Original article Determination of simple sugars, malic acid and total phenolic compounds in apple pomace by infrared spectroscopy and PLSR Mary D. Queji, 1 * Gilvan Wosiacki, 1 Gilce ´lia A. Cordeiro, 2 Patricio G. Peralta-Zamora 2 & Noemi Nagata 2 1 Food Engineering Department, Ponta Grossa State University, Ponta Grossa, Parana´, 84030-000 Brazil 2 Chemical Department, Parana´ Federal University, Curitiba, Parana´, 81531-980 Brazil (Received 15 May 2009; Accepted in revised form 17 December 2009) Summary The aim of this work was to develop a fast, versatile, inexpensive and environmentally safe analytical method to quantify simple sugars, malic acid and total phenolic compounds in apple pomace, considering its potential use as a raw material with value instead of as an industrial waste. Diffuse reflectance infrared spectroscopy (DRIFTS) measurements of twenty-six samples of apple pomace were analysed by partial least squares regression (PLSR), using several signal pre-processing methods. Multivariate models developed with four to five latent variables (LVs) and based in the MIR (mid-infrared) region had good prediction for the determination of sucrose, fructose, malic acid and total phenolic compounds, with average errors between 3.9% and 6.6%. By contrast, glucose was better determined by models developed in the NIR (near-infrared) region and using six LVs, yielding an average error lower than 7.4%. These results confirmed the feasibility of the multivariate spectroscopic approach as an alternative for expensive and time-consuming conventional chemical methods. Keywords Apple pomace, DRIFTS-FTIR, fructose, glucose, malic acid, multivariate calibration, phenolic compounds, sucrose. Introduction In the last few decades, the adequate disposal of industrial residues has become increasingly imperative, mainly because of the high environmental impact of many sub-products. Agricultural wastes generally con- tain a high content of organic matter, thus necessitating the development of environmental friendly strategies, preferentially to increase the value and potential use of these by-products. Studies on the valorisation of by-products from the food industry has been reported recently (Llorach et al., 2003; Botella et al., 2005; Yoon et al., 2005; Mabrouk & El Ahwany, 2008; Vendruscolo et al., 2008), revealing advantages related not only to the reduction of treat- ment and disposal costs but also to the production of value-added products. In the industrial processing of apple for the produc- tion of apple juice, fermented (vinegar) and distilled beverages, around 20–40% of the raw material becomes pomace, the main by-product (Downing, 1989). In 2005, approximately 204 000 tons of apples were processed in Brazil, generating approximately 61 000 tons of apple pomace, a residual material that is used mainly as an organic fertiliser or for animal feed (Souza Filho et al., 2008). The composition of its soluble solids (near 40% (w ⁄ w) on a dry weight) consists of a mixture of simple sugars (fructose, glucose and sucrose) and organic acids (mainly malic acid) (Downing, 1989). Even though the physical and chemical features of apple pomace are influenced by the fruit variety, the processing and analytical methods used in their characterisation and the presence of sugars, organic acids and phenolic compounds suggest their potential as a substrate for biotechnological purposes (Kennedy et al., 1999). Apple pomace can be used as a substrate for the production of fungal chitosan (Streit et al., 2009), components of aroma and other volatile compounds (Almosnino & Belin, 1991; Roberts et al., 2004), organic acids (Shojaosadati & Babaeipour, 2002; Gullo´n et al., 2008a), enzymes (Berovic & Ostroversnik, 1997; Villas- Boas et al., 2002), supplement for animal feed (Villas- Boas et al., 2003) and ethanol having better quality than that obtained by the use of sugar-cane (Ngadi & Correia, 1992; Chatanta et al., 2008). Additionally, apple pomace has been studied for its potential use as a nutritional and functional food additive, mainly due to its high antioxidant capacity and fiber content (Sudha et al., 2007; Garcı´a et al., 2009; Vitali et al., 2009). The use of apple pomace as a biotechnological sub- strate is largely dependent on its chemical composition. *Correspondent: Fax: +55 42 99743068; e-mail: melqueji@hotmail.com International Journal of Food Science and Technology 2010, 45, 602–609 602 doi:10.1111/j.1365-2621.2010.02173.x Ó 2010 The Authors. Journal compilation Ó 2010 Institute of Food Science and Technology