问题 问答题 简答题

豆腐乳生产中怎样防止菌种退化?

答案

参考答案:

优良的菌种是获得优质发酵产品的前提和保证,在实际生产中获得一株优良的菌种,有时是非常不容易的,即使选到,如果保藏不适当,还会出现退化,如生产中菌种在试管斜面经多次传代,经常表现生长变慢,生长不齐、不产孢子,甚至“光秃”;或出现生产性能下降、抗逆性差等现象,这些都是菌种退化的表现。其原因可能是菌种自发突变;多核菌丝的霉菌不纯,传代后导致核分离;培养条件的变化;杂菌的污染等。

防止菌种退化的措施主要是:①尽量减少传代次数,减少自发突变的概率;②充分利用孢子接种,因霉菌菌丝都是有多个细胞核,而孢子是休眠体,一般是单核的,抗逆性强,不易退化;③采用合适的菌种保藏方法,如采用石蜡油封藏、沙土管封藏,最好是冷冻干燥保藏,斜面冷藏只适合菌种的短期保藏;④选择合适的培养条件,菌种培养和保藏中,应避免条件的经常变换(营养、温度等);⑤生产中用菌应定期分离纯化和复壮。

阅读理解

It's not the flashiest car in the world. Not even close. But the 1971 Volkswagen named Helioscan do something most cars can't: nm on solar energy – energy from the sun's light and heat!

Joshua Bechtold, 14, and the other students at the Riverside School in Lyndonville, Vermont, worked many months to get Helios ready for the 1999  American Tour de Sol ("Sol" is the Latin word for "sun"). They named their car after Helios, the sun god in Greek mythology(神话).

The 4-year-old Tour de Sol encourages the use of "green", or environmentally friendly, cars to help reduce pollution and save energy. It’s not a race. Cars are  judged on fuel efficiency(耗油量) rather than speed. In the week-long event, 44 cars took the 350-mile tour from Waterbury, Connecticut, to Lake George, New York. Of the 23 student cars, Helios was the only one built by middle school students.

A teacher drove Helios, but the children talked with people wherever they stopped along the mad. "That was my favorite part," says Anna Browne, 15. "We explained how the car runs.”

Due in part to old, inefficient batteries(电池), Helios finished fourth - out of four - in its kind, the sun-powered class. "We were there for the fun of it," Anna says. "We're proud of Helios," says Ariel Gleicher, 14. "It's a car that's good for the environment."

64. What is special about the car Helios in the text?       

A. It was built by middle school students.

B. It has an attractive design.

C. It was made in 1971.

D. It won the fourth prize.

65. How many sun-powered cars took part in the race?      

A. 1.       B. 4.     C. 23.    D. 44.

66. What would be the best title for the text.'?       

A. The Making of Helios

B. 1999 American Tour de Sol

C. Sun-powered Cars on the Road

D. Use of Green Cars in Connecticut

67. The students felt proud of Helios because______.

A. it could run as far as 350 miles     B. it was favored by many children

C. it had high-quality batteries         D. it was driven by clean energy

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