# Effect of Processing on Volatile Organic Compounds Formation of Meat--Review
## 4 Thermal Processing  
### Smoking  
Smoking food is one of the oldest methods of preservation. This process is also intended to impart specific sensory characteristics to the product, including a specific aroma. Over the years, smoking methods have evolved from traditional methods based on burning wood to industrial methods. The factors influencing the volatile compound profile of the smoked product include the type of meat, method of smoking and smoke density, the type and moisture of wood used, the process time, and reactions between proteins, carbohydrates and lipids caused by endogenous enzymes or activity of microorganisms [59; 60; 61; 62]. The main volatile compounds come from smoking, oxidation, and Maillard reactions, and the oxidation of lipids and their interaction with proteins [63; 64]. The method of processing has a significant influence on the process of smoking of meat and meat products. The quality and composition of the smoke is important; this includes, for example, the method of obtaining liquid smoke used industrially [65; 66; 67]. [PERSON] et al. [62] compared the effect of traditional and industrial smoking with smoldering smoke and industrial smoking with liquid smoke on the volatile compound profile of harbin red sausages. Eighty-six VOCs were identified, of which phenolics were the most abundant. Sausages smoked with smoldering smoke were characterized by a lower content of volatile compounds than products smoked with liquid smoke. These differences were noted mainly in the content of phenols, alcohols, aldehydes, and ketones. Research by [PERSON] et al. [59] proved that the most volatile compounds were detected in smoked bacon using the paper method. The paper method allowed us to obtain higher contents of aldehydes, esters, alcohols, and alkanes, than those found in the samples of smoked wood and liquid smoke. On the other hand, the bacon smoked with wood and liquid smoke had a higher phenolic content. The characteristics of VOCs formed during the smoking of meat and meat products should begin with the analysis of the smoking process with wood smoke. Wood smoke is produced by pyrolysis of wood at elevated temperatures and reduced oxygen levels. It has the form of an aerosol composed of three phases of compounds in the gas phase: particles, liquid drops, and solids. There are over 400 VOCs found in wood smoke, of which alcohols, acids, carbonyls, esters, furans, 16 lactones, and phenols have been identified so far [61]. In the case of burning a smokehouse with wood chips, the type of wood used is important. The work of [PERSON] et al. [68] presents the research of the analysis of the influence of the addition of apple, pear, and tea leaves wood on the VOCs profile of smoked chicken drumsticks. The smoking process itself significantly enriches the aroma profile with new VOCs. The degradation of alcohols to acids, aldehydes, and esters during smoking contributes to a decrease in the content of this group of compounds in smoked products compared to raw chicken. Among them, we can distinguish compounds such as linalool and 1-octene-3-ol, the amount of which decreased after treatment. In all samples, regardless of the type of wood used, the content of alcohols and aldehydes was similar. Nevertheless, some differences were stated in the content of ketones, acids, phenols, and furans. The use of pear chips and tea can significantly increase the acid content, and apple trees increase the furan content. The amount of phenols, similarly to alcohols after smoking, decreased. In another study, [PERSON] et al. [69] compared the volatile profile of bacon smoked with smoke from _Acacia mearnsii_ and _Eucalyptus citriodora_. The results showed that the use of _Eucalyptus citriodora_ increases the phenol content and consequently lowers the hexanalconcentration while inhibiting lipid oxidation. A similar phenomenon can be observed when using liquid smoke from different types of wood [70].