3.6.2

DISINFECTION OF RICE STRAW FORAGE WITH SATURATED STEAM


ZHINONG MAO1, ZHENG ZHAO1 and RUHAI XING1

1Dalian Animal and Plant Quarantine Service, Dalian, Liaoning 116001, PR China

Background and Objectives
For quarantine reasons, products made of rice straw without proper treatment are not allowed to enter Japan from mainland of China. In order to open Japanese market of rice straw forage, heat treatment tests were conducted. As rice straw forage was in big condensed bales, it is not easy for heat to penetrate into the center of the bales. Several heating methods such as moist heat wind, high frequency, microwave and vacuum saturated steam were compared and vacuum saturated steam method was finally chosen.

Materials and Methods
Fundamental Study: Rice straws pieces about 4mm in length were inoculated with indicating pathogen Cochliobolus miyabeanus and then were dried. These rice straw pieces were treated for 10min., 20min., 30min. and 40min. with saturated steam of 62; for 5min., 10min., 15min. and 20min. with saturated steam of 65; for 3min., 6min., 9min. and 12min. with saturated steam of 68; for 2min., 4min., and 6min. with saturated steam of 71; for 5/60min., 1min., 2min. and 3min. with saturated steam of 74. Treated rice straw pieces were cultured on PDA medium at 25 for 5 days. At each temperature, the longest treating time that the pathogen was not completely killed was considered as longest survival time (LST), and the shortest treating time that the pathogen was completely killed was considered as shortest death time (SDT). The thermal death time (TDT) at each temperature was given by extracting the square root of product of LST and SDT.
Production Test: Based on the results of fundamental study, disinfection test of commercial rice straw forage has been conducted three times. The dimension of tested cut rice straw bale is 25cm25cm80cm and weight is 201kg. Indicating pathogen Cochliobolus miyabeanus was introduced into rice straw bale and temperature sensors were set to the bales' geometric center. During the treating, disinfection rate was calculated by temperature and TDT curve. The first two test's treating procedures were as follow: vacuuming for 5 min., then treating with saturated steam of 100 for 5 min., vacuuming again for 5 min., and then treating with saturated steam of 100 till theoretic disinfection rate over 100%, finally vacuuming for 15min., The third test's treating procedures was as follow: vacuuming for 5 min., then treating with saturated steam of 100 for 5 min., vacuuming again for 5 min., and then treating with saturated steam of 100 for 10min., finally vacuuming for 15min.. Vacuum intensity after each vacuuming should be no less than 720mmHg. Weight of each tested bale was measured before and after treatment respectively, and nutrient content analysis for treated and untreated rice straw was conducted.

Results and conclusions
The space of set temperatures of 62,65,68,71and 74C were measured with standard thermal meter and the actual temperatures were 63.4,65.5,66.9,71.0 and 75.0C, respectively. The TDT values at those five temperatures were 14.14min., 12.25min., 7.35min., 2.83min. and 0.29min., respectively. TDT curve, log(Y)=10.4001-0.1447X, was made by linear regression of logarithm of TDT and temperature, and the correlation coefficient r=-0.9454. In order to guarantee the security of application of the TDT curve, the TDT curve was moved up 3 standard as log(Y)=11.1843-0.1447X.
The time that theoretic disinfection rate reached 100% in the first two production tests were 22min. and 26min., respectively. All introduced Cochliobolus miyabeanus were killed in whole three production tests. The weight of treated rice straw bales was slightly increased by about 0.5kg and no significant changes of nutrient content were observed.
Due to repeat of vacuuming and steaming, disinfection of rice straw forage with vacuum saturated steam is rapid and the results of disinfection can be guaranteed. As this method has been widely used in chemical production, the technology and equipment are pretty mature and the prospect of its use in forage industry is splendid.

References
1. Kazuo Tanabe, A. Kawai and T. Sugimoto, 1990,Res. Bull. Pl. Prot. Japan, 26:57-60.
2. Kazuo Tanabe, A. Kawai and Nishio, 1989,T., Res. Bull. Pl. Prot. Japan, 25:55-57.