Sample Handling Affects Accuracy of HS-SPME GC for the Study of Volatile Aroma Compounds in Apple Tissue

HS-SPME coupled to GC has been widely used for analysis of volatile aroma compounds (VACs) in apple tissue. Several factors influence the accuracy of these methodologies such as SPME conditions, GC conditions, sample preparation and sample handling. In the present work, the effect of sample handling was evaluated. Storage time, storage temperature, vial material, and addition of sodium chloride prior or after sample storage were assessed to optimize the conditions for a reproducible and accurate HS-SPME analysis of VACs in apple tissue. Freshly made apple juice was placed in plastic or glass vials (20mL juice in 20mL plastic vials –conical cap-, or 2mL juice in 4 mL glass vials –SPME vials-),  stored for 10 or 30 days at three different temperatures: 0, -20, or -70 °C. Sodium chloride was added (36 % w/v) prior or posterior to storage. For VACs analysis, tissue was thawed and those samples stored in 20mL plastic vials were pipetted into 4 mL glass vials (2mL of juice into SPME vials). For those samples stored without sodium chloride, this was added prior analysis. VACs were analyzed by HS-SPME (65 µm PDMS/DVB fiber; 45min fiber exposure time) with a GC-MS instrument. Storage time of 30 days was the factor with the greatest effect on the VACs content in apple tissue, independently of temperature or vial. Concentration of 2-hexenal, 1-butanol, and 2-metyl propyl acetate, considerably increased after 30 days of storage; while concentration of hexanol, butyl acetate, 2-metyl butyl acetate highly decreased after 30 days of storage. At 10 days of storage, there was an effect on VACs concentration due to the type of vial: plastic vials increased values of hexanal, hexanol, 2-metyl-1-butanol, and decreased values of hexyl acetate, butyl acetate, 2-metyl butyl acetate; whereas glass vials didn´t affect VACs, at any temperature (0, -20 or -70°C). On glass vials, addition of salt prior to storage, helped to maintain VACs concentration. From this study we recommend to store samples for no more than 10 days, using glass vials (SMPE vials), and adding sodium chloride prior to storage, at temperatures below -20°C. The optimal storage conditions for the reproducible and accurate analysis of headspace apple juice VACs requires further studies to elucidate the complex interactions between the many participating factors.