Effect of pretreatment of lignocellulosic substrates on physiological and biochemical characteristics of some species of edible and medical mushrooms

Abstract

The development of new effective substrates for mushroom cultivation is relevant not only in order to obtain high-quality food products, but also as a need for rational use of natural resources. Chemical pretreatment can modify the chemical composition of various types of lignocellulosic materials and remove the growth-inhibiting compounds which can affect the physiological and biochemical processes in fungi. The present work provides the first comprehensive assessment of growth and biochemical characteristics of xylotrophic basidiomycetes cultivated on substrates that contain chemical pretreated lignocellulosic materials. Edible mushrooms Hericium erinaceus (Bull.) Persoon and Flammulina velutipes (Curtis) Singer and medicinal mushroom Ganoderma lucidum (Curtis) P. Karst. were selected for study. Pine (Pinus sylvestris L.) and beech (Fagus orientalis Lipsky) sawdust of particle size 0.24-0.315 mm were used. Pretreatment of lignocellulosic material with 2 % wt. solutions of hydrochloric acid, sulfuric acid, sodium hydroxide and hydrogen peroxide was carried out in autoclave at 120 С for 60 min. Sodium hydroxide pretreatment resulted in a statistically significant (p < 0,05) increase in the easily hydrolysable carbohydrates content in pine sawdust by 5 % and in beach sawdust by 4 % compared to control. Acid pretreatment instead led to an increase in hardly hydrolysable carbohydrates content in lignocellulosic materials by 20 %. Hydrogen peroxide pretreatment had no significant effect on composition of pine and beech sawdust. Chemically pretreated pine and beach sawdust were used as the basis for substrates for cultivation of xylotrophic basidiomycetes. Substrates, in addition to sawdust, contained 10 % wheat bran and 1 % CaCO3. Alkaline and acid pretreatments contributed to increased bioavailability of lignocellulosic materials for the vegetative growth of G. lucidum. The colony of G. lucidum after nine days of growth on untreated pine sawdust was 43±4 mm in diameter, on hydrochloric acid-, sulfuric acid-, sodium hydroxide- and hydrogen peroxide-pretreated pine sawdust were 58±7, 52±3, 63±4 and 49±7 mm in diameter, respectively. Acid pretreatments of sawdust improved the growth rate and density of F. velutipes and H. erinaceus vegetative mycelium. Hydrochloric acid pretreatment of pine sawdust led to an increase in the colony diameters of F. velutipes and H. erinaceus from 51±1 and 19±2 mm to 62±1 и 23±4 mm, respectively, of beach sawdust — from 48±4 and 19±2 mm to 50±4 и 32±3 mm, respectively. No growth of H. erinaceus occurred on substrate with alkali-pretreated pine sawdust. Fruit bodies of H. erinaceus were harvested from substrates containing acid-pretreated and untreated pine and beach sawdust. Acid pretreatments of pine and beach sawdust decreased the cultivation time twice. The second wave of fructification occurred during the cultivation of H. erinaceus only on substrates containing hydrochloric acid-pretreated sawdust. This provided an increase in yield of fruit bodies compared to control. The total yield of fruit bodies of the first and second waves cultivated on substrates with pretreated pine sawdust was 8.0 % dry wt., on substrate with untreated sawdust — 3.9 % dry wt. The yield of fruit bodies reduced significantly during cultivation of H. erinaceus on a substrate with beech sawdust after sulfuric acid pretreatment. For the first time, a significant difference in the protein and polysaccharide contents of fruit bodies of the first and second waves is shown. In the case of growing mushrooms on a substrate with hydrochloric acid-pretreated sawdust fruit bodies of the second wave had 1.3 times higher protein content and 1.7 times higher polysaccharide content than fruit bodies of the first wave. Thus, a comparative study of different pretreatments of lignocellulosic materials revealed a high efficiency of hydrochloric acid pretreatment step in preparing pine sawdust substrates for cultivation of H. erinaceus.