outcome <- c(1,0,0,1,1)
string <- c('I love pasta','hello world', '1+1 = 2','pasta madness', 'pizza madness')

df = df=data.frame(outcome,string)


> df
  outcome        string
1       1  I love pasta
2       0   hello world
3       0       1+1 = 2
4       1 pasta madness
5       1 pizza madness

outcome <- c(1,0,0,1,1)
string <- c('I love pasta','hello world', '1+1 = 2','pasta madness', 'pizza madness')

df <- data.frame(outcome=factor(outcome,levels=c(0,1)),string, stringsAsFactors=FALSE)

library(dplyr)
library(tidyr)
inp <- df %>% mutate(string=strsplit(string,split=" ")) %>% unnest(string)
##   outcome  string
##1        1       I
##2        1    love
##3        1   pasta
##4        0   hello
##5        0   world
##6        0     1+1
##7        0       =
##8        0       2
##9        1   pasta
##10       1 madness
##11       1   pizza
##12       1 madness

library(randomForest)
mm <- model.matrix(outcome~string,inp)
rf <- randomForest(mm, inp$outcome, importance=TRUE)
imp <- importance(rf)
##                     0        1 MeanDecreaseAccuracy MeanDecreaseGini
##(Intercept)   0.000000 0.000000             0.000000        0.0000000
##string1+1     0.000000 0.000000             0.000000        0.3802400
##string2       0.000000 0.000000             0.000000        0.4514319
##stringhello   0.000000 0.000000             0.000000        0.4152465
##stringI       0.000000 0.000000             0.000000        0.2947108
##stringlove    0.000000 0.000000             0.000000        0.2944955
##stringmadness 4.811252 5.449195             5.610477        0.5733814
##stringpasta   4.759957 5.281133             5.368852        0.6651675
##stringpizza   0.000000 0.000000             0.000000        0.3025495
##stringworld   0.000000 0.000000             0.000000        0.4183821