Journal of Chemical Engineering of Japan, Vol. 37, No. 2, pp. 253–260, 2004 Research Paper Copyright © 2004 The Society of Chemical Engineers, Japan 253 Sub-critical Water Hydrolysis Treatment for Waste Squid Entrails and Production of Amino Acids, Organic Acids, and Fatty Acids Hiroyuki YOSHIDA and Omid TAVAKOLI Department of Chemical Engineering, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai-shi, Osaka 599-8531, Japan Keywords: Sub-critical Water, Squid Wastes, Amino Acid, Organic Acid, Fatty Acid The study presented in this paper is focused on processing squid wastes by using sub-critical water extraction and hydrolysis to produce valuable substances. Using a small-scale batch reactor (9.0 cm 3 ), squid entrails were treated with sub- and supercritical water without oxidants. Reaction conditions in- volved temperatures of 443–673 K, pressures 0.792–30.0 MPa, and reaction times 1–50 minutes. Fat and oil were extracted into sub-critical water very efficiently, and soluble proteins, organic acids, and amino acids were produced in the aqueous phase by a hydrolysis reaction. Even in the short reaction time of 5 min, more than 99% of solid were converted to those products in the temperature, which was higher than 550 K. Results showed a maximum total yield of sixteen amino acids of 0.1031 kg/kg dry en- trails at 473 K (5 min), and a maximum total yield of seven organic acids of 0.055 kg/kg dry entrails at 553 K (40 min). Also the oil extracted with hexane included useful fatty acids. Our experimental results indicated that the sub-critical water treatment method is very efficient for converting waste squid en- trails to valuable materials as resources for many other industries. Introduction Seafood processing and related waste discharges have become a serious problem since damping of those kinds of wastes has been banned by the London treaty of 1996. In Japan, about 1.3 million tons of seafood wastes are produced a year. After the London treaty, almost all those wastes have been incinerated and the running cost of which is $500–800 per ton of wastes, which is very expensive. Therefore an efficient process for converting seafood wastes to useful materials is strongly needed to achieve zero emissions. Yoshida et al. (1999) showed that sub-critical water hydrolysis is an efficient method for production of useful substances such as amino acids, organic acids, fatty acids, oil, and so on, from fish wastes. Kang et al. (2001) used sub- and supercritical water to produce amino acid from fish wastes. Other applications of sub- and supercritical water treatment have also reported. Kabyemela et al . (1999) decomposed glucose, fructose and cellobiose. Sasaki et al. (2000) reported hydrolysis of cellulose, and Taylor et al. (2001) investigated hydrolysis of methyl t-butyl ether. Also some technologies of sub- and supercritical water conditions with oxidants have been reported, such as treatment of wastewaters con- taining organic pollutants including hazardous organic (Modell, 1989), pyridine (Crain et al., 1993), Azo dye Received on December 18, 2002. Correspondence concerning this article should be addressed to H. Yoshida (E-mail address: yoshida@chemeng.osakafu-u.ac.jp). (Donlagic and Levec, 1997), linear alkylbenzene sulfonates (Mantzavinos et al., 2000), municipal sew- age sludge (Goto et al ., 1999), phenol (Savage et al ., 1995; Portela et al ., 2001), and aqueous polyvinyl al- cohol (Won et al., 2001). There are three important reasons to use sub-criti- cal water treatment. Firstly, the main aim in supercritical water oxidation is that wastes must be decomposed to CO 2 , N 2 , and water. On the other hand the sub-critical water hydrolysis produces many use- ful and valuable organic substances. This is because the ion product of water shows maximum value around 520–550 K in the sub-critical region. Secondly, sub- critical water acts as a powerful solvent and exhibits important effects in its physical properties. This is be- cause the dielectric constant decreases with increas- ing temperature. Moreover, sub-critical water is an excellent extraction solvent for polar and non-polar organic substances, and a much broader range of polarities can be achieved in this condition. Thirdly, this process is more economic than, not only supercritical water treatment, but also the incineration processes. The supercritical water treatment process is normally carried out at very high temperatures (higher than 1000 K) and high pressures, and the ap- paratus decays quickly by strong oxidation even if very expensive Hastelloy is used. The incineration process is also very expensive, as we already mentioned. Since the squid entrails contain a lot of cadmium and other harmful metal cations; their treatment has