Microwave pretreatment allows accurate fatty acid analysis of small fresh weight (100 g) dried alfalfa, ryegrass, and winter rye samples

Accurate analysis of forage fatty acid (FA) profile and content, particularly polyunsaturated FA (PUFA), such as alpha-linolenic acid (ALA) and linoleic acid (LA), is vital for ongoing research optimizing the desired FA content of ruminant-derived foods. Lyophilization (i.e., freeze-drying) is regarded as the gold standard for preserving labile constituents, such as PUFA, in fresh forage samples. This method, however, is expensive, time consuming, and generally impractical for the large number and size of forage samples in agronomic studies.The objective of the first experiment was to evaluate the efficacy of a brief (1 min) microwave pretreatment prior to forced hot air drying (FHA) for preserving alfalfa (Medicago sativa L.) and Italian ryegrass (Lolium multiflorum L.) forage samples, relative to both a freeze-drying control and FHA drying alone. In a second experiment, similar drying methods were examined in winter rye (Secale cereale L.), as well as the decline of ALA proportion of total FA in ground samples stored 72 weeks.In the first experiment, small fresh weight samples (100 g) receiving 1 min microwave pretreatment before FHA drying were found equivalent to freeze-dried samples in FA content and profile, and both were greater in ΣPUFA than samples that received FHA alone. Large fresh weight samples (400–500 g) receiving FHA alone, a common practice in agronomic studies, contained 1.5–2.5 fold lower total ALA content and 1.3–1.6 fold lower LA content than the freeze-dried control, while large fresh weight samples (400–500 g) receiving 1 or 2 min microwave pretreatment contained 1.2–2.2 fold lower total ALA content and 1.1–1.5 fold lower LA content than the freeze-dried control. The second experiment revealed a reduction in ALA proportion of total FA of stored samples over a long period of time regardless of preparation method; however, FHA samples had a greater rate of reduction.This study confirms that FHA drying alone is not adequate for forage sample preservation for FA analysis, and may lead to a significant underestimation of ALA, the principal FA in plants. This distorts measurements of other FA reported on a proportional basis (g 100 g−1 total FA). In conclusion, a simple and inexpensive 1 min microwave pretreatment prior to FHA drying results in FA analysis comparable to freeze-dried samples, provided that samples are of a small fresh weight (100 g).