Evaluation Method of Tuna Freshness Using SHG Imaging

As the global popularity of raw food cultures, such as sushi, continues to increase, the freshness of sashimi has become a crucial factor directly linked to both its taste and safety. Traditionally, subjective assessments based on the five senses or physicochemical testing methods have been widely used. However, for more consistent and reliable evaluations, a quantitative method for assessing freshness is highly desirable. To address this need, we developed a non-invasive, non-destructive freshness assessment technique using polarized Second Harmonic Generation (SHG) microscopy. Tuna samples were stored for 0 to 72 hours, and SHG images were acquired at time points of 0, 12, 24, 48, and 72 hours. Through image analysis, we observed the degradation of muscle fibers over time. Our results demonstrated that muscle fragmentation progressed between 12 and 24 hours, and by 72 hours, most muscle tissue had disintegrated, leaving collagen fibers intact. This structural transformation, when quantitatively analyzed, demonstrated the potential for scientifically evaluating sashimi freshness.