Vitamin C distribution in acerola fruit by near infrared hyperspectral imaging

Abstract

The study aims at developing a methodology for qualitative mapping of nutraceutical compounds in fruit by near infrared hyperspectral imaging (NIR-HSI), focusing on vitamin C mapping in acerola (Malpighia emarginata D.C.), a Brazilian super-fruit characterised by its high content of ascorbic acid. Despite the fact that the spectral approach has often been applied to agricultural crops, research on acerola is very limited. So far, it is known that ascorbic acid decreases in acerola during ripening, but there is no information about its distribution in the fruit from green to red ripe maturity stages. Towards this aim, hyperspectral images of ten sliced acerola, picked at three maturity stages, were acquired using a SisuChema NIR-HSI system. On the pre-processed images, combined in a comprehensive matrix, principal component analysis was computed to select relevant components for classical least square (CLS) regression. CLS allowed distribution maps of ascorbic acid to be obtained (non-negativity, LOF = 1.9%), using, as reference spectra, acerola juices enriched with 0% and 5% of vitamin C powder. The pixels correlated with 5%-enriched juice showed a reduction from 29% to 6.5% according to colour changes, confirming a vitamin C decrease along the ripening stages. These results demonstrated the reliability of NIR-HSI for the evaluation of vitamin C distribution inside the different acerola areas. The presented approach presents the basis for qualitative mapping of nutraceutical compounds in fruits.Keywords: acerola, vitamin C, near infrared hyperspectral imaging (NIR-HSI), classical least square (CLS) Introduction Acerola (Malpighia emarginata D.C.) is a round-shaped fruit, with diameter varying from 3 cm to 6 cm and a very thin protective peel that quickly ripens and encases its fleshy and succulent pulp. At the initial ripening stage, the fruit has a full green colour, changing to yellow-reddish and finally to red or purple when completely ripened, depending on the genotype. The colour of the fruits is not only a sign of pigment changes, but is also linked to complex biochemical changes occurring during ripening, which involve all its main compounds. Indeed, acerola is rich in many nutrients, such as protein, carotenes, thiamine, riboflavin, niacin, proteins, calcium and phosphorus, but its main appealing feature is related to its high vitamin C content. 1 Vitamin C is the generic term for all compounds exhibiting the biological activity of l-ascorbic acid, 2 including ascorbic acid and dehydroascorbic acid, and is one of the most important nutritional quality factors in many horticultural crops. Vitamin C biological activity is relevant in the development and maintenance of the human body's health status. 3 An intake of 100-200 mg per day has been suggested, since stress in modern life is known to increase the requirement for antioxidant compounds. 2 Vitamin C is a six-carbon keto-lactone, a strong reducing agent, which serves as an antioxidant and as a cofactor in hydroxylation reactions. Hydrogen donation from...

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