As the previous lignocellulosic feedstock, the reactions with apple pomace were carried out under the same reaction conditions (Figure ). It should be noted that the experiment series with apple pomace showed no trend of increase of the soluble fraction for the reaction conditions of the first four treatments in Figure . A possible reason could be non-structural inhibitors in the substrate which are blocking the acid effect and partial encapsulation in rigid structures derived from kernel fragments where the energy input is not sufficient in the same way as for wheat straw. The hypothesis of non-structural substances which hinder the reaction is supported by comparison with wheat straw where the content of extractives is low and the percentage of the soluble fraction is high. The experiment which increased the soluble fraction of apple pomace by 5 % compared to the condition without catalyst was the one with the harshest reaction conditions of 255 min milling duration and 6 % acid content.