# Bioconversion of Apple Pomace to _Meyerozyma guilliermondii_ and _Scheffersomyces stipitis_ Biomass
## 3 Results and Discussion
### Sugars, Organic Acids, and Alcohols in the AP Hydrolysates
In our studies, approximately 750 mL of liquid hydrolysate was obtained from 100 g of apple pomace (AP), indicating that approximately 133 g of AP was used to obtain 1 L of hydrolysate. Comparing this with the total sugars and galacturonic acid concentrations presented in Table 2, it appears that for the AE50 hydrolysate (39.197 g/L), approximately 29.47% of the AP was converted into the analysed sugars and galacturonic acid. In contrast, only about 0.6% of the raw material for the W sample was hydrolysed into the analysed components presented in Table 2.  
The citric acid content in the samples after aqueous pretreatment (Table 3) was 0.188 g/L, which is more than four times higher than in the sample after acid pretreatment--0.043 g/L. Following enzyme treatment, the citric acid content ranged from 0.085 g/L to 0.215 g/L, depending on the enzymatic hydrolysis variant. The highest content of citric acid in the enzymatic hydrolysates was observed in the WE30 sample (0.215 g/L), while the lowest was in the AE50 sample (0.085 g/L). Additionally, it was noted that in samples subjected to acid pretreatment and enzyme hydrolysis, the citric acid content was at least twice as high as in the initial sample before enzyme treatment. For instance, in sample A, the citric acid concentration was 0.043 g/L, compared to 0.085 g/L in the AE50 hydrolysate and 0.101 g/L in the AE30 sample. The amount of succinic acid in the samples after pretreatment varied; for aqueous pretreatment (W sample), it was highest at 0.127 g/L, and for acid pretreatment (A), highest at 0.091 g/L. After enzymatic hydrolysis, the succinic acid content of the samples ranged from 0.007 g/L to 0.09 g/L. The lowest content of this compound in the enzymatic hydrolysates was observed in the WE30 sample and the highest in the AE30 sample. Inthe case of succinic acid, it was noted that, with aqueous pretreatment, the presence of this acid is always lower in the subsequent enzymatic hydrolysate, regardless of the enzymatic hydrolysis temperature. After pretreatment of AP with water (W) and acid (A), 1.27 g/L and 1.20 g/L of lactic acid were obtained in the medium, respectively. Enzymatic hydrolysis slightly affected the presence of this acid in the samples, and its concentration ranged from 1.11 g/L (AE50 sample) to 1.24 g/L (AE30). The formic acid (FA) content in the pretreatment samples was, for the aqueous treatment, equal to 0.067 g/L, and for the acid treatment, it was 0.083 g/L. In the samples after enzymatic hydrolysis, the amount of this acid decreased and fluctuated between 0.042 g/L and 0.065 g/L. As formic acid is also a product of the decomposition of sugars during acid-thermal treatment, higher contents of this compound were observed for samples after such treatment. The dependence of FA concentrations between the hydrolysate samples was as follows: AE30 > AE50 > WE40 > WE50. The raw materials and products derived from apples are rich in organic acids; therefore, the results of Table 2 are supported by the other data [8; 75; 76; 77; 78]. In the aqueous pretreatment sample, an acetic acid content of 0.262 g/L was recorded, more than five times lower than that in the acid pretreatment sample of 1.115 g/L. In the samples after enzymatic hydrolysis, the citric acid content ranged from 0.858 g/L (WE30 sample) to 1.141 g/L for the AE50 hydrolysate. Higher values of more than 1 g/L were observed for samples after acid pretreatment: AE30--1.178 g/L and AE50--1.141 g/L. The literature states that the elevated temperature of the lignocellulosic matrix treatment favours some organic acid formation, such as acetic or formic acid; our experiments also observed this phenomenon. The negative effect of elevated organic acids concentration, usually greater than 1 g/L, on yeast metabolism was also widely reported [79; 80; 81; 82; 83; 84; 85; 86]. Nonsignificant differences (\(p>0.05\)) in glycerol contents (Table 4) were observed for the W and A samples of pretreated AP (0.668 \(\pm\) 0.0206 g/l and 0.679 \(\pm\) 0.0249 g/L, respectively).