700*400*50 will fit entirely into memory, so read all of the values at one time and then do whatever you need to do for writing.
[type,sheetname] = xlsfinfo('test.xlsx');
m = size(sheetname,2);
alldata = cell(1, m);
for i=1:1:m;
Sheet = char(sheetname(1,i)) ;
num{i} = xlsread(filename,Sheet,xlRange);
end
Now you can process the cell array num .
num_mat = cat(3, num{i});
Now num_mat will be a 700 x 400 x m numeric array (provided the same amount was written on each sheet). You can loop,
for row = 1 : 700
for col = 1 : 400
thisdata = reshape( num_mat(row, col, :), [], 1);
... and now it is a vector containing data for the 50 sheets, that you could write out
end
end
