It is not obligatory to do ghusl from janaabah when one wants to eat, rather it is Sunnah to do wudoo’ when one wants to eat. This is a matter on which there is consensus among the scholars.
The evidence for that is the hadeeth of ‘Aa’ishah (may Allaah be pleased with her), that the Prophet (peace and blessings of Allaah be upon him), when he wanted to eat or sleep at the time when he was junub (in a state of impurity following intercourse), he would do wudoo’. (Narrated by Muslim, al-Hayd, 461). Shaykh Ibn ‘Uthaymeen said in al-Sharh al-Mumti’: “It seems to me that the one who is junub should not sleep without doing wudoo’, this is mustahabb, as is also the case with regard to eating and drinking. Some of the scholars said: it is not makrooh for him to eat or drink without doing wudoo’ when he is junub.”