Our ambivalence about food may be due in part to the influence of the Importance of Not Being Earnest rule. Excessive zeal on any subject is embarrassing, and getting all earnest and emotional about something as trivial as food is, well, frankly rather silly.
But it seems to me that our uneasiness about food and foodieness involves something more than this. There is a hint here of a more general discomfort about sensual pleasures. Flaunting one's passion for good food, and talking openly about the pleasure of eating it, is not embarrassing just because it is over-earnest but also because it is somehow a bit obscene.
It has been said that the English have a puritanical streak, but I'm not sure this is quite accurate. Sex, for example, is not regarded as sinful, but as private and personal and therefore a bit embarrassing. Jokes about sex, even quite explicit ones, are acceptable; earnest or fervent talk about the same intimate physical details is obscene. The sensual pleasures of eating, it seems to me, are in the same category - not exactly a taboo subject, but one that should only be talked about in a light-hearted, unserious, jokey manner.
Foodies (or foreigners) who dwell too lyrically, too erotically, on the delights of a perfectly executed, voluptuously creamy sauce bearnaise, will make us squirm, blush and look away. To avoid offending, all they need do is lighten up a bit, laugh at themselves, not take the whole thing quite so seriously. Without such ironic detachment, foodie-talk becomes a form of 'gastro-porn' (the term normally refers to lavishly illustrated foodie magazines and cookbooks, with detailed, mouth-watering descriptions of each luscious dish - but can equally be applied to over-enthusiastic foodie conversation).