation, added the research team, was performed by a conventional method that involves mixing both phases at 75 ± 5 °C by magnetic stirring and phase inversion to change the water-to-oil ratio by increasing the volume of the surfactant-water external phase and correspondingly decreasing the volume of internal phase.
The validation of the method was performed against a baking chocolate standard reference material (SRM 2384) and recoveries ranged from 88.6 per cent for potassium to 105.5 per cent for zinc, they said.
“The main advantages of the proposed method over traditional digestion methods are that it does not require a long sample treatment or large amounts of organic solvents or inorganic acids and it is simple and shows good accuracy and reproducibility,” found the research team.
The authors stressed that it would ideal if a single emulsion could be used for the determination of all minerals. However, they said that this was not possible due to the surfactant metal content and the different levels of each metal in the chocolate samples.
As the challenges and main requirements for emulsion application in atomic spectrometry involve the attainment of stable emulsions with low viscosity, the proposed method including emulsification and subsequent metal determination for FAAS fulfilled these requisites and proved to be sensitive, reproducible, simple, and economical, they added.
The mineral present in the highest concentration is potassium (K), with amounts varying from 2495 to 6361 mcg g_1. “Interestingly, the higher the cocoa content the higher the K level,” said the authors.
The amount of sodium was found to be directly related to the content of milk and cocoa liqueur in the chocolates. However it also presented great variation among the brands, the team reported.
Chocolates contain amounts of calcium varying from 324 to 4533 mcg g_1, with the highest levels in white chocolates, said the authors, while they found that chocolate contains levels from 365 to 1834 mcg g_1of magnesium, for white and dark chocolate, respectively.
The minor component contents, iron, and zinc, ranged from 1.2 to 140 mcg g_1 and from 8 to 23 mcg g_1, respectively. For both metals, the higher the cocoa content the higher the metal content, said the researchers.
Source: Food Control
Published online ahead of print: doi:10.1016/j.foodchem.2010.07.043
Title: Determination of sodium, potassium, calcium, magnesium, zinc and iron in emulsified chocolate samples by flame atomic absorption spectrometry
Authors: C.V.S. Ieggli, D. Bohrer, PC do Nascimento, L.M. de Carvalho