Effect of drying room environment on color of green raisin in Xinjiang

Xinjiang is the largest cultivated and output region of grape in China. Green raisins are the main varieties of raisins in Xinjiang, which is also a specialty of Xinjiang. Green raisins are highly respected by customer due to its fresh, attractive green color and sweet flavor. The price of green raisin is 2-3 times as much as red brown raisins, obviously green raisins have higher economic value. The unique environment and natural conditions of Xinjiang are conducive to fresh grape cultivation and raisin production. Thompson Seedless is the main dry grape variety in Xinjiang, the planting area of Thompson Seedless is in the range from 40 000 to 50 000 hectares. The annual production of raisins in Xinjiang ranges from 150 000 to 180 000 t, the yield of raisins in Xinjiang reached to more than 95% and more than 8% of the country's and the word's total output, respectively. Turpan and Hami are the main producing areas of raisins in China and also the largest producing areas of green raisins in the world. The special environmental conditions of drying room in Turpan and Hami areas of Xinjiang are the key factor for green raisin production. In this article we made systematic detection and analysis to decipher the key index of green raisins, including the whole drying period the temperature, humidity and light environment index between drying room and control (sun-dried). The following conclusions have been drawn: The extremely high temperature and dry ecological conditions in Turpan, and the appropriate temperature /humidity which created by the specific structure of air drying room in this special ecological environment are the key factors for green raisin production. The sun drying indices such as maximum daily temperature (30.5-36.3 ℃), mean daily temperature (25.8-30.0 ℃) and differential value (DIF) (7.8-17.2 ℃) during the period of drying process can effectively reduced by the drying room, the 3 key indices range from variation of sun drying respectively are 40.8-68.0, 27.0-34.0 and 27.7-50.0 ℃. The humidity in the air drying room is not easily affected by the external humidity, the another key factor for green raisins production is humidity stabilized at 15.0%-35.0% .The low light and ultraviolet ray intensity in the drying room is one of the important factors for green raisins production. Compared with sun drying condition the light and UV intensity are respectively over 2 000 and 121.5 μmol/(m2•s), the light intensity and UV intensity in the air drying room are controlled below 3.3 and 1.46 μmol/(m2•s). The color of raisin is mainly controlled by the content of chlorophyll a, which made by air drying is significantly higher than in brown raisins which makes by sun drying, and there is no significant difference in chlorophyll b content between the two groups. According to the comprehensive analysis, low light intensity, stable temperature and humidity are the key factors for the formation of green raisins. The results of this study can provide a theoretical basis for drying room structure improvement and industrial product condition control.