There was no statistically significant difference in concentration of log change from baseline fluoride levels in biofilm fluid at each individual time-point ( 5 , 15 , 30 and 60 minutes ) for the 2-minute brushing time compared with the 40-second brushing time , but significant differences were observed for 15 , 30 and 60 minutes in favour of 2-minute brushing time when log biofilm fluid value was analysed .