but operated below target for the higher vitamin C level. Even though the batch process delivered the best performance, it is recognized that WSB represents only a small fraction of total blended products distributed by the Public Law (P.L.) 480 Title II program.
The target vitamin C values for conventional and higher levels of fortification are 40 and 90 mg per 100 g, respectively. Many factors combine to produce variation in the measured values of vitamin C in samples from production runs. When processes are in control, vitamin C is approximately normally distributed. Consequently, 99.7 percent of the values will be within 3 S.D. (standard deviations) of the mean. When making control charts, extremes such as the mean ±3 S.D., are called the upper and lower control limits (UCL and LCL).
Specifications (SPECS) refer to the limits that define acceptable product. Generally these are determined by the customer, and any product outside these limits is reprocessed or discarded. Appropriate SPECS depend on the product and the property being measured. For example, a 12-ounce soft drink product might have a lower SPEC only for volume (if the can contains less than 12 ounces the customer has not received what has been paid for, but excess product is acceptable). On the other hand, the sugar content would have upper and lower SPECS; a drink that is either too sweet or not sweet enough is unacceptable. For some products an 80 percent rule is used: a product that contains at least 80 percent of the quantity advertised is acceptable. It is important to note that SPECS refer to the acceptability of a product, not to the mean and standard deviation of the process that yields the product. Very often, a target value is used, which is in the middle of the upper and lower SPECS. It is also important to note that under current USDA-CCC procurement procedures for P.L. 480 Title II blended commodities that specifications are written for the manufacturing process (e.g. how much of the nutrient must be added during manufacture), not on the nutrient content of the final product.
Process capability refers to measures that express how well the production process satisfies the SPECS. These measures take several forms but generally can be translated into the proportion of product that is unacceptable. There are two ways to ensure that this proportion is small: (1) the process mean should be close to the target value, and (2) the standard deviation of the process should be small. In many processes, the first is relatively easy to achieve by adjusting a component. For example, if the process mean for vitamin C in conventional CSB is 27 mg/100 g, the equipment should be adjusted to add approximately 50 percent more premix in order to achieve the desired level of 40 mg/100 g.