Of these, 3 factors: pH, oxidation reactions, and chemical catalytic reactions can be directly attributed to the presence of trace minerals (zinc, copper, manganese, iron, selenium, iodine, cobalt, others), especially when included in conventional inorganic forms, such as sulfates and oxides. However, regarding pH it might be surprising to learn that vitamin premixes alone are somewhat acidic (pH 5), whereas inclusion of conventional trace mineral sources decreases VTM pH somewhat to 4.2; and chelated/organic forms actually acidify VTM to pH 2.2 (reference Shurson et al, 2011, AnFdSciTech 163:200-206).  So pH alone is not a good enough reason to justify specialty or organic sources of trace minerals over conventional sources.  

So what about mineral reactivity?  In the Shurson 2011 reference, researchers at the University of Minnesota compared conventional/inorganic trace mineral sources vs. complexes/”organic” sources in a VTM formulated for sow feeds, which included choline.  Premixes were stored for 120 d in an environmentally-controlled dark room at 31C and ambient (22-38%) relative humidity—considered low stress conditions for most feed mills and producers.  Organic trace minerals prevented up to 40-50% of the vitamin storage losses observed in the premixes formulated with conventional sources of trace minerals.  This tells us that factors other than pH control are responsible for the “protective” effect of complexed trace minerals.

Trace Mineral Considerations

Depending on mineral source and application, trace minerals make up from 40% (conventional inorganic sources) to 63% (complexed/organic sources) of the weight of active ingredients in a VTM.  When formulating a VTM or trace mineral premix, nutritionists must consider final levels needed in the feed, possible interactions with other feed and premix ingredient, reactivity, source, cost per unit mineral, bioavailability, particle size, and bulk density. dsm-firmenich handles more than 20 sources of zinc alone in our premix plants, each with specific handling and mixing properties, and in the case of chelated/complexed sources, each has specific manufacturer feeding guidelines.

Water Activity: the true culprit?

In the above list of premix stability factors, water activity (“A sub w”) and moisture content are critical factors for vitamin stability.  Water activity is defined as the vapor pressure of a given product (VTM in this case) compared to pure water—the higher the vapor pressure, the higher the water activity and thus reactive water present. The deleterious effect of high relative humidity in storage is well-known, as is the negative effect of including choline (hygroscopic) in humid conditions.  Moisture and premix humidity are known to activate conventional trace mineral sources such as copper sulfate, which has led many formulators to remove choline from their premixes.  When consulting with dsm-firmenich regarding VTM formulations, we have several management tips to assure quality:

• Remove choline from a VTM if possible; add choline separately. If that’s not possible, manage storage humidity
• Shorten storage time, especially in the hot/humid summer months
• Consider chelated/complexed/reduced reactivity trace mineral sources where stability is an issue
• For special applications such as dispersible premixes, dsm-firmenich has identified certain additives and carrier systems which can be used to manage “A sub W” and resulting redox reactions