Pepsin & Digestibility: Where It Came From and Why It Matters

What pepsin is:

Pepsin is a stomach enzyme that starts breaking proteins into smaller peptides during gastric digestion. It was first isolated and named in 1836 by the German physiologist Theodor Schwann, making it the first enzyme purified from animal tissue.

From biology to a lab test. Because pepsin’s natural job is to cleave proteins in acidic conditions, laboratories later adapted it into an in-vitro (test-tube) assay: grind/defat a feed ingredient, incubate it with dilute hydrochloric acid + pepsin, and measure how much nitrogen (or protein) dissolves. The more that dissolves, the higher the pepsin “digestibility” under gastric-like conditions. Modern descriptions of this approach appear in AOAC Official Method 971.09 and ISO 6655:1997

When did pepsin testing become “a thing” for animal proteins?

• 1960s–70s: US feed and pet-food definitions began to reference pepsin digestibility, making it a recognised quality screen. AAFCO ingredient definitions from that era include minimum digestibility expectations and limits on pepsin-indigestible fractions (proposed 1968, published 1970 for several animal by-product definitions).

• 1973 (rev. 1999): AOAC 971.09 codified a widely used filtration method for pepsin digestibility of animal-protein feeds.

• 1997 (confirmed 2018): ISO 6655 standardised determination of soluble nitrogen after pepsin treatment for animal feeding stuffs, aligning global labs around comparable procedures.

By the 1990s, pepsin digestibility results were routinely appearing on COAs and specs for rendered meals like meat-and-bone meal (MBM), fishmeal, etc., and buyer specifications frequently included thresholds for pepsin-indigestible matter. Examples in AAFCO documents still cite limits such as “not more than 12% pepsin-indigestible residue and not more than 9% of crude protein pepsin-indigestible” for certain animal-protein products.

What the pepsin test actually measures.

• A lab imitates the stomach phase of digestion (acid + pepsin, controlled temperature/time).

• It then quantifies soluble nitrogen (or, inversely, the insoluble residue) to estimate how much of the ingredient’s protein would be susceptible to gastric proteolysis.

• It’s a screen for protein quality and heat/processing damage; it doesn’t replace full amino-acid digestibility data but correlates well when differences are large. Method details (enzyme strength, incubation time) matter and can shift absolute values, which is why standards specify conditions.

How it became possible to measure reliably. Two things made pepsin testing practical at scale:

1. Purified enzyme & controlled acid conditions (lab-ready pepsin and standardised HCl solutions), and

2. Harmonised methods (AOAC 971.09, ISO 6655) so that different labs could reproduce results with known precision—key for procurement and QA decisions.

Why this matters for MBM and similar animal proteins. Rendered meals vary with raw-material mix and time/temperature in the cooker. Over-severe processing can reduce pepsin solubility and damage amino acids (e.g., reactive lysine), even if crude protein looks fine. That’s why buyers track pepsin digestibility alongside ash/Ca/P and, for high-value applications, pair it with amino-acid profiles or reactive-lysine

Fun Facts

• The First Enzyme, StarPepsin, was the first enzyme ever discovered (1836). Scientists knew digestion “just worked” — pepsin finally explained how.

  
  

• Nature’s Powerful Shredder. In the right acidic conditions, pepsin can dissolve almost any animal protein — except keratin, which is why hair and feathers survive the stomach trip.

• Heat Can Fool the LabelOver-cooked protein still counts in crude protein tests, but pepsin says, “Nope — I can’t digest that anymore.”