The noun 'garlic' here is being used in a generic sense. The definite article is required for this in French. http://french.about.com/library/weekly/aa082401.htm
It is not 'de l'ail' (the partitive article) because that would convey a different meaning in French to what is intended here. There's no 'the' in the translation because it's not necessary in English, but in French it is.