Moisture Content: Bread & Water

September 13, 2007

Requests for a magic screw press that will produce press cake of some impossibly low moisture content, like 10% or 30%, still come in. These are generally referred to Pressing News #045, Four Kinds of Water. This helps explain the limits of dewatering capacity of pressure machines such as screw presses.

A simpler explanation is that materials have two kinds of water, free water and organic water. Bread has no free water: none of us, as kids, playing with bread, ever squeezed a drop of water out of it. Nor will a screw press. Yet, if we put a kilo of bread in an oven, set low, just under the boiling point of water, we will find only 620 grams of dry solids remaining the next morning. We would say that the oven evaporated 380 grams of water. Thus, bread has 38% moisture.

Clearly this 38% moisture is not free water. No amount of pressure will squeeze water out of the bread. So, it must be something else, which we call organic water.

What we mean by the term organic water is H2O molecules that are actually components of much larger organic molecules. Molecules of organic material are those long chain molecules of immense molecular weight, with lots of H's, O's, and C's, perhaps bound to some other elements. Pressure alone will not break loose the H2O's: it takes a chemical change caused by either heat or a chemical reaction.

Here is where it gets fun. We took one kilo of fresh bread and added, arbitrarily, 2,800 grams of water. If you think about it, you can see that we had our original 620 grams of solids, plus 3,180 grams of water. We dried some of it in an oven, and it measured about 84% moisture content.

We put 3.8 kilos of bread and water in a blender, and we ended up with a mass from which we could squeeze out some water. When we ran it through a screw press, we had problems because the mass tended to blind (cover over) the screen. Still, we eventually squeezed out a thick liquid in the form of press liquor.

We ended up with press cake that measured 77% moisture, and press liquor that showed a solids content of 11%. We could squeeze only about 2,100 grams of press liquor, although we had added 2,800 grams of water.

Why is it that the press cannot squeeze out the full 2,800 grams of water we added? Why can't the screw press get it back down to the 38% moisture material that we started with? The answer is that some of the water combined with the solids in the bread in such a way that simple mechanical squeezing will no longer remove this water.

You cannot squeeze wet bread any drier than 77% moisture. There is no magic screw press.

And, why are there 11% solids in the press liquor? With an additional test in the lab, we found that the 11% solids were made up of 7% suspended solids and about 4o Brix dissolved solids.

The soluble solids in the original bread have, like table sugar, diffused into the 2,800 grams of water that we added. A screw press cannot separate dissolved solids, so many of the dissolved solids in the bread flow out with the press liquor. Pressing News #169, Material Balance, explains how this impacts screw press results.

The 7% suspended solids in the press liquor were squeezed through the screen of the press during the pressing action. This is about a third of all of the suspended solids we started with. The amount going through the screen of the press is so high because the fibers in bread come from finely milled flour. Once mixed with water, these tiny particles slip right through the tightest screens available in screw presses.

[In gathering the data for this report, we cut the crust off the bread. Crust measures only 25% moisture, because of the chemical change that occurs during the baking process.]

PS If your kid is hunting for a subject for his PhD thesis, give him this one.

Issue 191