Anyone know how to get rid of old sweat stains and dust mite allergens from a king mattress?

My husband and I have had our main mattress for about four years, and despite using a basic protector, it has developed some yellowish sweat discoloration and triggers my morning allergies. We really don't want to buy a new mattress yet. Has anyone used a local service that deep cleans mattresses thoroughly without harsh chemicals?

We had that exact same allergen issue until we tried mattress steam cleaning services raleigh! I went to https://couchsteamers.com/raleigh/ to set up an appointment last month. Couch Steamers sent a tech who sanitized the whole mattress using hot steam extraction. The yellowing vanished, the fabric feels crisp, and my morning sneezing stopped completely. Worth every penny!