Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2014
Publication Date: 3/1/2015
Citation: Delwiche, S.R., Vinyard, B.T., Bettge, A.D. 2015. Repeatability precision of the falling number procedure under standard and modified methodologies. Cereal Chemistry. 92(2):177-184.
Interpretive Summary: When wheat is traded, beginning at the country elevator level and continuing through all downstream stages, one of the determining factors of price is the soundness of the starch contained in the seed endosperm. Under adverse weather conditions occurring during the late stages of plant maturation, the seed within the ripening wheat head may begin the process of germination. Known as pre-harvest sprouting, this enzyme-induced condition of nature has a deleterious effect on the products made from wheat. An analytical method known as ‘Falling Number’ (FN) has been used for more than a half century to gauge the degree of enzyme activity that is responsible for starch degradation. Essentially, the FN method characterizes the viscosity of a meal:water mixture under tightly controlled conditions of mixing and heating, with values typically ranging between 150 (low quality) and 450 (high quality). Sales contracts often specify a minimum value for FN, typically 300, below which the consignment becomes heavily reduced in price. Although universally used, the FN method has its critics because, as with all analytical procedures, it has an inherent level of imprecision. The level becomes troublesome to buyer and seller when FN values fall within close proximity of the specified minimum. A study was conducted to quantify the precision: first, under best laboratory practices, second, with slight changes to the wheat meal:water mixture quantities, and last, with the addition of a surfactant to enhance mixing. The findings indicate that the method has good precision, with coefficients of variation (a dimensionless figure of merit, expressed as a percent, with values near zero indicating highest precision) typically being on the order of 3% for the method as conventionally specified, and 2 to 3% when the meal and water quantities are increased. Such levels of precision should act as reassurance that the method itself is adequate when good laboratory practices are followed, and that precision may be improved by making slight changes to the meal and water formulation while keeping the instrument hardware untouched. These findings have direct application to the wheat trade and processing industries that utilize this method extensively as a factor in setting the price of grain consignments.
Technical Abstract: The falling number (FN) procedure is used worldwide to assess the integrity of the starch stored within wheat seed. As an indirect measurement of the activity level of alpha-amylase, FN relies on a dedicated viscometer that measures the amount of time needed for a metal stirring rod of precise geometry to descend a fixed distance through a column of water-flour or water-meal slurry that undergoes enzyme-activated starch hydrolysis under controlled mixing and heating conditions. For U.S. wheat, FN values of 300 seconds and above generally indicate soundness in the condition of the seed starches, whereas values less than 300 often indicate that some seeds have broken dormancy, which deleteriously affects bread-, cake-, and noodle-making quality of products derived from their flour. Domestic and especially overseas sales contracts will often specify a minimum FN value for consignments, thus making it critical to ensure that the FN procedure be highly precise. The study described herein examined the level of repeatability precision of the FN procedure under strictly controlled laboratory conditions as a means to establish precision levels arising alone from the random nature of the viscous properties of starchy meal undergoing mixing and heating. Six representative samples of Pacific northwest grown soft white wheat, ranging in FN between 168 s and 404 s, were repeatedly measured using the conventional FN procedure and three modifications thereof, with the modifications being increased meal and water amount (8 g meal + 30 mL H2O instead of 7 g + 25 mL) or the addition of a polysorbate surfactant (0.1% Tween 20) to the mixture water. Three hundred grams of each sample were ground, and dual-column FN tests were repeatedly run four times for each of four treatments (conventional plus three modifications). The grinding and FN procedures were performed four times on each sample. Minimal FN differences among grinds allowed for their pooling, thus yielding 16 FN runs for each sample and treatment. Estimated variances and coefficients of variation (CV) were determined for each treatment-sample combination. The results indicated that CVs between 1% and 4% were achieved for all treatments and samples. The treatment modification of an augmented test sample size improved precision, whereas incorporating a surfactant had negligible effect. Precision and treatment (conventional vs. augmented) findings were corroborated by two external laboratories using two of the six original samples, and by the main laboratory on an independent set of 14 wheat meal samples of commercial origin.