Food inflation slipped into the negative zone, falling to minus 3.36 per cent for the week ended December 24, as the prices of essential items like vegetables, onions, potatoes and wheat declined.This is the first time in almost six years that food inflation has fallen, on an annual basis.Food inflation, as measured by Wholesale Price Index, had stood at 0.42 per cent in the previous week, and at almost 21 per cent in the corresponding week of 2010.According to official data released, today, onions became 73.74 per cent cheaper, year-on-year, during the week under review, while potato prices were down 34.01 per cent.Wheat prices fell 3.41 per cent. Overall, vegetables became 50.22 per cent cheaper during the week under review.