Journal of Hazardous Materials B137 (2006) 1643–1648 Hydrothermal decomposition of yeast cells for production of proteins and amino acids Wiwat Lamoolphak a , Motonobu Goto c , Mitsuru Sasaki c , Manop Suphantharika b , Chirakarn Muangnapoh a , Chattip Prommuag a , Artiwan Shotipruk a,∗ a Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Patumwan, Payathai Road, Bangkok 10330, Thailand b Department of Biotechnology, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400, Thailand c Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 850–8555, Japan Received 23 November 2005; received in revised form 1 May 2006; accepted 2 May 2006 Available online 16 May 2006 Abstract This study examines hydrothermal decomposition of Baker’s yeast cells, used as a model for spent Brewer’s yeast waste, into protein and amino acids. The reaction was carried out in a closed batch reactor at various temperatures between 100 and 250 ◦ C. The reaction products were separated into water-soluble and solid residue. The results demonstrated that the amount of yeast residue decreased with increasing hydrolysis temperature. After 20 min reaction in water at 250 ◦ C, 78% of yeast was decomposed. The highest amount of protein produced was also obtained at this condition and was found to be 0.16 mg/mg dry yeast. The highest amount of amino acids (0.063 mg/mg dry yeast) was found at the lowest temperature tested after 15 min. The hydrolysis product obtained at 200 ◦ C was tested as a nutrient source for yeast growth. The growth of yeast cells in the culture medium containing 2 w/v% of this product was comparable to that of the cells grown in the medium containing commercial yeast extract at the same concentration. These results demonstrated the feasibility of using subcritical water to potentially decompose proteinaceous waste such as spent Brewer’s yeast while recovering more useful products. © 2006 Elsevier B.V. All rights reserved. Keywords: Subcritical water; Proteins; Amino acids; Hydrolysis; Baker’s yeast; Spent Brewer’s yeast 1. Introduction Spent Brewer’s yeast, the by product from the brewing indus- try, is being produced in large amount annually from main beer manufacturers due to increased volume of beer production. It is generally sold primarily as inexpensive animal feed after inac- tivation by heat, and much of this by product is considered industrial organic waste that causes a great deal of concerns. Such wastes are generally incinerated or put into landfill, in which case, the remaining proteins and amino acids, and other useful substances were not recovered. In addition, incineration of organic waste often gives toxic emission whose distribution degree is even higher than that of organic solid waste. Attempts have been made to recover higher value protein and amino acid products from spent Brewer’s yeast [1] by employing var- ∗ Corresponding author. E-mail address: artiwan.s@chula.ac.th (A. Shotipruk). ious processes such as autolysis, plasmolysis [2,3] in organic salt solution or non-polar organic solvent, acid or alkali cat- alyzed hydrolysis, or enzymatic hydrolysis [1,4]. Plasmolysis and alkali or acid hydrolysis involves use of harmful chemicals and washing off these chemicals leads to generation of wastew- ater. Autolysis and enzymatic hydrolysis is therefore preferred, however autolysis requires long process time and the enzymes are usually costly to be practical in large scale. Recently, hydrothermal conversion of organic wastes into more valuable substances using sub- or supercritical water with- out oxidants has been investigated. In this process, the ion product and dielectric constant of water play an important role. At the temperature below the critical point, the ion product of subcritical water, K w , increases, for example, to approximately 10 -11 at 250.3 ◦ C and 25 MPa. The value then decreases to 10 -16 at the critical point (374.2 ◦ C) [5–7]. Two major types of reaction take place in sub-or supercritical water: oxidation and hydrolysis. Supercritical water oxidation is a process that con- verts organic materials completely into carbon dioxide, water, 0304-3894/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2006.05.029