B. P. Carter1 and M. T. GallowayDecagon DevicesPullman, WA, U.S.A.
C. F. MorrisUSDA-ARS Western Wheat Quality
Laboratory, Washington State University
Pullman, WA, U.S.A.
G. L. WeaverArdent MillsOmaha, NE, U.S.A.
A. H. CarterWashington State UniversityPullman, WA, U.S.A.
Wheat (Triticum aestivum L.) is one of the most widely pro-
duced grain crops in the world, and wheat stores are considered an indi-cator of the economic stability of a country (20). To produce a viable food ingredient, most wheat is milled into white flour. Wheat typically is classified as either hard or soft, each of which has unique end uses. Hard wheat grain typi-cally contains higher levels of protein, requires a harder grind during milling, produces flour with coarser particle sizes, and is used for bread production (20). Soft wheat grain typically contains lower levels of protein, produces flour with finer par-ticle sizes and less damaged starch, and is used for cookie and cracker production (17,20). To have value as an ingredient, flour must perform well during processing, but it also must maintain its quality while being stored prior to use.
Milling is a multistep process used to transform wheat grain and consists of grinding the grain into powder by passing it through a series of rolls and then sieving the powder to sepa-rate the bran from the white flour (17). White flour is primarily produced from the endosperm of the grain, with the bran and germ removed, whereas whole wheat flour includes the bran and germ. Whole wheat flour contains more nutrients, but white flour has a longer shelf life and is easier to work with as an ingredient due to less discoloration and better processing
properties (7). Farina is a coarser milled product that consists of white flour and small amounts of germ and commonly is utilized as a hot breakfast cereal (8).
Prior to milling, the wheat grain must be tempered with moisture to soften the endosperm and toughen the bran, which facilitates grinding of the grain and separation of the bran and germ from the white flour (17). In tempering, sufficient liquid water is added to raise the moisture level of the grain to 12–17%; the moisture is then allowed to equili-brate in the grain for 16–24 hr before milling. However, standard practices do not include a test to determine whether moisture equilibrium has been achieved but instead depend on preset soaking times (17). The result-ing variability necessitates temper-ing optimization for a given milling facility to maximize flour yield while maintaining flour quality. This re-quires careful balance, as Kweon et al. (12) found that tempering condi-tions impact milling performance
and flour functionality, with flour produced from lower mois-ture tempering having a greater yield but poorer quality.
To have value as an ingredient, flour must possess good end-use qualities that remain stable while the flour is stored prior to use (5). Degradative reactions that could potentially end the shelf life of flour include microbial spoilage, caking and clump-ing, nutrient losses, color loss, and rancidity (7,16). The two extrinsic influences that most significantly impact the rate of flour shelf-life loss are temperature and moisture level (3,10). Moisture content is a common requirement on flour specifica-tion sheets: 13.5% is ideal for soft wheat, and 14% is ideal for hard wheat (Glen Weaver, personal communication). Moisture content provides useful information about the purity level of the flour and works well as a standard of identity but, unfortu-nately, is not very helpful in assessing the rate of shelf-life loss (13). The shelf-life loss factors listed above are better correlated to water activity, a thermodynamic measurement of the energy of water (2). Water activity measurement typically is accom-plished in 3–5 min using easy-to-use instrumentation, is more repeatable than moisture content analysis, and can be verified using saturated or unsaturated salt solutions (9). In addition, water activity helps form the basis for the U.S. Food and Drug Administration’s definition of potentially hazardous foods (19). Consequently, including water activity in flour specifications is
1 Corresponding author. Brady Carter, Decagon Devices, 2365 NE Hopkins Crt, Pullman, WA 99163, U.S.A. E-mail: [email protected]; Tel: +1.509.332.2756.
more critical to ensuring quality and shelf life than is moisture content, yet water activity currently does not appear on flour specification sheets.
The purpose of this study was to provide an argument for mak-ing water activity level a commonly requested specification for flour. More specifically, the study investigated the impact of particle size, tempering conditions, and storage conditions on the water activity, moisture content, and moisture sorption properties of wheat grain, flour, and farina. The information generated in this study should help explain some of the confu-sion concerning recommended moisture levels for flour, high-light the impact of storage conditions on flour moisture, and determine whether water activity may be a preferable metric for tracking moisture in grain and grain-based products.
Water Activity and Moisture Sorption AnalysisCommercial hard red spring and soft white winter wheat
grain was obtained and processed by the USDA Western Wheat Quality Lab in Pullman, WA. Samples (50 g) of dry whole wheat (DWW) were set aside for water activity and moisture sorption isotherm testing. The remaining hard and soft grain samples were tempered by adding enough water to raise the moisture level to either 15.5 or 17.0%, followed by tumbling for 20 min and soaking for 16 hr. As with DWW, 50 g samples of tempered whole wheat (TWW) at each moisture level were set aside for water activity and moisture sorption isotherm testing. The re-maining tempered wheat grain was milled using a modified experimental mill (Quadrumat, C.W. Brabender Instruments) (11). Break flour and farina samples were obtained from the mill (Fig. 1) and analyzed at Decagon Devices, Inc. in Pullman, WA.
Three replicates each of both hard and soft DWW, TWW, farina, and flour at each tempering level (not including DWW) were analyzed for moisture content, using AACC Approved Method 44-15.02 (1), and water activity at 25°C, using a water activity meter (AquaLab Series 4TE, Decagon Devices). Report-ed water activities and moisture contents were averaged across replicates. In addition, three replicates of each sample were ana-lyzed for equilibration time and maximum moisture sorption when changing from 30 to 65% RH and from 30 to 90% RH at 25°C (weight change [%dm/dt] trigger setting [0.008/3 events]), using the dynamic vapor sorption (DVS) method in the vapor sorption analyzer (AquaLab, Decagon Devices). Equilibration time was identified as the time required (in hours) for the flour sample to move from the initial water activity level and reach equilibrium, as indicated by meeting the weight change trigger,
at the final water activity level. Maximum moisture sorption was defined as the moisture content level achieved at either 65 or 95% RH. Mean equilibration times and maximum mois-ture sorption values were averaged across triplicate analyses.
Whole grain, flour, and farina samples were also analyzed in triplicate using the dynamic dew point isotherm (DDI) method in the vapor sorption analyzer (AquaLab, Decagon Devices) at 25°C, with an initial water activity (aw) of 0.10, a final aw of 0.90, and a flow rate of 80 mL/min (6). To compare the relative hy-groscopicity of the hard and soft flour and farina samples, the linear slopes of the adsorption curves from 0.10 to 0.80 aw were compared. For whole wheat samples, the critical water activity (RHc) level was identified as the water activity level associated with a sharp inflection in the DDI curve and was determined by finding the first maximum in the Savitzky-Golay second de-rivative curve (6,18). Mean RHc values averaged across tripli-cate DDI curves were reported.
A two-way ANOVA was used to determine whether particle size (whole wheat versus farina versus flour), hardness level, and tempering level were significant sources of variation in moisture content, water activity, isotherm slope during adsorp-tion, equilibration time, and maximum moisture sorption (15). For treatments that were shown to be significant, Tukey’s mul-tiple means comparison was used to determine which treatment levels were significantly different.
Water Activity of Whole Grain, Flour, and FarinaAs expected, DWW had the lowest water activity, whereas
TWW had the highest water activity (Table I). The water activi-ties of hard wheat grain, flour, and farina were always higher than their soft wheat counterparts, regardless of the tempering level (P = 0.0001) (Tables I and II). The water activity level of 17% TWW was higher than the RHc level for mold growth
Fig. 1. Sample types utilized for this study (clockwise from upper left): whole grain, farina, and break flour.
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(0.70 aw) for both hard and soft wheat but at 15.5% tempering was higher only for hard wheat (4). The typical 16–24 hr hold time for tempered wheat should not be long enough for mold to grow. However, if the hold time is extended, any tempered wheat with aw > 0.70 would likely experience mold growth.
The water activities and moisture contents of flour and farina were always lower than those of TWW due to removal of mois-ture during the milling process (Table II). Based on Tukey’s comparisons, flour and farina had similar (P > 0.05) water ac-tivities after milling for a given hardness and tempering level, with the exception of soft white wheat farina at 15.5% temper-ing, which had slightly lower water activity than flour. The wa-ter activities (<0.70) and moisture contents (13.5% for soft wheat and 14.0% for hard wheat) of both flour and farina at 15.5% tem-pering were within acceptable limits. At 17.0% tempering, the water activities of the hard flour and farina were above the criti-cal 0.70 level and below this level for soft flour and farina. The moisture contents for all products at 17.0% tempering would be considered too high.
A suggested water activity specification for flour and farina would be 0.62–0.68. As indicated in this study, this water activ-ity range corresponds with ideal moisture levels for both hard and soft wheat flours. In addition, this water activity range would assure no mold growth and minimize the rate of rancidity (4,14).
Moisture Sorption of Whole Grain, Flour, and FarinaThe slopes of the dynamic isotherms for the flour and farina
samples were not significantly different (P = 0.148), indicating equivalent levels of hygroscopicity (Fig. 2). The sorption curves were essentially linear up to 0.75 aw. The shape of the DDI curves for the whole grain samples differed from those of the flour and farina samples and initially were almost flat (Fig. 2). This is typical of materials with hard coatings that limit the pen-etration of water into the interior. However, at higher water ac-tivities, the sorption isotherm curves of the whole grain samples showed sudden changes in sorption properties when the samples began to absorb a lot more moisture, causing an inflection in the DDI curve. The water activity associated with this change
was identified as the RHc and represents the point at which wa-ter begins to penetrate the pericarp of the grain. RHc occurred at a lower water activity level for soft grain (0.743) than for hard grain (0.795) (P = 0.0001). A review of Table I shows that the RHc values for soft and hard wheat grain were similar to the water activities of the 17.0% tempered whole grain.
Based on the connection between the RHc value of whole wheat grain and current tempering moisture levels, it would be feasible to temper grain to a constant water activity level rather than to a moisture level. Based on the preliminary results of this study, a recommended water activity specification for tempered wheat would be 0.75. This water activity level would achieve tem-pering conditions similar to those currently being used but with more consistency because water activity is a more precise mea-sure than moisture content and can easily be monitored with instrumentation to determine when tempering is complete. Finally, tempering the grain to 0.75 aw could be achieved by vapor equilibration to 75% RH (using saturated NaCl), which is more uniform than adding liquid water. The equilibration time should be similar to current tempering hold times, so mold growth should not have time to begin. Finally, having a consistent initial water activity level for tempered wheat would consistently result in flour or farina water activities <0.70, pre-venting mold growth and reducing the rate of rancidity.
The DVS results shown in Figure 3 indicate that at 0.65 aw, differences between hard and soft flour, as well as hard and soft farina were not significant (P = 0.798). However, the 14.3% av-erage moisture content of the hard and soft flours was signifi-cantly higher than the 13.6% average moisture content of the hard and soft farina samples (P = 0.009). At 0.90 aw, the mois-ture content was roughly 20%, again with no significant differ-ences between hard and soft flours or hard and soft farinas (P = 0.441). In addition, flour moisture content was not significantly different from farina moisture content (P = 0.488). The time required to move to 0.65 aw from 0.30 aw was longer for hard flour (15.1 hr), but not significantly longer (P = 0.471), due to high variability in equilibration times for hard flour. The equili-bration times were similar for soft flour, soft farina, and hard
Fig. 2. Dynamic isotherms for hard and soft whole grain, flour, and farina at 25°C. Inflection points in the dynamic curves (highlighted by black dots) indi-cate the critical water activity (RHc) levels for whole grain samples. The slopes of the dynamic curves for flour and farina indicate relative hygroscopicity.
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farina at ≈5 hr, a further indication that the hard flour equili-bration time was incorrect. Equilibration times at 0.90 aw were significantly different (P = 0.008) among the sample types, with the longest for hard farina at 23.4 hr, followed by soft farina at 17.3 hr, hard flour at 14.5 hr, and soft flour, which had the short-est time at 11.0 min.
The results indicate RH levels of 60–70% should not be prob-lematic, because water activity would be below the mold growth limit and moisture levels would be acceptable for both flour and farina. However, exposure to lower RH would result in moisture loss to potentially unacceptable moisture levels. If flour or farina is exposed to high RH (>70%), the water activity would be above the limit for microbial growth, and the moisture levels would be unacceptable. Soft flour would reach unsafe water activity levels the fastest, but all products would reach unacceptable water ac-tivity and moisture levels in only 24 hr (Fig. 3). This change in water activity and moisture content only applies to product that is exposed to ambient conditions. Moisture movement through a mass of bulk stored product once the surface has come to equi-librium and the subsequent changes in water activity and mois-ture content have occurred are not part of this discussion.
ConclusionsObserving the water activity changes in wheat as it was trans-
formed from dry whole grain to tempered whole grain and then milled into flour or farina provided insight into the basis for cur-rent handling practices. The current system depends heavily on tracking moisture content changes during milling rather than on water activity. Considering that the current suggested mois-ture content levels for flour and farina correspond to water ac-tivity levels right at the cutoff point for mold growth, it behooves the flour industry to consider including a water activity specifi-cation to ensure microbial safety. In addition, because lower water activity levels are better correlated to lower rates of ran-
cidity than moisture content, it would make more sense to focus on optimizing the water activity level and then confirm that the moisture content is acceptable rather than rely solely on a mois-ture content specification.
The results of this study indicate the water activity of DWW with acceptable moisture content levels is below the microbial growth limit and low enough that the seed coat can effectively block moisture penetration. The water activity of TWW is above the critical water activity level of the seed coat, causing a reduc-tion in its resistance to water penetration and allowing softening of the grain in preparation for milling. Finally, the water activity level of finished flour is just below the cutoff limit for microbial growth. However, all flour types tested are equally hygroscopic and susceptible to water activity levels rising above the micro-bial growth limit when exposed to high RH during storage, which will occur in less than 24 hr.
Based on this survey of the water activity of wheat milling con-stituents, it appears that, through trial and error, those who have set current recommended moisture content levels for grains and flours fortunately, if unknowingly, found the right water activity level to maximize stability. Moisture content measurements in grain and flour certainly have their purpose as a standard of identity. However, logic suggests that because water activity is the parameter driving stability, directly measuring water activ-ity would be a more effective method of ensuring a consistently stable product.
AcknowledgmentsWe thank Decagon Devices, Inc. for providing the funding for this
study, as well as Andy Galbraith and other support staff at Decagon Devices for facilitating data collection. We also thank the USDA Western Wheat Quality Lab for providing whole grain wheat, tem-pered whole grain wheat, flour, and farina. Finally, thank you to Shyam Sablani for providing feedback.
Fig. 3. Average maximum moisture sorption and equilibration times for hard and soft wheat flour and farina samples when equilibrating to 0.65 and 0.90 water activity (aw).
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Brady P. Carter is a research scientist with Decagon Devices, a world leader in water activity measurement. He specializes in water activ-ity and moisture sorption applications. Brady earned his Ph.D. and M.S. degrees in cereal chemistry and crop science from Washington State University and a B.A. degree in botany from Weber State Uni-versity. He has 14 years of experience in research and development, and prior to taking a position with Decagon Devices, he was an as-sistant scientist at Washington State University, focusing on wheat end-use quality. At Decagon, Brady oversees development of new moisture analysis instrumentation and provides support for current instrumentation. He has been the instructor for water activity semi-nars in more than 23 countries and provided on-site water activity training for companies around the world. He has authored more than 20 white papers on water activity, moisture sorption isotherms, and complete moisture analysis and has participated in hundreds of ex-tension presentations and given presentations at numerous scientific conferences. Brady can be reached at [email protected].
Mary T. Galloway has been an R & D lab scientist with Decagon Devices for nearly five years. Her work in the Product Testing Tech-nologies group primarily focuses on using and testing instrumen-tation that measures water activity and its influence on physical properties. She has contributed to a variety of projects, including application studies using dynamic dew point (DDI) and dynamic vapor sorption (DVS) isotherms on various food and pharmaceutical products to determine critical water activities, transition points, and kinetics; texture analysis; and determining best sampling practices. Her other responsibilities include testing prototype instruments for proof of concept, usability, and accuracy. Mary holds a B.S. degree with emphases in chemistry, math, and engineering from Washing-ton State University. Mary can be reached at [email protected].
Craig F. Morris is the director of the USDA-ARS Western Wheat Quali-ty Laboratory (WWQL) located at Washington State University (WSU). He is an adjunct professor at WSU in the Department of Crop and Soil Sciences, the WSU/University of Idaho School of Food Science, and the Department of Soil and Crop Sciences at Colorado State Univer-sity. Craig is an AACCI Fellow and an honorary research professor of
the National Wheat Improvement Center, Chinese Academy of Agricul-tural Sciences. He served on the AACCI Board of Directors and served three terms as associate editor and is currently editor-in-chief of Cereal Chemistry. Craig cofounded and leads the Pacific Northwest Wheat Quality Council. He has published more than 160 research pa-pers and book chapters and has been awarded six patents. He holds degrees from Iowa State University (B.S.) and Kansas State University (M.S. and Ph.D.). Craig is an AACCI member and can be reached at [email protected].
Glen L. Weaver is a research fellow for Ardent Mills. Glen’s academic background is in food science. He has spent more than 40 years work-ing for ConAgra Foods and Ardent Mills in various capacities, focus-ing on food safety, quality management, technical support, and new technology development. He is active in AACCI, IFT, IAOM, the Wheat Quality Council, and the American Baker’s Association (FTRAC/Food Technical Regulatory Advisory Committee), as well as other organiza-tions. His primary interest is developing genetics and IP systems, and others, to yield solutions for grain-based ingredients and processes. He stresses the importance of effective communication and linkages in the supply chain, including universities, government, and private technology providers. Glen is an AACCI member and can be reached at [email protected].
Arron H. Carter is an associate professor and winter wheat breeder at Washington State University. His main goal is to develop soft white and hard red winter wheat cultivars for the state of Washington and the Pacific Northwest. His main objectives are high yield potential and agronomic adaptability, durable biotic and abiotic resistance, and excellent end-use quality. His research program focuses on identify-ing genes for stripe rust resistance, end-use quality, winter survival, and abiotic stress resistance. He also focuses on other diseases pres-sures relevant to regional areas, as well as regional adaptation such as emergence from deep planting. Arron uses a variety of tools to accomplish his breeding objectives, including field selection, doubled haploid production, high-throughput field phenotyping, marker- assisted selection, genotyping by sequencing, and genomic selec-tion. Arron can be reached at [email protected].
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