Charak Samhita
पक्वाशयं तु प्राप्तस्य शोष्यमाणस्य वह्निना। परिपिण्डितपक्वस्य वायुः स्यात् कटुभावतः॥११॥
When the pakva part (the non-absorbable remnant part after the absorption of the nutrients), reaches the pakvashaya (the colon), the drying effect of agni converts it into a solid mass. There also occurs the release of vata which is katu (pungent) state
english translation
pakvAzayaM tu prAptasya zoSyamANasya vahninA। paripiNDitapakvasya vAyuH syAt kaTubhAvataH॥11॥
hk transliteration by Sanscript