Role of Water Activity in the Spoilage of Alfalfa Hay

Abstract

Moisture sorption isotherms of alfalfa stems and leaves obtained from the first and third cutting of the season were constructed. Important differences in the relationship between moisture content and water activity values for alfalfa stems and leaves were observed. The extent of those differences was influenced by the initial moisture content and maturity of the alfalfa plant. Water activity, not moisture content, determined the nature of fungal growth on alfalfa stems and leaves. At water activity of 1.0 (moisture content, 51.7 to 65.3%), fungal growth was characterized by extensive mycelial development. Reducing the water activity to .93 (moisture content, 28.1 to 41.7%) dramatically increased fungal fruiting body development and reduced mycelial development. Even though alfalfa stems are nutritionally inferior to alfalfa leaves, fungal fruiting bodies appeared sooner on the stems at water activity values less than .81. Addition of a rich nutrient source, yeast extract, to the stems and leaves did not change that relationship. Analysis of the experimental results reported showed that alfalfa stems have a greater spoilage potential than alfalfa leaves. Implications of these findings to the spoilage process and the nutritional quality of baled alfalfa hay are discussed.