Calcium fortification is a growing trend in the food industry. Milk and dairy products have a healthy image and are ideal vehicles for delivery of additional calcium and other minerals. Calcium has a vital role in bone health and concern about the risk of osteoporosis has heightened the interest in calcium fortified foods. An adequate calcium intake has been associated with reduced risk of hypertension, colon cancer and kidney stones, although more work is needed to obtain more definitive links between calcium and disease. Calcium has an essential role in muscle contraction, blood clotting, and hormone regulation and enzyme activation. As more consumers are made aware of the health benefits of calcium, an increasing number of new calcium-fortified products are being marketed.
Strategies used for calcium fortification range from the addition of soluble calcium in the form of mineral and/or organic salts, the use of insoluble calcium salts as fine particles in suspension often with additional gums or thickeners, or utilize calcium associated with protein. Although the addition of insoluble calcium salts has been used, there are problems encountered with the use of this strategy. There is risk of sedimentation of the calcium salt in the liquid milk. When insoluble calcium sources are dry blended with milk powder, there is risk of precipitation on re constitution of milk powder containing soluble calcium salts in hot water. This will depend on the source of soluble calcium used and the temperature of the water used for reconstitution.
When using soluble calcium salts for fortification of milk, there is a need to consider the complex equilibrium between the proteins and minerals in milk. In a typical milk with a total calcium concentration of 30 mmol·L –1, the concentration within the colloidal phase is 20 mmol·L –1 and the serum phase contains a total of 10 mmol·L –1 calcium at pH 6.70. Of the serum calcium, 2 mmol·L –1 is free calcium . Holt has proposed a model structure for the colloidal phase of milk based on the formation of nano-clusters with a core / shell structure. Various workers have demonstrated that calcium and other minerals can move from phase to phase when conditions such as the pH, ionic strength or temperature of milk is changed, or when calcium chelating salts are added. Within the serum phase there is a further partition between free ionic forms of the salts and salts complexedwithother components. Re-distribution of these salts from one phase to the other can also give rise to a re-distribution of the casein proteins between the phases of milk.
The interactions of proteins and calcium have a marked influence on the heat stability of milk, an important functional property of milk. Generally the addition of calcium salts to milk causes an increase in serum calcium ions, an increase in calcium in the colloidal phase, a decrease in pH, and a decrease in heat stability. Reduction of the calcium ion concentration by the addition of phosphate or citrate generally causes an increase in heat stability.
Most researchers have used calcium sequestering agents for stabilization of calcium-fortified milks but very few of the strategies claim to be able to create a calcium fortified milk that can withstand high heat treatments. Some notable exceptions are milk fortified with a soluble calcium source that is made stable to the heat treatment used in yoghurt manufacture through the addition of citrates and pH adjustment. Citrates have also been used for the production of calcium-fortified UHT milk but in two of these cases the calcium and milk are sterilised separatelyandthenmixed aseptically when cool and in the third carrageenans are also needed for stability. The use of calcium phosphate nanoclusters derived from milk, has been described for food and pharmaceutical applications.
The aim of this work was to develop a strategy involving the addition of a soluble calcium salt in combination with orthophosphate and pH manipulation for the manufacture of calcium-fortified milk powders. The target was milk powders with an additional 8 g of calcium per kg of powder. Reconstitution of these powders will give skim milks (100 g total solids·kg –1) and full-cream milks (125g total solids·kg –1)with a calcium level in excess of 2000 mg·L –1.
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