ORIGINAL PAPER Argon Plasma Treatment of Tapioca Starch Using a Semi-continuous Downer Reactor Weerawut Chaiwat 1 & Rungtiwa Wongsagonsup 2 & Nutticha Tangpanichyanon 2 & Tanchanok Jariyaporn 2 & Panakamol Deeyai 3,4 & Manop Suphantharika 5 & Asira Fuongfuchat 6 & Mudtorlep Nisoa 7 & Somsak Dangtip 8,9 Received: 4 September 2015 /Accepted: 17 February 2016 # Springer Science+Business Media New York 2016 Abstract To develop a novel modification process of tapioca starch using low-pressure argon plasma treatment in a large-scale production, a semi-continuous downer reactor was designed to provide a production rate of 0.1 – 0.5 kg per cycle. Physicochemical and rheological properties of plasma-treated starch were investigated in order to predict the phenomena oc- curred during the plasma treatment. Native tapioca starch (NTS) was plasma-treated for 1, 3, or 6 cycles, which were referred to as PTS-1, PTS-3, or PTS-6 samples, respectively. Plasma treatment of the NTS resulted in a significant decrease (P < 0.05) in paste clarity and Rapid Visco Analyzer (RVA) breakdown viscosity, and an increase in gel strength of starch. The PTS-1 had the lowest paste clarity of 39.85 %T at 650 nm and breakdown viscosity of 29.71 Rapid Visco Unit (RVU), and the highest gel strength, e.g., G′ = 5.47ω 0.32 . With increasing the number of treat- ment cycles, i.e., the PTS-3 and PTS-6, the paste clarity and breakdown viscosity significantly increased (P < 0.05), while the gel strength decreased. The Fourier transform infrared (FTIR) spectroscopy via relative areas of C–O–C peaks indicated that the PTS-1 had a significant increase (P < 0.05) in the C–O–C cross-linked bonds compared to the NTS, but the effect of depo- lymerization could further suppress the cross-linking reaction when the number of treatment cycle was increased. Taking an advantage of a very short residence time (less than 0.3 s) in the plasma downer reactor, the semi-continuous process of starch modification could be developed for a commercial production of non-chemically modified starch with a relatively low degree of cross-linking. Keywords Modified tapioca starch . Low-pressure argon plasma . Semi-continuous downer reactor system . Rheological properties . Cross-linking . Depolymerization Introduction Tapioca starch, one of the major industrial starches in Thailand, is widely used in food and non-food industries due * Somsak Dangtip somsak.dan@mahidol.ac.th 1 Environmental Engineering and Disaster Management Program, School of Interdisciplinary Studies, Mahidol University, Kanchanaburi Campus, Kanchanaburi 71150, Thailand 2 Food Technology Program, School of Interdisciplinary Studies, Mahidol University, Kanchanburi Campus, Kanchanaburi 71150, Thailand 3 Materials Science and Engineering Program, Faculty of Science, Mahidol University, Bangkok 10400, Thailand 4 Present address: Department of Industrial Physics and Medical Instrument, Faculty of Applied Science, King Mongkut’ s University of Technology North Bangkok, Bangkok 10800, Thailand 5 Department of Biotechnology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand 6 National Metal and Material Technology Center (MTEC), National Science and Technology Development Agency (NSTDA), Ministry of Science and Technology, Klong Luang, Pathum Thani 12120, Thailand 7 Plasma Agricultural Application Laboratory, School of Science, Walailak University, Nakhon Si Thammarat 80160, Thailand 8 Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand 9 NANOTEC CoE at Mahidol University, Faculty of Science, Mahidol University, Bangkok 10400, Thailand Food Bioprocess Technol DOI 10.1007/s11947-016-1701-6