Effects of high temperature and short time heat shock treatment on the callus formation of sweet potato tuber

Abstract: Sweet potato is a kind of vegetable and grain crop which is grown world-wide, and it is also the second largest potato crop in China. During the harvest and storage process, sweet potato is damaged by machinery and susceptible to pathogenic microorganisms, resulting in great economic losses caused by the deterioration of sweet potato. Therefore, callus pretreatment of potato crops before storage has become the main methods to reduce the storage loss of potato crops. To improve the existing problem of traditional callus treatment (35 ℃ heated in storage condition for more than 2 d), such as high energy consumption, low efficiency, instability, in sweet potato callus formation technology, the purpose of this research was to study the effects of different heat shock treatments on the formation of sweet potato root callus healing, the sweet potatoes were treated with High Temperature Short Time Callus (HTSTC) at 55-70 ℃ lasting 10-20 min with hot air by self-designed high-temperature-short-time healing equipment. After the callus healing was completed at (13±1)℃ for 7 d, the appearance color, the thickness of the phelloderm layer, the content of lignin and the mass loss rate of sweet potato were measured, and the suberin deposition was observed under a microscope. The results showed that after callus heat shock treatment at 65 ℃ for 15 min, the lignin accumulation and suberin deposition were the best among HTSTC groups. The HTSTC treatment under 65 ℃ for 15 min groups showed a stronger blue fluorescence intensity and lager blue fluorescence region than other HTSTC groups, which was similar to the fluorescence intensity of traditional callus treatment. On the other hand, the lignin content and cork layer thickness of 65 ℃ for 15 min HTSTC groups were 12.27% and 19.41% higher than those sweet potatoes under traditional 35 ℃ healing for 2 d callus treatment. The traditional callus method would lead to a serious mass loss problem by dehydration and respiration acceleration. Compared with traditional callus method, the mass loss of 65 ℃ for 15 min HTSTC treatment group was only (3.78 ± 0.34)%, which could effectively reduce the mass loss of damaged sweet potatoes. In addition, the optimal HTSTC treated sweet potato could increase the L* and reduce partial browning better compared with traditional callus treatment on sweet potatoes. It was found that the callus color of the 65 ℃ heat for 15 min was gray green without obvious browning, which was an acceptable appearance for consumers. In order to verify the experimental results, the HTSTC treatment temperature and time were optimized and calculated by response surface design method. Through response surface simulation optimization, the optimal heat shock treatment condition is at 67.01 ℃ for 12.69 min, which was very close to the actual experimental parameter of 65 ℃ for 15 min. The R2 of cork layer thickness, mass loss rate, and lignin content in response surface were 0.999, 0.998, and 0.997 respectively. In conclusion, the experimental results are very close to the response surface optimization results. The 65 ℃ heat for 15 min HTSTC treatment could not only achieve better result of calls formation than traditional callus treatment, but also ensure the lowest mass loss rate and maintain the best appearance quality, which could become a new scientific method for rapid, efficient, energy saving, and quality assurance callus treatment of postharvest sweet potato and other root crops.