Effect of Roasting Temperature and Time on Precursors, Volatile Components and Flavour Quality of Cocoa Beans During Nib Roasting

The effect of roasting temperature and time on precursors, volatile components and quality of cocoa beans during nib roasting was studied using response surface methodology (RSM) which consisted two independent variables of temperature (110-170˚C) and time (5-65 min). Steam Distillation Extraction (SDE) method was used to extract and gas chromatograph-mass spectrometry equipped with an ICIS data system was used to identity the volatile compounds. The pH linearly increased during roasting period at 120˚C. The colour was used for 1.20-1.80 Fermentation index value. The moisture content of the nib decreased (to 1.1%) as the roasting time and temperature were increased. Fifty three volatile compounds had been detected in roasted nib. Among them, pyrazines and esters were two major groups which constituted of 34 different compounds. Tetramethylpyrazine, trimethylpyrazine, phenethyl acetate, isoamyl acetate, 3-methylbutyl acetate, 2-methylbutyl acetate, phenylacetaldehyde, and benzaldehyde were present in all treatments. The pyrazines formation increased as roasting time and temperature were increased. When roasting time was increased from 5 to 35 and 65 min at 140°C, the number of pyrazines increased from 4 to 10 and 11, respectively. The ratio value of esters increased when the roasting time was increased from 15 to 65 min (at 110-120°C). However, the ratio value of carbonyls linearly decreased with an increasing roasting temperature at shorter time (5-25 min). The ratio value of phenols was enormously reduced at the highest roasting temperature (170°C) with the longest roasting time (65 min) while that of alcohols slightly decreased as roasting temperature and time were increased. With the increase in roasting temperature and time, the concentration of free amino acids, reducing and non-reducing sugars had decreased. The concentration of total free amino acids decreased by 23.0% and 73.3% in the range of 1287.34 - 76.43 mg/100g. Sugars concentration decreased from 24.87 to 12.98 and 11.30 mg/l00g respectively as roasting temperature was increased from 110 to 140 and 170°C. The nib roasted at 140°C had higher response percent of cocoa taste, moderate response of bitter taste and low response of sour; astringent and absent of smoky. However, the nib roasted at 170°C had the lowest percent of cocoa taste but higher in bitter and high roast taste.

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