Bhagavad Gita Verse 10, Chapter 17

yaatayaamam gatarasam pooti paryushitam cha yat |
ucchishtamapi chaamedhyam bhoojanam taamasapriyam || 10 ||
 
That which is stale, without essence, putrid and is left over, also that which is pre-tasted and impure, such food is dear to the taamasic.
 
yaatayaamam : stale
gatarasam : without essence
pooti : putrid
paryushitam : leftover
cha : and
yat : that which
ucchishtam : pre-tasted
api : also
cha : and
amedhyam : impure
bhoojanam : food
taamasapriyam : dear to taamasic
 
Having covered the characteristics of food dear to saatvic and raajasic people, Shri Krishna now describes the nature of food dear to taamasic people. The first quality he describes is "yaata yaamam". A "yaama" is equal to three hours. So any food that was cooked more than three hours ago is considered "yaata yaamam" or stale. "Gata rasam" refers to food that has lost its essence, its nutritional value, its "rasa". Any junk food or processed food comes under this classification, as does food that has not been fully cooked.
 
It may surprise us to see Shri Krishna explicitly mention putrid or rotten food in this shloka, since we may think no one would ever eat such food. But there are people in this world who prefer eating food that has an odour to it. He also adds to the list leftover food and food that has already been tasted by someone else. The last item in the list is "amedhyam", which is food that cannot be offered to the deity for worship. He expects us to have the understanding to know what we can offer to the deity and what we should not.
 
So, having gone through the various characteristics of food in these three shlokas, what is the practical implication for us? The first step towards making our antaha karana saatvic, our intellect, our mind our thoughts saatvic, is to watch what we eat. If we are habituated to eating extremely spicy food, we will not have the stability of mind needed to make other parts of of life saatvic. By slowly adopting food that is predominantly saatvic, we lay the foundation for improving the texture of our entire personality.