Laboratory Technical Supplement
Federal Milk Market Administrator Seattle, WA
G.T. Shen October 2000
True Protein and Other Solids technical details for keeping pace with federal order reform
As of January 2000, the USDA adopted a new reference method (i.e. "true" protein) for determining protein in raw milk. In addition, some federal orders shifted to a three-component pricing system based on butterfat, true protein, and "other solids". These changes have important implications on how infrared analyzers are calibrated and how mathematically derived terms such as total solids and solids-non-fat are calculated. This supplement deals with the technical adjustments needed to properly implement these changes.
For clarification as to the rationale for the shift to true protein, consult Protein Testing - FAQ's. If you wish to learn more about why there are no winners or losers with respect to protein payment, regardless of NPN level (non-protein nitrogen), please consult Protein Changes - Myths and Facts.
Other Solids (OS)
In contrast to fat and protein, this component is a mathematical construct (an infrared analyzer cannot "see" other solids). In the past, all of the following relationships have been successfully used to estimate OS:
OS = total solids fat total protein
OS = SNF total protein
OS = lactose + ash
The January 2000 conversion to true protein, however, complicated the situation. The above relationships may or may not hold strictly true any longer, depending on ones infrared calibration approach. We expand on this below.
True Protein
In multi-component markets, all testing of milk received as of 2000 is based on "true" protein and associated fat/OS/TS/SNF reference values. As a result, in January of 2000, producers saw their protein levels decline by an average of 0.19% below previous levels based on total nitrogen. The 0.19% represents the average level of non-protein nitrogen found in raw milk.
This decline in protein must be compensated for since none of the other reference methods changed. In other words, an extra 0.19% must be carried in all "solids" categories (i.e. TS, SNF, OS). This means that traditional relationships such as [fat + protein + lactose + ash = TS] will no longer be strictly correct. For most labs that calibrate directly for TS, SNF, or OS, there is little to worry about. Understanding the following two scenarios will ensure that your lab and pay test handling routines will be 100% correct.
Scenario I -- Labs calibrating IR instruments directly for TS, SNF, or OSAs of the reform date, these labs should simply recalibrate using true protein and associated TS/SNF/OS reference values. This holds whether the lab regularly performs a strict MLR (multiple linear regression) or adjusts some type of fixed solids calibration equation via LSR (least squares regression -- likely consequences of this will be a reduced protein bias and an enlarged TS/SNF/OS bias relative to past calibrations). Thus, the problem takes care of itself. Other solids can be obtained directly from an OS calibration, or via:
OS = TS fat true protein
OS = SNF true protein
If you wish to compute OS from lactose and ash, however, please use one of the following:
OS = lactose + constant ash* + 0.19 = lactose + 0.91
OS = lactose + variable ash** + 0.19 = lactose + ash eqn + 0.19
**if a constant ash factor is used, the MA recommends a value of 0.72
**ash correlation with true protein (as used by MA): variable ash = (True Protein x 0.0596) + 0.5379
supplemental footnote: note that in the federal order language, solids-non-fat (SNF) is denoted nonfat solids
Scenario II Labs calibrating IR instruments via LSR or MLR on fat, protein, and lactose only
Labs in this scenario will have correct fat, protein, and lactose values, but all solids measures obtained by conventional summation will be deficient by the average NPN (non-protein nitrogen) fraction, 0.19%. Thus, one of the following sets of equations should be adhered to:
Labs preferring constant ash factor (recommended value of 0.72):
OS = lactose + 0.72 + 0.19 = lactose + 0.91
SNF = true protein + lactose + 0.72 +0.19 = true protein + lactose + 0.91
TS = fat + true protein + lactose + 0.72 + 0.19 = fat + true protein + lactose + 0.91
Labs preferring variable ash factor (ash eqn same as above):
OS = lactose + ash eqn + 0.19
SNF = true protein + lactose + ash eqn + 0.19
TS = fat + true protein + lactose + ash eqn + 0.19
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