Bhagavad Gita Chapter 17, Verse 10

यातयामं गतरसं पूति पर्युषितं च यत् | उच्छिष्टमपि चामेध्यं बोजनं तामसप्रियम् ||

yāta-yāmaṁ gata-rasaṁ pūti paryuṣitaṁ ca yat ucchiṣṭam api cāmedhyaṁ bhojanaṁ tāmasa-priyam

Food that is stale, tasteless, putrid, rotten, unclean, and impure—such food is dear to tamasic people.

Krishna describes tamasic food—food that is dear to tamasic people. These foods are 'yāta-yāmaṁ gata-rasaṁ pūti paryuṣitaṁ'—stale (yāta-yāmam), tasteless (gata-rasam), putrid (pūti), and rotten (paryuṣitam). They're also 'ucchiṣṭam api cāmedhyam'—remnants (ucchiṣṭam) and unclean (amedhyam). These are foods that are old, stale, processed beyond recognition, or impure. They're 'tāmasa-priyam'—dear to tamasic people—because they match the tamasic nature's tendency toward ignorance, inertia, and disregard for health. Tamasic food promotes dullness, lethargy, and ignorance. Understanding this helps you recognize tamasic food and avoid it to support your growth.