Calcium solubility digestion procedure

Experiments were conducted to determine if varying the conditions in the in vitro digestion procedure would affect post-digestion calcium solubility and in some cases, exchangeability. This was done with two purposes to test the use of the in vitro digestion procedure for studying factors which might influence calcium bioavailability and to use the results to modify the standard procedure. 

Choice of Potential Bioavailability Criterion. It is usually assumed that calcium must be soluble and probably ionized in order to be available for absorption ( ). For the in vitro procedure, as a first approximation we chose calcium solubility after centrifugation at 18,000 x g as the measure of potential bioavailability (Figure 1). We assumed that this would probably overestimate the available calcium and later work based on fractionation might define the bioavailable calcium more precisely. The data in Table IV illustrate how the choice of criterion for "solubility" could affect the in vitro estimate of potential availability, even if in vitro conditions closely resembled in vivo conditions. Since our in vitro criterion unexpectedly underestimated calcium bioavailability for two of the three foods in the direct in vivo - in vitro comparison (8), it was necessary to determine the in vitro digestion conditions which might be limiting solubility before addressing the choice of appropriate criterion. 

The term "standard" is not meant to denote a digestion procedure which should be routinely used to determine potentially available calcium. Since the number of foods tested so far is limited, it will be more useful to think of the "standard" procedure as a set of conditions to be used to see how well we understand food chemistry and calcium solubility in the gastrointestinal environment. It should be used for measuring the relative solubility of calcium from foods and meals, but mostly in the context of comparisons with in vivo results to define factors which require further study. 

The edible salts of calcium and magnesium do not precipitate pectins until the degree of esterification of the pectin molecule has been reduced below 8.2% methoxyl content on the basis of 100% calcium pectate (10, 17). In 1935 a patent (16) was granted on the use of soluble salts of alkaline earth metals, such as calcium chloride, to precipitate pectic substances which had received a partial de-esterification treatment. Olsen and Stuewer obtained a patent (IS) on a digestion-extraction procedure, termed pickling and carried out at less than 50 C. below pH 1 for a period sufficient to produce pectin precipitable by calcium salts at pH 4. The metal-free pectinic acids can be isolated from either the aluminum or calcium salts by treatment with acidified alcohol in accordance with methods well known for years. 

---