Question: Why did they switch from pork sausage to beef in hamburgers?
About the article "Hamburger", section "Frank and Charles Menches".

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Product ranges from Beef, Buffalo, Pork, Mutton, Rabbit and Poultry.
The processed and semi-cooked products include corn beef, meat loaf, sausages, curries, bacon, ham, cutlet-mix, chicken-n-ham and salami.
Sausages, one of the most popular of the MPI products range come in varieties such as cocktail sausage, pork sausage, chicken pepperoni sausage, masala sausage and the world famous kransky.
They also export a rissole (labelled as continental sausage) to Australia.
Currently, the company producing more than 65 varieties of items.  
Falukorv is a large traditional Swedish sausage made of a grated mixture of pork and beef or veal with potato flour and mild spices.
The sausage got its name from Falun, the city from where it originates, after being introduced by German immigrants who came to work in the region's mines.
Isterband is made of pork, barley groats and potato and is lightly smoked. 
 "Kolbász" is cooked Hungarian sausage, usually smoked.
The best known and most popular versions are: "Gyulai" sausage is named after the Hungarian town of Gyula and also has PGI protection.
It is slow cooked while being beech wood smoked.
It is made from pork, 'szalonna' (Hungarian bacon fat), garlic, pepper, caraway, and a Hungarian red paprika.
At the World Exhibition of Food in Brussels 1935, the Gyulai kolbász was awarded a gold diploma.
The sausage may be cut into thin slices and eaten alone or with bread.
They are also added to many Hungarian dishes including lecsó and potato/egg casserole (rakott krumpli).
"Csabai" sausage is made in the town Békéscsaba, and has Protected Geographical Status (PGI) protection.
It is similar to Gyulai, but somewhat spicier.
There are several variations in size and type, but it is a spicy sausage with a lot of paprika.
"Csemege kolbász" is an mildly spiced cooked smoked sausage "Cserkész kolbász" is a cooked smoked sausage made from beef and pork.
"Debreceni kolbász" is usually unsmoked or more mildly smoked, with a strong paprika flavour and used for cooking.
"Lecsókolbász", a spicy cooked smoked sausage made specifically for serving as part of the dish lecsó, a vegetable stew with peppers and tomatoes.  
Lao sausage, also known as Laotian sausage / Sai Oua / Sai Ua (Lao: ໄສ້ອັ່ວ) or Sai Gork / Sai Kork (Lao: ໄສ້ກອກ), usually refers to a popular type of Lao sausage made from coarsely chopped fatty pork seasoned with lemongrass, galangal, kaffir lime leaves, shallots, cilantro, chillies, garlic, salt and fish sauce.
Another variant is known as "soured Lao sausage".
It uses cooked Lao sticky rice as an additional ingredient in the filling, which is then allowed to "sour" at room temperature for a couple of days.
Other types of Lao sausages are "Mam" (beef liver sausage) and "Sai Gork Leuat" (blood sausage). 
Bologna sausage, also known as baloney (), boloney or polony, is a sausage derived from and similar to the Italian mortadella, a finely ground pork sausage containing cubes of lard, originally from the Italian city of Bologna, ).
U.S. Government regulations require American bologna to be finely ground and without visible pieces of lard.
Bologna can alternatively be made out of chicken, turkey, beef, pork, venison or soy protein.
Polony is the name for a large sausage made from a mixture of beef and pork that is popular in South Africa and was very popular in Britain during the 17th and 18th centuries.
Polony is now eaten less often.
Polony can be stored for long periods, sometimes many weeks.
Polony is highly seasoned before being hot smoked.
It is then cooked in boiling water when required.