Exo-polygalacturonase (exo-PG) hydrolyzes pectin acids and liberates mono-galacturonate, which plays an important role in juice extraction, and has rarely been reported. Exo-PG (AfumExoPG28A) from Aspergillus fumigatus belongs to the glycoside hydrolase 28 family. In this study, its gene was cloned and the protein was expressed and secreted in Pichia pastoris with a maximal activity of 4.44 U/ml. The optimal temperature and pH of AfumExoPG28A were 55°C and 4.0, respectively. The enzyme exhibited activity over almost the entire acidic pH range (>20.0% activity at pH 2.5-6.5) and remained stable at pH 2.5-10.0 for 24 h. The Km and Vmax values of AfumExoPG28A were calculated by the substrate of polygalacturonic acid as 25.4 mg/ml and 23.6 U/mg, respectively. Addition of AfumExoPG28A (0.8 U/mg) increased the light transmittance and juice yield of plantain pulp by 11.7% and 9%, respectively. Combining AfumExoPG28A (0.8 U/mg) with an endo-PG (0.8 U/mg) from our laboratory, the enzymes increased the light transmittance and juice yield of plantain pulp by 45.7% and 10%, respectively. Thus, the enzyme's potential value in juice production was revealed by the remarkable acidic properties and catalytic activity in fruit pulp.

A filamentous fungus producing polygalacturonase (29.2 U/mL) was isolated from orchard soil in northern Guangxi, and identified as Trichoderma koningiopsis. Using polygalacturonic acid as the substrate, the optimum reaction temperature of the enzyme was 50 ℃, and it maintained more than 70% of the maximum activity at 50 ℃ for 1 h, which was identified as a medium-high temperature tolerant enzyme. The enzyme had unique weakly acidic catalytic characteristics. The optimum reaction pH was 5.0, and it retained 73% of its maximum activity at pH 6.0, and was tolerant to pH 2.5–8.0. Zn2+ partially activated the enzyme activity, while Mn2+ inhibited the enzyme. It could depectinize five weakly acid fruit pulps (papaya, banana, banana, and white- and red-fleshed pitaya) but to different extents. The yield of juice of banana pulp increased by 24.4%, the viscosity of papaya pulp decreased by 82.5%, and the transmittance of red-fleshed pitaya pulp increased by 31.9%, indicating good depectinization efficiency for the three fruits. The polygalacturonase-producing strain has good prospects for its application in juice production from tropical and subtropical perishable fruits.

An endo-polygalacturonase (endo-PGase) exhibiting excellent performance during acidic fruit juice production would be highly attractive to the fruit juice industry. However, candidate endo-PGases for this purpose have rarely been reported. In this study, we expressed a gene from Penicillium oxalicum in Pichia pastoris. The recombinant enzyme PoxaEnPG28C had an optimal enzyme activity at pH 4.5 and 45°C and was stable at pH 3.0-6.5 and < 45°C. The enzyme had a specific activity of 4,377.65 ± 55.37 U/mg towards polygalacturonic acid, and the Km and Vmax values of PoxaEnPG28C were calculated as 1.64 g/l and 6127.45 U/mg, respectively. PoxaEnPG28C increased the light transmittance of orange, lemon, strawberry and hawthorn juice by 13.9 ± 0.3%, 29.4 ± 3.8%, 95.7 ± 10.2% and 79.8 ± 1.7%, respectively; it reduced the viscosity of the same juices by 25.7 ± 1.6%, 52.0 ± 4.5%, 48.2 ± 0.7% and 80.5 ± 2.3%, respectively, and it increased the yield of the juices by 24.5 ± 0.7%, 12.7 ± 2.2%, 48.5 ± 4.2% and 104.5 ± 6.4%, respectively. Thus, PoxaEnPG28C could be considered an excellent candidate enzyme for acidic fruit juice production. Remarkably, fruit juice production using hawthorn as an material was reported for the first time.

从桂北果园土壤中筛选到一株产聚半乳糖醛酸酶(酶活力为29.2U·mL -1 )的丝状真菌，鉴定该菌为拟康宁木霉（Trichoderma koningiopsis）。以聚半乳糖醛酸为底物，测得该酶最适反应温度为50 ℃，且在50 ℃条件下保温1 h，仍能保持70%以上最大酶活力，属于中高温酶。该酶具有独特的弱酸性催化特征，最适反应pH 5.0，且在pH 6.0仍具有73%最大酶活力，并在pH 2.5～8.0条件下具有一定耐受性。Zn 2+ 对酶活性具有部分激活作用，Mn 2+ 则对该酶有一定抑制作用。针对5种弱酸性水果果浆（木瓜、香蕉、芭蕉及火龙果（红心和白心））脱胶实验结果显示，该酶对五种水果的脱胶效果都有不同程度的提高，其中芭蕉果浆出汁率提高了24.4%，木瓜果浆粘度下降了82.5%，红心火龙果果浆透光率提高了31.9%，三种水果脱胶效果显著。本研究挖掘出一种新型产聚半乳糖醛酸酶菌株，对于热带、亚热带易腐烂水果的果汁生产具有较好的应用前景。