Abstract:

The study aims to determine the fresh-keeping effects of modified atmosphere (MA) packages on fresh walnuts. With ‘Xiangling’ fresh walnuts as the test materials, five kinds of plastic composite packaging bags were used, including PA/PE bag -200 μm (T1), PA/PE bag -180 μm (T2), Al/PET bag -210μm (T3), PET/PE bag -160 μm (smooth surface) (T4) and PET/PE bag -160 μm (surface mesh) (T5), and plastic film packing -6 μm (T6) was used as the control. Fresh walnuts were stored at low temperature, and the changes of respiratory intensity, fat content, protein content, SOD activity, POD activity, acid value, peroxide value and iodine value were measured regularly. In the respiratory intensity test, except T3, the respiratory intensity of walnuts under T6 and other treatments decreased from 135 CO₂ mg/(kg·h) to less than 60 CO₂ mg/(kg·h) within 12 days from the beginning of the storage, then almost kept stable in the middle and late stages of the storage. The respiratory intensity under T4 and T5 was between 30-40 CO₂ mg/(kg·h) after 15 days of storage. The fat content of walnuts under T3 within 30 days of storage was 34.2 g/100 g; the fat content under T1, T2, T4 and T5 decreased continuously at the early stage of storage, and the fat content under T4 and T5 was 45.8 g/100 g and 46.1 g/100 g respectively on the 60th day of the storage. When fresh walnuts were stored for 60 days, the protein content under T4 and T5 was the highest, reaching 17.6 g/100 g and 17.1 g/100 g, respectively. During the storage period, the acid value of walnuts under T6 increased from 0.24 mg KOH/g to 4.3 mg KOH/g, and that under T4 and T5 increased from 0.24 mg KOH/g to less than 2.7 mg KOH/g. The SOD activity of walnuts under T6 reached 140.1 U/g and that under T1 and T2 reached 135.2 U/g on the 30th day of the storage. The walnut SOD activity under T3, T4 and T5 reached 139.9 U/g, 149.7 U/g and 150.0 U/g, respectively, on the 45th day of the storage. The POD activity of walnuts under T6 and T3 reached the lowest value on the 30th day of the storage, which was 10.5 U/g and 10.7 U/g, respectively, and the POD activity under the other 4 treatments dropped to the lowest value on the 45th day, which was 10.8 U/g, 11.0 U/g, 10.1 U/g, and 10.3 U/g, respectively. During the whole storage period, the MDA content of fresh walnuts continued to increase, and on the 60th day of the storage, the MDA content of T4 was 6.25 mmol/g and that of T5 was 6.08 mmol/g, both lower than that of other treatments, but the difference was not significant. Among the six tested packaging materials, PET/PE-160μm package has the best fresh-keeping effect and can be popularized.

Key words: fresh walnuts, fresh-keeping, modified atmosphere (MA) packages, cold storage, quality