Comparative assessment of sugar in concentrated and nectar grape juices by refractometry, spectrophotometry and chromatography

Caldas, Bárbara Sthéfani; Constantino, Leonel Vinicius; Silva, Cyntia Helena Gomes Alves; Madeira, Tiago Bervelieri; Nixdorf, Suzana Lucy

Palavras-chave: arbohydrates, fructose, glucose, authenticity, HPLC-RID.

Resumo: Significant growth in juices production and consumption demand control methods, capable to measure fruit and sugars proportions for each kind of product. Basic consumer rights, require clear and appropriate composition and risks information’s on packaging. Despite this, Brazilians are often misleader, starting with juice’s nomenclature. By law, whole and concentrate juices must contain 100% of fruit without any addition. On the other hand, nectar, beverage ready for consumption, should be elaborated with a minimum of 50% of fruit, by adding water, sucrose, acidifiers and preservatives. Therefore, this study aimed to verify whether total sugars amounts of concentrate and nectar grape juices are comparable and consistent, as determined by 3 different techniques/methods – refractometry, spectrophotometry and chromatography, checking their applicability. Refraction (RF), the most applied in industrial control, employs oBrix direct reading in refractometer. Spectrophotometry measures glucose at 490 nm by conventional phenol-sulfuric method (FS-ESP). While, high-performance liquid chromatography determines fructose, glucose and sucrose levels using a refractive index detector (RID-HPLC). For purposes of total sugars concentration direct comparison, HPLC-RID levels were obtained by the sum of individual carbohydrates. The average of total sugars amount employing RF, ESP-FS, HPLC-RID was considered similar (Tukey p <0.05) for nectar (14.85±0.44 g 100 mL-1) and concentrate grape juice (61.74±2.60 g 100 mL-1), except for concentrate analyzed by ESP-FS (80.47±8.6 g 100 mL-1). The smallest variation levels was observed by RF (0.5 to 3.0%), followed by HPLC-RID (3.4 to 6.3%), with ESP-FS (8.4 to 17.2%) presenting the greatest dispersion. Thus, RF proved to be suitable for routine determination of total sugars, considering the precision, practicality and simplicity of use. HPLC-RID however has an additional differential besides providing total sugars content. Based on the chromatographic profile allows distinguishing between whole and concentrating juices, made only by fruit within fructose and glucose proportions naturally found in grape itself, from products added sucrose as nectar. This method consists therefore in an alternative tool that could be use in case of counterfeit suspected, to attest for authenticity and legislation compliance.

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