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Ind. Eng. Chem. Res. 2010, 49, 8470–8476
Chemical Production of Pectic Oligosaccharides from Orange Peel Wastes Martina Martı´nez, Remedios Ya´n˜ez, Jose´ Luis Alonso´,* and Juan Carlos Parajo´ Department of Chemical Engineering, Faculty of Science, UniVersity of Vigo (Campus Ourense), As Lagoas, 32004 Ourense, Spain
Orange peel wastes (OPW) were extracted with water to remove soluble material, and the resulting solid phase (which was obtained at a yield of 48.8 kg/100 kg of OPW, oven-dry basis) was employed as a substrate for the manufacture of pectic oligosaccharides by nonisothermal processing with hot, compressed water (autohydrolysis or hydrothermal treatments). The effects of treatments on the composition of both liquors and spent solids were assessed in terms of the severity factor (Ro). Operating under selected conditions (maximum temperature of 160 °C, corresponding to Ro ) 288 min), the oligosaccharide yield (including oligogalacturonides, arabinooligosaccharides, and galactooligosaccharides) accounted for 25.1 kg/100 kg of extracted OPW, whereas limited concentrations monosaccharides and nonsaccharide compounds were present in the reaction media. 1. Introduction Prebiotics have been defined as “selectively fermented ingredients that allow specific changes, in both the composition and/or activity in the gastrointestinal microbiota that confers benefits upon host well-being and health”.1 Several oligosaccharides with prebiotic properties (such as fructooligosaccharides, galactooligosaccharides, or lactulose) are commercially available, but there is an increasing interest in the identification and development of new prebiotic ingredients with added functionality.2 Pectic oligosaccharides (POS) are good candidates as prebiotics,3 and possess additional functionalities, including repression of liver lipid accumulation in rats, activity as antibacterial agents, protection of colonocytes against E. coli verocytotoxins, and stimulation of apoptosis in human colonic adenocarcinoma cells.4 Pectin is commercially obtained from citrus peel, but its market demand is low in comparison with the world availability of citrus-processing residue.5 Orange peel wastes (OPW) are abundant in Brazil, United States, Mexico, and Spain. Approximately, 13.5 million tons of OPW were produced worldwide in 2008, but their market is limited because of the high water content (about 86%) and drying problems when using common industrial devices. On the other hand, OPW are not easily disposable6 and are usually used as cattle feed.7 OPW are rich in both soluble and insoluble carbohydrates. Sugars (mainly glucose, fructose, and sucrose) can be extracted with water at room temperature, leading to solutions that can be fermented by microorganisms into a variety of chemicals or fuels such as ethanol;8 whereas insoluble polysaccharides (including pectin, cellulose, and hemicelluloses)9 could be suitable substrates for the production of oligosaccharides with prebiotic potential. The structure of pectins is complex, being possible to recognize three structural polymers: homogalacturonan (HG), rhamnogalacturonan I (RGI), and rhamnogalacturonan II (RGII). HG is the most abundant type (about 60% of the total pectin) and consists of a backbone of R-(1,4)-linked D-galacturonic acid residues. The structural units can be esterified at C-6, and/or O-acetylated at O-2 and/or O-3. The RGI backbone is made up * To whom correspondence should be addressed. Fax: +34 988387001. E-mail:
[email protected].
of units of the dimer R-(1,2)-L-rhamnose-R-(1,4)-D-galacturonic acid. In this polymer, the rhamnosyl residues can be substituted at O-4 with neutral sugars side chains, mainly composed of galactosyl and/or arabinosyl residues. RGII is a region within HG, containing clusters of four different side chains.10 These structural polymers are highly susceptible to enzymatic hydrolysis,11-14 acid hydrolysis,9 or hydrothermal treatments.15 Hydrothermal treatments, carried out with hot compressed water, have been successfully employed for producing oligosaccharides from a variety of raw materials.16-19 This methodology shows favorable features, such as (i) environmentally friendly character (water and feedstock are the only reagents), (ii) corrosion problems are avoided, since no mineral acid is added to the reaction media, (iii) ability for generating oligosaccharides in a single stage at favorable yields, (iv) limited production of undesired sugar-degradation products, (v) faster reaction than enzymatic hydrolysis, and (vi) coproduction of spent solids enriched in cellulose, showing the ability to be utilized for production of metabolites (such as ethanol) by fermentation.15,20,21 This work deals with the production of oligosaccharides from OPW by nonisothermal hydrothermal treatments. The effects of temperature on the composition of spent solids and liquors were evaluated and interpreted in terms of the severity factor (Ro), enabling the selection of operational conditions leading to optimal oligosaccharide production with limited concentration of undesired, nonsaccharide compounds. 2. Materials and Methods 2.1. Raw Material. OPW samples were kindly supplied by Indulleida S.A. (Lleida, Spain). To avoid compositional differences, samples were homogenized in a single lot and stored at -18 °C until use. Aliquots from this lot were defrosted, milled, and subjected to analytical determinations or to processing according to the methods explained below. 2.2. Aqueous Extraction of Orange Peel Wastes. OPW samples were suspended in water (using 10 g of water/g of ovendry OPW), contacted for 15 min at room temperature in a 2 L stirred tank, and centrifuged. This treatment was repeated two additional times using 2 g of water/g of oven-dry OPW. All liquid and solid phases were analyzed for composition (see below), and the insoluble material (denoted SHT or solid for
10.1021/ie101066m 2010 American Chemical Society Published on Web 07/30/2010
Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010
Figure 1. Temperature profile followed in hydrothermal processing.
hydrothermal treatment) was used as a raw material for hydrothermal processing. 2.3. Hydrothermal Treatment of SHT. A 3.75 L stainless steel Parr reactor (Parr Instruments Company, Moline, Illinois) was used for hydrothermal processing of SHT. The reactor was fitted with two four-blade turbine impellers, heated by an external fabric mantle, and cooled by tap water circulating through an internal stainless steel loop. Temperature was controlled through a PID controller, model 4842 (Parr Instruments Company, Moline, Illinois). In each experiment, SHT was mixed with water at a liquid to solid ratio (LSR) of 12 kg of water/kg of dry feedstock, and the reactor was heated to the desired maximal temperature (in the range 140-200 °C). When the target temperature was attained, the reactor was rapidly cooled down, and the liquid and solid phases were recovered by centrifugation. The solid phase was washed with water at a liquid to solid mass ratio of 2 kg/kg (oven-dry SHT basis), centrifuged to recover the washing liquors, and dried at 60 °C. During the treatment, the suspensions were stirred at 150 rpm. Autohydrolysis liquors, washing liquors, and the solid phases were quantified and analyzed as described below. The temperature profile followed during the heating and cooling process of the reactor is shown in Figure 1. The effects of time and temperature on SHT autohydrolysis were interpreted in terms of the severity factor (Ro), defined as Ro )
- 100 ∫ exp[ T(t)14.75 ] dt t
0
(1)
where t is time, T is temperature (°C), and 14.75 is the value reported for an empirical parameter related with the activation energy of the reaction. Time zero was taken when temperature reached was 60 °C. The calculation of Ro enables the comparisons between different reaction conditions defined by different temperature profiles. 2.4. Analysis of Raw Material, Solids for Hydrothermal Treatment, and Autohydrolyzed Solids. Aliquots of OPW were milled to particle sizes
