Foods in the mode of ignorance or darkness are essentially those that are not fresh. Any food cooked more than three hours before it is eaten (except prasādam, food offered to the Lord) is considered to be in the mode of darkness. Because they are decomposing, such foods give a bad odor, which often attracts people in this mode but repulses those in the mode of goodness.
Remnants of food may be eaten only when they are part of a meal that was first offered to the Supreme Lord or first eaten by saintly persons, especially the spiritual master. Otherwise the remnants of food are considered to be in the mode of darkness, and they increase infection or disease. Such foodstuffs, although very palatable to persons in the mode of darkness, are neither liked nor even touched by those in the mode of goodness. The best food is the remnant of what is offered to the Supreme Personality of Godhead. In Bhagavad-gītā the Supreme Lord says that He accepts preparations of vegetables, flour and milk when offered with devotion. Patraṁ puṣpaṁ phalaṁ toyam. Of course, devotion and love are the chief things which the Supreme Personality of Godhead accepts. But it is also mentioned that the prasādam should be prepared in a particular way. Any food prepared by the injunction of the scripture offered to the Supreme Personality of Godhead can be taken even if prepared long, long ago, because such food is transcendental. Therefore to make food antiseptic, eatable and palatable for all persons, one should offer food to the Supreme Personality of Godhead.
Ref >> Srila Prabhupada Vani.