Adhesion of food powders during coating and the effects of alkalization and roasting conditions on cocoa volatile compounds.

摘要

The effects of powder resistivity, coating voltage, relative humidity and coating density on adhesion were investigated in both nonelectrostatic and electrostatic coating. Cocoa powder with a high resistivity (1.15 x 1013 Om) showed a stronger electrostatic adhesion than starch powder with a medium resistivity (2.56 x1010 Om) and NaCl powder with a low resistivity (7.31x105 Om). The adhesion of starch and cocoa powders coated at 0, 40 and 95 kV increased with increasing voltage. The adhesion at 0 kV was in the range of the theoretically calculated van der Waals force, and the measured adhesion forces at 40 kV and 95 kV were in the range of the theoretically calculated electrostatic image force at 40 kV and 95 kV respectively. For nonelectrostatic coating (0 kV), there was no significant change in adhesion when relative humidity (RH) increased from 30% to 60%, while adhesion increased when RH increased from 60% to 80%. For electrostatic coating, the adhesion decreased when RH increased from 30% to 60%, but the adhesion at 80% RH, close to the theoretically calculated capillary force value, was larger than the adhesion at 30% and 60% RH. For both nonelectrostatic and electrostatic coating, the adhesion force decreased as coating density increased to 1.0 mg/cm2, but there was no significant change from 1.0 mg/cm2 to 2.0 mg/cm2.;Cocoa beans were alkalized before or after roasting and made into cocoa liquor before analyzing by selected ion flow tube-mass spectrometry (SIFT-MS). In both alkalized-before-roasting and alkalized-after-roasting samples, there were significantly higher concentrations of alkylpyrazines for the samples with pH above 7.0 than pH below 7.0. At pH 8, the concentrations of 2,3-, 2,5- and 2,6-dimethylpyrazine (DMP), 2,3,5-trimethylpyrazine (TrMP), 2,3,5,6-tetramethylpyrazine (TMP) and 2,3-diethyl-5-methylpyrazine (EMP) in the samples alkalized-before-roasting were higher than those in the samples alkalized-after-roasting. Volatiles increased under conditions that promoted the Maillard reaction. The partition coefficient was not significantly affected by pH from 5.2 to 8.0. The ratios of TrMP/DMP and DMP/TMP increased while the ratio of TMP/TrMP decreased as the pH increased. The concentrations of Strecker aldehydes and other volatiles followed a similar pattern as that of the alkylpyrazines. High pH favors the production of alkylpyrazines and Strecker aldehydes.;Alkalized and unalkalized Don Homero cocoa beans were roasted at 120, 150 and 170 °C in a rotary roaster. The real-time and end-of-roasting concentrations of cocoa volatiles in the headspace of the roaster were analyzed by SIFT-MS. The concentrations of total alcohol, acids, aldehydes, esters, ketones and alkylpyrazines reached peak concentrations within the first 15 min roasting. The concentrations of alkylpyrazines and Strecker aldehydes increased as the roasting temperature increased from 120 to 170 °C. For most of the volatile compounds compared, there was no significant difference between Arriba and Don Homero beans, but Arriba beans showed higher concentrations of 2-heptanone, acetone, ethyl acetate, methylbutanal, phenylacetaldehyde and trimethylpyrazine than Don Homero beans. For unalkalized Don Homero beans (pH 5.7), the time to peak concentration decreased from 13.5 to 7.4 min for pyrazines, and from 12.7 to 7.4 min for aldehydes as the roasting temperature increased from 120 to 170 °C. Also, at 150 °C roasting, the time to peak concentration was shortened from 9.0 to 5.1 min for pyrazines, and from 9.1 to 5.0 min for aldehydes as the pH increased from 5.7 to 8.7.