When you are writing a letter to a prof
1- never be personal ( How are you, hope you are fine etc) UNLESS you know him for years and you are friends.
Stuff like that only happens in Iranian culture
2- go straight to the point! brief and concise! in one or maximum two lines
3- never apologize! What's this" I apologize for taking your time etc!" that is lame! remove it from your letter
4- Never claim stuff like "there was NOTHING ". First of all do your best and search google as much as you can
second of all, you can say "To my knowledge there was nothing ..."
5- Never force the person to respond to you soon by saying "Looking forward to hearing from you .. bla bla". What are you? you are not paying him to work for you, so be patient.
Now, my suggestion:
subject:Application of food biotechnology in prevent of obesity
Body:
Dear Dr.MM,
With regards to application of food biotechnology in prevention of obesity I was wondering what are the hot topics and trends [in developing countries / etc]
In fact, I am looking for some key terms so I can expand my research while searching at google scholar
Thank you,
Mohsen Movaviyan ( complete name, not abbreviation)
AND BELIEVE ME MOST LIKELY HE WOULD NOT EVEN BOTHER TO ANSWER, CUZ you can easily search google scholar and find tons of papers and read them and learn about more key terms
example:
http://scholar.google.ca/scholar?q=%22food+biotechnology%22+in+prevention+of+obesity&btnG=&hl=en&as_sdt=0%2C5
Over 600 articles not enough?
good luck and take my advice, trust me :)