Sentence:  More or lesser amounts of <WSD>middlings</WSD> are always formed, and must be reprocessed to salvage the good seed it contains. The <WSD>middlings</WSD> fraction can be re-separated over the gravity separator to salvage the good seed. ... To salvage its good seed, <WSD>middlings</WSD> are re-separated over the gravity. This spreads the <WSD>middlings</WSD> out further over the deck, and produces a closer separation of good seed from the undesirable material. 
Meaning of the word marked with <WSD> tags:  Low-grade or coarse flour; coarse wheat mixed with bran.