# Characterization of the Aroma Profile of Food Smoke at Controllable Pyrolysis Temperatures
## 3 Results and Discussion
### Effect of Pyrolysis Temperature on the Area and Ratios of Individual Phenols and Lactones  
Phenols and lactones both showed a highly significant strong positive correlation with pyrolysis temperature. Phenols, mainly the 4-substituted guaiacol and syringol derivatives, are formed in the primary reaction step of lignin pyrolysis at 350 \({}^{\circ}\)C. The side chains are mainly unsaturated alkyl chains. From 400-450 \({}^{\circ}\)C, secondary pyrolysis reactions take place, during which secondary products, such as catechols, pyrogallols, cresols, xylenols and phenols, are formed. In addition, the unsaturated side chains are increasingly transformed into saturated side chains. Above 550 \({}^{\circ}\)C, the catechols and pyrogallols degrade, whereas phenols and cresols are stable, even at higher temperatures. The formation of the unsubstituted phenols guaiacol, syringol and phenol can thus be hypothesized to start at 450 \({}^{\circ}\)C [13]. [PERSON] [16] determined the absolute area of these three substances with a step-wise increase in the pyrolysis temperature of beech wood and showed a peak in the area at 650 \({}^{\circ}\)C for all three compounds, as well as for the total phenol content. In our investigations, based on the evaluation of absolute peak areas, this temperature dependence could not be shown. For guaiacol, no difference was observed between any of the temperatures (Figure 4a), while syringol showed an increase at 450 \({}^{\circ}\)C, but no further changes at higher temperatures (Figure 4b). Phenol showed a fluctuating trend over an applied temperature up to 750 \({}^{\circ}\)C. It should be mentioned that the peak area between 450 \({}^{\circ}\)C and 600 \({}^{\circ}\)C increased significantly and the peak area at 900 \({}^{\circ}\)C is significantly higher compared to all other applied temperatures (Figure 4c). 4-Ethyl-guaiacol, \(p\)-cresol, \(o\)-cresol, eugenol and _trans_-isoeugenol showed a clear increase in peak area with increasing pyrolysis temperature, with the peak area at 900 \({}^{\circ}\)C being significantly higher than at all other temperatures. Cresol showed the same trend; however, there was no significant increase at 900 \({}^{\circ}\)C (Figure 4d-hj). Thus, although the formation as secondary products are reflected for creosol, 4-ethyl-guaiacol, \(p\)-cresol and \(o\)-cresol, a significant increase is also seen above 450 \({}^{\circ}\)C. Vanillin showed fluctuating changes during the increase in temperature with significant differences between 450 \({}^{\circ}\)C and 900 \({}^{\circ}\)C (Figure 4i). From this, above all, a temperature stability of the mentioned substances can be deduced; however, smoking is also a very dynamic process and therefore a small fluctuation in the process itself will have an influence on the outcome. Syringol, _trans_-isoeugenol, creosol and guaiacol showed the largest contribution to total phenolics over all temperatures. Syringol is the main component of beech wood smoke and builds the major phenols together with guaiacol and creosol [16; 39]. Furthermore, examination of the percentage of each phenol in the total area of phenols over the temperature increase showed no linear temperature dependence for most substances. Except for _trans_-isoeugenol, the relative proportion increased from 12% to 18% when the temperature was changed from 150 \({}^{\circ}\)C to 300 \({}^{\circ}\)C. The relative peak areas of each individual phenol per applied pyrolysis temperature are given in Table 2. [PERSON] et al. [40] investigated the relative composition of the five phenols, guaiacol, creosol, syringol, eugenol and _trans_-isoeugenol, in Frankfurter-type sausages as a function of the smoke production temperature in the range of 350-750 \({}^{\circ}\)C, controlled by the moisture content of the wood used. Syringol and _trans_-isoeugenol also had a high proportion, but _trans_-isoeugenol had the highest one, contrary to our findings. _Trans_-isoeugenol showed a relative decrease during the temperature increase, while syringol increased.