Tofu for bentos

I really really love eating plain boring old boiled tofu, and I'd like to incorporate it into my bentos. However, when I tried to, by lunch time it had gone all gross and sour and I ended up having to chuck it away T_T Does tofu just not keep that well, or was I doing something wrong? If I want to bring tofu to school in my bento I usually end up having to fry it, which I hate, because it just loses that nice texture >.< Does anyone have suggestions? Also, lately my mother and her friends seem to be getting very upset about tofu manufacturers putting gypsum into tofu. I'm a little confused; I thought gypsum was the coagulant you use in making tofu (or nigari). Should we be concerned?