Study of the thermal degradation of citrus seeds V. Herna ´ ndez-Montoya a,b , M.A. Montes-Mora ´n b , M.P. Elizalde-Gonza ´lez a, * a Centro de Quı´mica, Instituto de Ciencias, Universidad Auto ´noma de Puebla, Apdo. Postal J-55, Puebla 72570, Pue, Mexico b Instituto Nacional del Carbo ´n, CSIC, Apartado 73, E-33080 Oviedo, Spain article info Article history: Received 25 February 2009 Received in revised form 7 May 2009 Accepted 12 May 2009 Published online 12 June 2009 Keywords: Citrus seeds Orange Lemon Grapefruit Thermal degradation abstract The citrus seeds are one of the principal residues in the juice industry and their utilization can decrease significantly the problems of their final disposal. In this work the thermal degradation of three Mexican citrus seeds: orange (Citrus sinensis), lemon (Citrus Limon) and grapefruit (Citrus paradisi) was studied in nitrogen atmosphere. The two components (embryo and husk) of the seeds were characterized separately. The results showed that the thermal effects are very similar between the three embryos and the three husks. The embryos show higher degradability, superior content of nitrogen and higher heating value than the husks. The thermal degradation of the components of the three seeds is completed at 600  C and it is considered to be a global process derived from the decom- position of their principal components (cellulose, hemicellulose and lignin). The results suggest that mixing the three entire seeds will not lead to a severe deviation from their individual thermal behavior and that the industry could apply them for carbonization purposes. ª 2009 Elsevier Ltd. All rights reserved. 1. Introduction Citrus are recognized as one of the world’s major fruit crops. They are harvested in many countries with tropical or subtropical climate and the principal producers are Brazil, China, Japan, Mexico, Pakistan, USA and countries of the Mediterranean region. The citrus production worldwide is around 105 Mt per annum and the fruits of this genus are mainly processed to make juice. The residues of the juice industry, such as peels, seeds and pulps, which represent about 50% of the raw processed fruit, are a potential source of valuable by-products [1]. Reuse of citrus residues has increased and recently the antioxidant activity of the orange, lemon and grapefruit seeds was reported [2]. Other authors have found limonoids in citrus seeds and this component exhibited a variety of biological activities like insecticidal, insect antifeedant, etc. [3,4]. Also, the citrus seeds have been used as minor sources of oils. Specifically, the oil of citrus seeds from Pakistan, Nigeria and Vietnam [1], Mexico [5], Brazil [6], Spain [7] and Egypt [8] has been characterized. The results show that the oil char- acteristics such the density (r), refractive index (RI) and iodine value (IV) of orange seeds (same genus) from Egypt and Pakistan are very similar (r: 0.93 vs 0. 92 mg ml 1 ; RI: 1.4681 vs 1.4641; IV: 99.2 vs 99.85). But, when the comparison was performed between diverse genus of citrus seeds from the same source (orange vs lemon vs grapefruit, etc.), some characteristics of the oil differed. For example, the acid value (AV), unsaponifiable matter (UM) and color red units (CRU) of the orange vs grapefruit seeds of Pakistan (AV: 0.5 vs 0.66; UM: 0.5 vs 0.39; CRU: 3.00 vs 1.40) were dissimilar [1]. Also, differences were observed in the oil of the citrus seeds of Egypt, where the AV was higher in the grapefruit (0.90) than in the orange seed (0.21) [8]. * Corresponding author. Tel.: þ52 2222295500; fax: þ52 2222295525. E-mail address: melizald@siu.buap.mx (M.P. Elizalde-Gonza ´ lez). Available at www.sciencedirect.com http://www.elsevier.com/locate/biombioe 0961-9534/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2009.05.016 biomass and bioenergy 33 (2009) 1295–1299