Viability of near infrared spectroscopy for a rapid analysis of the bioactive compounds in intact cocoa bean husk

Highlights

• NIRS allows the predictions of total sugars, theobromine and total phenols in CBH.

• Both spectra of intact cocoa beans and unground husk are useful for calibrations.

• NIRS can be convenient for factories interested in CBH as source of bioactive compound.

Abstract

The potential of the cocoa bean husk (CBH) as a natural source of bioactive compounds is ever-increasing. In this work, its bioactive compounds and antioxidant activity were analyzed using near infrared spectroscopy in samples of CBH. Beans were harvested and fermented in a Mexican gene bank. Reference data on total sugars, total phenols, phenolic compounds, theobromine, and antioxidant activity were correlated with the intact husk and bean spectra. The Modified Partial Least Square regression method (MPLSR) was used to develop calibrations. Good calibration statistics were obtained for total sugars (r² = 0.90), theobromine (r² = 0.83) and total phenols (r² = 0.81) in data related to the CBH spectra, with a ratio of standard deviation/standard error of cross validation (RPD) of 3.16, 2.39 and 2.28, respectively. Acceptable calibrations for the estimation of bioactive compounds in CBH were obtained for the first time from the spectra of intact grain samples. Industries interested in bioactive compounds from CBH could use this technology as an easy and fast method to predict their contents, while avoiding the inconvenient de-husking process.

Introduction

The cocoa bean husk (CBH) is the main residue of the cocoa industry. Annual worldwide production is estimated at approximately 4,200,000 tones (FAOSTAT, 2018) and expected to increase given the demand for cocoa products. In the cocoa industry, CBH, also named shell, is separated from the cotyledons once the bean is fermented and dried, during or after the pre-roasting process.(Okiyama, Navarro, & Rodrigues, 2017). CBH has been considered a by-product, and traditionally has had limited applications, mainly as animal feed or organic soil fertilizer. In the search for alternatives for its valorization, as for other agro-industrial residues, the study of its composition and possible industrial applications has received more attention in recent years (Okiyama et al., 2017; Panak Balentic et al., 2018).

CBH represents between 12 and 20% of the total bean weight. It has a high fiber content (about 50–60% of its total weight), depending on whether it is roasted or not (Panak Balentic et al., 2018), along with minerals, proteins and all the essential amino acids. The low contents in soluble sugars and fat in unfermented CBH gives it a low calorific value, although the fat has an interesting profile, rich in palmitic, stearic and oleic fatty acids, which highlight its nutritional value. Furthermore, CBH is particularly rich in phenols and methylxanthines, compounds that are stored in the bean cotyledons but diffuse in part into the husk during the fermentation process, where they can accumulate in high concentrations (Arlorio, Coisson, Restani, & Martelli, 2001; Lecumberri et al., 2007). Phenols such as catechin, epicatechin and p-hydroxybenzoic acid have been identified (Arlorio et al., 2005; Hernández-Hernández, Viera-Alcaide, Morales-Sillero, Fernández-Bolaños, & Rodríguez-Gutiérrez, 2018), as well as theobromine and caffeine methylxanthines (Hernández-Hernández, Viera-Alcaide, et al., 2018). This composition in phenols and methylxanthines makes it an interesting source of bioactive compounds, given their antioxidant activity (Martínez et al., 2012). This by-product has been proposed as an inexpensive source of dietary fiber to help reduce calories and cholesterol levels and to control glucose levels in the blood. (Okiyama et al., 2017). Phenol-rich CBH extracts also have a powerful anti-cariogenic potential as phenols have anti-glucosyltransferase activity (Osawa et al., 2001). Hartati (2010) attributed health benefits to CBH's theobromine content due to its anti-cancer, diuretic, smooth-muscle relaxant and cardiac stimulant functions.

Therefore, its potential as a natural source of bioactive compounds is moving the interest of researchers and industry toward including CBH extracts as natural additives in food, pharmaceutical and cosmetic products in order to increase their bioactive characteristics. The production of different cocoa extracts has recently been patented (Okiyama et al., 2017; Panak Balentic et al., 2018). In a previous work, we proposed a totally physical method for the production of a natural extract from CBH on an industrial scale for the first time. This extract is rich in sugars (220 mg/g), phenols (55 mg/g) and theobromine (56 mg/g), and can be used directly, even though the compounds can be easily purified (Hernández-Hernández, Morales Sillero, et al., 2018).

Not all the raw materials destined for the cocoa industry are of similar interest in terms of their bioactive composition. It depends on their genetic basis, origin and processing (Hernández-Hernández, Morales Sillero, et al., 2018; Okiyama et al., 2017). Moreover, analytical methods which are considered appropriate for the identification and quantification of the bioactive compounds in CBH, such as high performance liquid chromatography (Arlorio et al., 2005; Hernández-Hernández, Morales Sillero, et al., 2018), are tedious and expensive for routine screening purposes. To our knowledge, indirect methods such as near infrared spectroscopy (NIRS) have not yet been explored for the quantification of the bioactive compounds in CBH. This technology is increasingly accepted for the routine analysis of antioxidants in many food, plant and agricultural products, saving analysis time and costs for both industry and research (Cozzolino, 2015). NIRS is based on the rule that the main components of each product, such as water, protein, fat and carbohydrates, exhibit electromagnetic absorption at wavelengths in the range 780–2500 nm. It is a powerful tool for characterizing and classifying foods according to quality standards. Sample preparation is usually quite simple and numerous parameters can be analyzed at the same time. Current NIR instruments allow fast and low cost measurements and utilize easy-to-use software for building calibration models which relate spectral data with individual chemical components (Alander, Bochko, Martinkauppi, Saranwong, & Mantere, 2013). Nowadays it is routinely used in different industries at a laboratory level and also at-line, on-line or in-line (Huang, Yu, Xu, & Ying, 2008).

Regarding CBH, NIRS has been applied for the rapid detection of husk in cocoa powders (Quelal-Vásconez et al., 2019) and for authentication according to geographic origin (Mandrile et al., 2019). The hypothesis of this work is that this technology can also be used in the food industry for the predicting the presence of any interesting bioactive compounds (sugars, phenols and methylxanthines). The content in total sugars includes not only monosaccharides but also potential neutral and acidic oligo and polysaccharides with antioxidant and biological properties like antioxidant fibers, phenolic glycoside modified pectin and prebiotic oligosaccharides previously identified in CBH and other lignocellulosic by-products (Hernández-Hernández, Viera-Alcaide, et al., 2018; Lama-Muñoz, Rodríguez-Gutiérrez, Rubio-Senent, & Fernandez-Bolaños, 2012; Rubio-Senent, Lama-Muñoz, Rodríguez-Gutiérrez, & Fernández-Bolaños, 2013).The potential for predicting these compounds from the spectra of both unground husk and intact cocoa beans was also evaluated.