Xiaojun SU ,Liu CAIKaizhong TIANYong SONG,Wenjia LIXingyao XIONG

1.Hunan Provincial Key Laboratory of Crop Germplasm Innovation and Utilization,Hunan Agricultural University,Changsha 410128,China；

2.Horticulture & Landscape College,Hunan Agricultural University,Changsha 410128,China;

3.College of Food Science and Technology,Hunan Agricultural University,Changsha 410128,China;

4.Institute of Vegetables and Flowers,Chinese Academy of Sciences,Beijing 100081,China;

5.Hunan Collaborative Innovation for Utilization of Botanical Functional Ingredients,Hunan Agricultural University,Changsha 410128,China

Responsible editor:Tingting XU Responsible proofreader:Xiaoyan WU

Cassava (Manihot esculenta Crantz) belongs to Euphorbiaceae and Manihot.It has thick fibrous roots,which are born in the seed stalk.In tropics and subtropics,cassava is a kind of perennial shrub plant.But in temperate regions,cassava is a kind of annual shrub plant[1-2].Cassava,also known as manioc,is one of the world’s three major potato crops (potato,sweet potato),and is also one of the 7 crops with annual output of more than 1.0×109t[3].Cassava (Manihot esculenta Crantz)was introduced to China in 1820s.It is firstly cultivated in Gaozhou County of Guangdong Province,and then introduced to Hainan Island.Cassava has now been widely distributed in southern China.Guangdong,Guangxi and Hainan have the most cultivation,followed by Fujian and Yunnan.In southern Jiangxi,Sichuan and Guizhou,cassava is also cultivated[4-5].

Cassava has now been considered worldwide to be a kind of economic crop and a kind of important industrial material with a relatively high comprehensive utilization value[6].The fuel ethanol,produced from cassava,is known as eco-friendly green gasoline,alternative to traditional gasoline,and it is also the most economical and practicable biomass energy resource[7].With economic growth and increasing energy shortage,the utilization value of cassava,as a non-food crop,is increasingly highlighted[8-9].The processing and development of cassava provides a tremendous development space for the industrialization of cassava.In addition,the comprehensive utilization of cassava will further increase the added value of cassava[10].The cassava industry in China has a broad prospect and must be developed with the increased national investment,promotion of improved varieties and high-yielding cultivation techniques and improvement of processing and comprehensive utilization levels.

Different cassava cultivar has different quality.They also have different ethanol-converting ability.In this study,11 cassava cultivars were selected.The difference in ethanol-converting ability among the 11 cassava cultivars was explored through determining their starch contents,aiming at providing a scientifically theoretical basis for the reasonable selection of uncooked materials.

Material and Methods

Test material

The cassava cultivars were provided by the cassava Experimental Station of Hunan Agricultural University.The glucoamylase,α-amylase and cellulase were purchased from the Genencor (Wuxi) Bio-Products Co.,Ltd.,and their activities were 8 000,2 000 and 2 000 U/g,respectively.The high temperature-tolerant and highactivity dry yeasts were purchased from the Angel Yeast Co.,Ltd.

Test methods

Yeast activation A certain amount of active dry yeast was weighed.Then 5-10 times of water and 2%of glucose were added.The yeasts were incubated in a water bath at 40 ℃for 30 min for use.

FermentationThe cassava was cut into pieces,dried,crushed and then passed through a 0.38 mm sieve.A certain amount (70 g) of cassava powder was weighed and mixed with 125 ml of water in a fermentation flask(500 ml).Then 200 U/g of glucoamylase,12 U/g of amylase,25 U/g cellulase,0.5 g of ammonium sulfate and 3.5 ×107cell/ml of activated yeasts were added to the flask.The solution was fully mixed.The initial pH of the fermentation solution was adjusted to 3.5.The fermentation was completed in a thermostatic shaker at 32 ℃.The fermentation solution was distilled 120 h later for determination.

The cassava was cut into pieces,dried,crushed and then passed through a 0.38 mm sieve.A certain amount(70 g)of cassava powder was weighed and mixed with 125 ml of water in a fermentation flask (500 ml).Then 12 U/g of amylase and 0.5 g of ammonium sulfate were added to the flask.The flask was then bathed in hot water(80-95°C)for 30-40 min.Whenthe temperature of the fermentation solution declined to 60 ℃,200 U/g of glucoamylase and 25 U/g of cellulase were added.When the temperature declined to 30 ℃,3.5×107cell/ml of activated yeast was added.The solution was fully mixed.The initial pH of the fermentation solution was adjusted to 3.5.The fermentation was completed in a thermostatic shaker at 32 ℃.The fermentation solution was distilled 120 h later for determination.

Table1 Starch contents in different Cassava cultivars of Manihot esculenta Crantz

Determination of ethanol yieldA certain amount (100 ml) of fermentation solution was mixed with an equal amount of water.The mixture was then distilled to collect 100 ml of distillate.The ethanol concentration and temperature of the distillate were determined with gravity method.The ethanol yield (%,v/v) was obtained by referring to the table.

Results and Analysis

Names of starch contents of cassava

The 11 cassava cultivars included H680,H360,H873,H629,H609,H800,SC205,SC5,Bread Manioc and Guijingyin983.As shown in Table1,the starch contents in the 11 cassava cultivars were all higher than 70%.The starch contents in SC9 and Break Manioc were as high as 82.33% and 82.06%,respectively.

Ethanol yield of fermenting uncooked cassava

Fig.1 showed the fermentation effect differed significantly among different cassava cultivars,resulting in the significant difference in ethanol concentration.H680 had the highest ethanol yield (9.7%),while H873 and H629 had the lowest ethanol yield(8.2%).The ethanol concentrations of all the cassava cultivars were all higher than 8%.Cai et al.[11]optimized the fermentation processes of uncooked cassava.Their ethanol concentration reached 15.7%,increased by almost 6% compared to that in our study.It was indicated the noncooked cassava was suitable for producing fuel ethanol with uncooked material fermentation process.Fig.2 showed the relationship between starch content and ethanol concentration.With the increase of starch content,the ethanol yield was not always increased.Bread Manioc had the highest starch content(82.06%),but its ethanol yield was only 8.73%,indicating a non-linear correlation between starch content and ethanol yield.All things considered,H680 was more suitable for producing fuel ethanol with uncooked material fermentation process.Its starch content was 74.8%,but its ethanol yield was 9.7%.

Ethanol yield of fermenting cooked cassava

As shown in Fig.3,the ethanol yield of cooked cassava was higher than that of uncooked material in overall.In addition,there was no significant difference in ethanol yield among different cooked cassava materials.H609 had the highest ethanol yield(13.8%),while H873 had the lowest ethanol yield (11.4%).The ethanol concentrations of all the other cassava cultivars were all higher than 12%.Fig.4 showed the relationship between starch content and ethanol concentration.As shown in Fig.4,with the increase of starch content,the ethanol yield was not always increased.H609 produced the most ethanol,but its starch content was only 70.27%.There was no proportionality between starch content and ethanol yield.Among the cooked cassava materials,H609 was more suitable for producing fuel ethanol.

Conclusion

In this study,the 11 cassava cultivars were selected,and their starch contents were determined.SC9 had the highest starch content(82.33%).A comparative test was conducted to investigate the difference in ethanol yield between uncooked and cooked cassava materials.The specific results were concluded as follows:

(1) There was a significant difference in ethanol-converting ability among different cassava cultivars.Moreover,the ethanol yield of cooked cassava material was higher than that of uncooked material.Among the uncooked cassava materials,H680 hadthe highest ethanol yield (9.7%).Among the cooked cassava materials,H609 produced the most ethanol(13.8%).

(2) Although the cooked cassava material had a higher ethanol-producing ability,the fermentation of cooked cassava material would consume enormous heat.Therefore,the uncooked cassava material had a greater development potential in fuel ethanol production.

(3) There was no proportionality between starch content and ethanol yield.So the assessment of ethanol producing ability of cassava should not only consider the starch content.Combining the ethanol yield and starch content,H680 could be used as the material for producing fuel ethanol with uncooked material fermentation process,and H609 could be used as the material for producing fuel ethanol with uncooked material fermentation process.Of course,the selection of cassava cultivar for producing fuel ethanol would be ultimately determined by the unit-area total effective starch content of the cultivar.

(4)In this paper,the ethanol-converting ability of the uncooked and cooked cassava materials was primarily discussed,aiming at providing a data reference for the screening of cultivars for producing fuel ethanol.

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