Meteorological conditions and rating method of quality formation of 'Cabernet Sauvignon' grape in eastern foothills of Helan Mountain

The quality of grape wine is largely determined by the quality of grapes, which is closely related to meteorological conditions during grape growing season. Under normal management conditions and agricultural technology, the method of evaluation of meteorological conditions for fruit growth is called quality meteorological evaluation of wine grape. Meteorological condition rating techniques and meteorological indexes research on wine grape quality formation provide the base lines of climatic conditions for rating commercial wine, vintage identification and hoarding. In this paper, five indexes were tested for total sugar, total acid, sugar-acid ratio, pH and tannin content of 101 'Cabernet Sauvignon' fruit samples collected at various sites in eastern Helan Mountain during 2003-2011. Test data for 51 samples were selected to analyze for correlations with meteorological data for fruit growth. Based on the analysis of biological effects of meteorological factors on quality, meteorological factors affecting each quality index were determined for significant level. Based on the determination coefficient of each meteorological factor to a quality index, five weight models of quality indexes with significant meteorological factors were constructed. Thus the quality indexes of 'Cabernet Sauvignon' grape were reckoned with the meteorological conditions for fruit growth period. Using determination coefficients of meteorological factors as weight, an integrated meteorological simulation model with 5 quality indicators was constructed and a comprehensive quality meteorological evaluation model was established by using the weight of individual quality indicators. Using related research results coupled with the winemaker suggestion, a comprehensive meteorological scoring model for wine grape quality was constructed based on the contribution of each quality index and wine quality. In relation to previous studies and the criteria of quality of grape and wine, 5 quality indicators and quality grades were classified. The threshold meteorological factors corresponding to the wine quality and classification criteria for grape climatic conditions of 'Cabernet Sauvignon' were also determined. It implied that based on the requirements of total sugar, total acid, sugar-acid ratio, pH and tannin content needed for brewing high quality wine, the 5 quality indicators and comprehensive meteorological grades of 'Cabernet Sauvignon' were divided into five grades. This included excellent (grade 5), very good (grade 4), good (grade 3), medium (grade 2) and poor (grade 1). Then based on the threshold of each quality grade and the comprehensive meteorological simulation equation of the above 5 quality indicators, the threshold of the meteorological factors affecting each quality grade was deduced. This was in turn used as the meteorological factor classification index for evaluating the climate condition for quality of 'Cabernet Sauvignon' grape. The effects of simulation and grape classification of the 5 quality indicators and 'Cabernet Sauvignon' grape in the foothills of Helan Mountain were tested using the 51 samples selected to establish the model. The results showed that all the models of the 5 quality indicators passed the 0.001 R-test and F-test, indicating that the estimated values were within the measured values and that the variation trends were consistent. Among these, the simulation effects of total sugar content, total acid content, sugar-acid ratio and tannin content were better (R ≥ 0.59 and small RMSE) than the effects of pH (where there was a relative large simulation dispersion). To predict the effects of the evaluation model and indexes by using the other 50 samples that were not included in the model development and meteorological data during fruit development, the effects of the grading were tested for 'Cabernet Sauvignon' grape quality in the eastern foothills of Helan Mountain. The results indicated that the contents of total sugar and total acid were close to the measured values. In addition to a relative large error for few samples of sugar-acid ratio and tannin content, the error for the other samples was relatively small and the overall trend was consistent with the measured values. pH was relatively stable and all the samples were within the appropriate range for brewing high-quality wine, and differences in yearly meteorological conditions were difficult to determine, indicating that the local climatic conditions were conducive for the pH needed for high quality wine brewing. From the comprehensive score and grade, 28 samples were the same as the actual level and only 4 samples differed at 2 levels. The rest of the samples were within level 1, which more accurately reflected the quality of raw materials. The meteorological evaluation indexes and models of 'Cabernet Sauvignon' provided a feasible tool for evaluating the quality of wine grape in the eastern foothills of Helan Mountain.