## 2 Results
### Analysis of Volatile Flavour Compounds in Yogurt  
#### 2.4.1 Analysis of Volatile Flavour Compounds  
Fermented dairy products share a common sour or tart taste. However, they each have unique flavours that enable them to be distinguished from each other. The volatile flavour compounds detected in all samples at the end of the fermentation using SPME-GC-MS are shown in Table S1. A total of 63 volatile flavour compounds were identified using the scan method and the standard spectrum library of NIST in all samples (A1-A5) and the commercial starter culture. The volatile flavour compounds were grouped by their chemical family as carboxylic acids, aldehydes, ketones, alcohols, esters and aromatic hydrocarbons. Carboxylic acids are very important for conferring strong odour notes to dairy products [19]. Nineteen carboxylic acids were identified in all samples and the control. Acetic acid is the main product of fermentation, indicative of glucose consumption/breakdown by the starter cultures [20]. At the end of fermentation, acetic acid concentrations of 1.8, 0.57 and 0.17 \(\mathrm{\SIUnitSymbolMicro g}\)/L were achieved in the A1, A2 and control treatments (Table S1). Acetic acid is the most important acid in yogurt and imparts strong, pungent, vinegary and acidic odours [21]. Hexanoic acid is also produced; it increases the yogurt aroma and significantly contributes to the 'rancid, cheese-like' odour and flavour [22]. High levels of hexanoic acid were detected in all samples and the control; in particular, a final concentration of 12.10 \(\mathrm{\SIUnitSymbolMicro g}\)/L was detected in the A4 samples, which was higher than the control (1.69 \(\mathrm{\SIUnitSymbolMicro g}\)/L). Octanoic acid can confer a 'fruit, caramel' flavour to yogurt [23]. As with hexanoic acid, high concentrations of octanoic acid were detected in the A4 samples. Aldehyde compounds have a low taste threshold, and even trace concentrations play an important role in the flavour of dairy products [10]. Acetaldehyde is recognized as an important flavour component, endowing a green apple or nutty flavour to yogurt [24]. Seven aldehyde compounds were detected in the volatile fraction of all samples and the control. High levels of acetaldehyde were detected in the A1, A3 and A4 samples at concentrations of 5.94, 2.79 and 22.33 \(\mathrm{\SIUnitSymbolMicro g}\)/L, respectively. The branched-chain aldehyde, 3-methylbutanal, is produced from isoleucine and leucine catabolism [25] and was detected in the A1, A4 and A5 samples at concentrations of 0.96, 0.96 and 1.02 \(\mathrm{\SIUnitSymbolMicro g}\)/L, respectively. Benzaldehyde may be produced from phenylacetaldehyde or cinnamic acid through \(\alpha\)-oxidation or \(\beta\)-oxidation [26] and was detected in all samples and the control; it is worth mentioning that the highest concentration of 2.40 \(\mathrm{\SIUnitSymbolMicro g}\)/L was found in the A1 samples. In total, eleven ketone compounds were identified in all samples and the control. 2,3-butanedione and aceta are volatile products of citrate metabolism, endowing buttery and creamy flavours to yogurt [27]; 2,3-butanedione was detected in samples A2 and A3, and at particularly high levels (4.72 \(\mathrm{\SIUnitSymbolMicro g}\)/L) in the A2 sample. Acettion was reduced from 2,3-butanedione and decarboxylated by \(\alpha\)-acetolate acid [28], and was detected in all samples at concentrations of 4.36, 16.59, 2.65, 21.51 and 7.86 \(\mathrm{\SIUnitSymbolMicro g}\)/L, respectively. With the exception of the A3 sample, the concentration of acetin in all samples was higher than in the control. 2-nonanone was also detected in all samples and the control. It is an important volatile compound that imparts a 'buttery, creamy, vanilla' flavour to yogurt [10]. Alcohol compounds are formed through lactose fermentation, amino acid metabolism, methyl ketones or aldehyde reduction [29]; seven alcohol compounds were identified. Amongst these, 1-nonanol is an important volatile flavour compound with a fatty odour that contributes to dairy product flavour [18].