These two terms are usually used in conjunction with discussions of protein solubility. NSI stands for Nitrogen Solubility Index and is expressed as a percentage of the protein nitrogen that is water soluble relative to the total protein nitrogen present. For example, if 75% of the total nitrogen contained in a protein is found by analysis to be water soluble, then the NSI is said to be 75. NSI is used to demonstrate total protein solubility. NSI values for MPC powders can be affected by:
- Age of the milk when the MPC was manufactured
- Manufacturing conditions
- Exposure to heat
- The presence of reactive minerals.
Fresh milk (0 to 24 hours old) will yield a more soluble MPC than will older milk. As milk sits in a silo, even at refrigerated temperatures, there are reactions that occur within the milk. Most of these reactions result in a decrease of milk protein solubility. A general rule of thumb is, fresher milk makes more soluble MPC. Manufacturing an MPC can be very tricky to those who don’t know what they are doing. There are subtle changes to MPC protein that will occur if the protein becomes too concentrated at the wrong time. Basic chemistry states that the more one concentrates molecules in a confined space, the faster they will react with each other. Many of these accelerated reactions prove detrimental to MPC solubility. As we have already covered, increased heat exposure during manufacture of MPC will result in a loss of solubility due to whey protein denaturation and calcium-casein interactions. MPC solubility can also be affected by the presence of reactive, free minerals. Calcium and magnesium ions will readily react with casein and cause the casein to lose solubility. Care needs to be taken during MPC manufacture to avoid liberating ionic calcium and magnesium.
WPNI, otherwise known as Whey Protein Nitrogen Index, is a measurement of whey protein solubility. As with NSI, WPNI is expressed as the amount of whey protein nitrogen that is water soluble relative to the total amount of protein nitrogen present. WPNI is usually used as a measuring stick to determine the amount of heat exposure a dairy powder has undergone. With higher heat exposure during manufacture, whey proteins will denature and become less water soluble, resulting in a powder with low WPNI value. A high WPNI value signifies less heat exposure and less whey protein denaturation. A low WPNI value signifies excessive heat exposure and a high degree of whey protein denaturation. For example, in nonfat dry milk (NFDM) with a total protein content of 35%, a low heat treated NFDM might have a WPNI result of 6.0 (6.0% of the whey protein nitrogen is water soluble versus the 35% total protein nitrogen). A high heat treated NFDM might yield a WPNI result of 1.2, signifying that most of the whey proteins in the skim milk have been heat denatured during the heating process. For an 80% protein MPC powder, the WPNI value should be much higher. In theory, an 80% protein MPC powder could contain as much as 18% whey protein nitrogen (soluble and insoluble). Therefore, a low heat exposure result for an MPC 80 powder would be somewhere in the range of 14 and higher while lower WPNI values would reflect increasing levels of heat exposure.