Just want to chip in and say, it might be a good idea to track your food/drink purchases as you buy them anyways.
Your method of track-what-i-eat doesn't include a huge area of $$ reduction, namely waste. Stuff we buy that collects dust in the corner of the pantry (I'm looking at you, fish sauce), veggies, bread, etc, that goes bad, or the truly awful throwing-away-leftovers move. In the end, bumps on bulk stuff like rice, oil, eggs, all evens out in the end, so I wouldn't worry about once-or-twice-a-year purchases throwing off your numbers too much.
Tracking what you eat tells you how much you NEED, but it won't tell you how much you're SPENDING. Both numbers seem handy to know, though.
Just my 2c =)