Por:
Guillermo Alberto Vélez Muriel
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Fecha:
2013
Arabinogalactan-proteins (AGPs) or proteoglycans in Green coffee beans (GCB) are characterized by their low levels of extraction. Previous work reported a yield of ~1,1% (w/w) of GCB using hot water extraction. This means over 92% AGP of the coffee beans remains in the residual cell wall material (RCWM) composed of cellulose and mannan. The aim of this research is to increase the extraction of AGP from green coffee beans, applying different extraction methods and to compare the methods based on their yield and the chemical composition of obtained fraction. AGP was extracted by: single extraction using (1) cold (4 °C) or (2) hot water (90°C), (3) enzymatic hydrolysis or by sequential extraction, starting with cold water, followed by (4) hot water or alkali (6M KOH) extraction and enzymatic hydrolysis. Extracted AGP was purified and recovered using a preparative anion exchange column (AEC). AGP fractions were identified with a Yariv radial gel diffusion assay.
The highest extraction yield (8,7% AGP) was obtained through sequential extraction using alkali. In this method, the alkali extraction resulting in extraction of 57% of the total AGP extracted with the sequence followed by the enzymatic treatment with a 37%. Two main AGPs were obtained with AEC one unbound and the other bound (both showed positive reaction to Yariv reagent test). In order to elucidate the reasons of having these two different AGPs, molecular size was determined using HPSEC, but clear differences were not observed. A main peak with a molecular weight of 287 kDa was measured for both. Analysis carried out in the composition of AGPs confirmed the majority content of arabinose and galactose and the presence of proteins and uronic acids to a less extent. Protein content was on average 12% (w/w).
The carbohydrate moiety accounted between 3 to 7% uronic acids with glucuronic acid being the most predominant. Effect of the extraction method on the structure was reflected in the variation of the ratio Galactose/Arabinose. For instance, AGP from cold water showed a ratio of 2.6 indicating its highly branched nature that eases its solubilization. In contrasts ratio of AGP from the enzymatic treatment in the sequential extraction was around 8, which means a poor branch structure. The difference in the ratios reflects the strength of the treatment applied in the composition of AGP. Therefore studies on the extraction of AGP from coffee beans have to be developed using milder methods to maintain the native structure of the AGP presences in the CWM and therefore reducing the side effects on its structure.
Fuente:
Biblioteca Virtual Banco de la República
Formatos de contenido:
Tesis
