Apple pomace ultrafiltration sludge – A novel substrate for fungal bioproduction of citric acid: Optimisation studies Gurpreet Singh Dhillon a , Satinder Kaur Brar a,⇑ , Mausam Verma b , Rajeshwar Dayal Tyagi a a INRS-ETE, Université du Québec, 490, Rue de la Couronne, Québec, Canada G1K 9A9 b Institut de recherche et de développement en agroenvironnement inc. (IRDA), 2700 rue Einstein, Québec, Canada G1P 3W8 article info Article history: Received 16 September 2010 Received in revised form 23 February 2011 Accepted 28 March 2011 Available online 1 April 2011 Keywords: Apple pomace ultrafiltration sludge Inducer Response surface methodology Submerged fermentation Total suspended solids abstract Ever-growing demand for citric acid (CA) and urgent need for alternative sources has served as a driving force for workers to search for novel and economical substrates. Submerged fermentation was conducted using apple (Malus domestica) pomace ultrafiltration sludge as an inexpensive substrate for CA biopro- duction, using Aspergillus niger NRRL567. The crucial parameters, such as total suspended solids and indu- cer concentration, were optimised by response surface methodology for higher CA production. The optimal CA concentrations of 44.9 g/100 g and 37.9 g/100 g dry substrate were obtained with 25 g/l of ini- tial total solids and 3% (v/v) methanol and 25 g/l of total solids and 3% (v/v) ethanol concentration, respectively, after the 144 h of fermentation. Results indicated that total solids concentration, and meth- anol as an inducer, were effective with respect to higher CA yield and also indicated the possibility of using apple pomace sludge as a potential substrate for economical production of CA. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Citric acid (CA) production by Aspergillus niger is one of the fin- est and most commercially utilised examples of higher accumula- tion of intermediate products during fungus metabolism. CA dominates the category of organic acids, with the global produc- tion estimated to be more than 1.6 million tons (Sauer, Porro, Matt- anovich, & Branduardi, 2008) and the volume of CA production by fermentation is constantly rising at a high annual rate of 5% (Fran- cielo, Patricia, & Fernanda, 2008), with future escalating trends. CA is an important multi-functional organic acid with a broad range of versatile applications, e.g. in food, pharmaceuticals and cosmetics. Currently, many advanced applications are coming to light, such as: (1) biomedicine, e.g. synthesis of biopolymers for culturing a variety of human cell lines; (2) nanotechnology, such as drug deliv- ery systems and; (3) agriculture, such as bioremediation of heavy metals due to its powerful sequestering action with various transi- tional metals (Dhillon, Brar, & Verma, 2010b). The world’s existing demand for CA is almost entirely (over 99%) met by fermentative processes, using A. niger in submerged or static liquid cultures (Dhillon, Brar, Verma, & Tyagi, 2010a). However, for the past few years, the CA market has been under tremendous pressure and continues to swing with reduction in prices. High energy, coupled with raw material costs, has squeezed CA production into an unprofitable market. The search for inexpensive substrates as an alternative to high cost substrates is vital to reduce the production cost of CA. In recent years, considerable interest has been focused on agro-industrial wastes for CA bioproduction (which serves to solve waste management problem faced by agro-industries). More- over, industries will also be benefitted by extra revenue from the value addition of agro-industrial wastes. Every year, thousands of tons of apple pomace (AP) and apple pomace sludge (APS) are generated by apple processing industries in Canada. In 2008–2009, out of world’s total apple production (69, 603, 640 tonnes), Canada contributed 455, 361 tonnes (more than 25% by Quebec alone) (Bhusan, Kalia, Sharma, Singh, & Ahuja, 2008; Food, 2008; Dhillon, Brar, Verma, & Tyagi, 2011; Statistics Canada 2010; Vendruscolo, Albuquerque, & Streit, 2008). The pro- cessing of apples will lead to production of 25–30% AP and 5–10% APS (Dhillon et al., 2011). The apple processing industries incur losses due to treatment of waste and transportation costs for dumping into landfills. In this context, CA production from nutri- ent-rich organic wastes, such as AP and APS (total carbon 51.9 g/ l, total nitrogen 2.94 g/l, carbohydrates 66.0 ± 1.7 g/l, lipids 5.9 ± 0.32 g/l, protein 33.8 ± 2.0 g/l) is a lucrative alternative for the apple processing industries (Dhillon et al., 2011). Due to the complexity of the metabolic state in fungus for the enhanced accumulation of desired product, there is an obvious need to optimise the important process parameters, depending 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.03.107 Abbreviations: APS, apple pomace ultrafiltration sludge; CA, citric acid; CCD, central composite design; EtOH, ethanol; MeOH, methanol; RSM, response surface methodology; TS, total suspended solids. ⇑ Corresponding author. Tel.: +1 418 654 3116; fax: +1 418 654 2600. E-mail address: satinder.brar@ete.inrs.ca (S.K. Brar). Food Chemistry 128 (2011) 864–871 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem