Graduate Thesis Or Dissertation
 

Impact of Preharvest Weathering, Without Sprouting, on the Quality of Soft White Winter Wheat in the U.S. Pacific Northwest

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  • When wheat is exposed to rain in the period before harvest, it sometimes causes the problem of pre-harvest sprouting (PHS). Wheat that undergoes sprouting has poor end-use quality and is less valuable. In recent years, the U. S. Pacific Northwest (PNW) has experienced increased summer showers sometimes causing widespread PHS. The potential for these showers to continue to occur in future years has prompted wheat breeders to focus on breeding for resistance to PHS. If breeders are successful, and increased summer rainfall becomes persistent, then the likelihood that harvest-ripe grain will be weathered by rain but not suffer PHS is increased. There is some concern that pre-harvest weathering (PHW) might cause quality losses in soft white winter wheat (SWW) grown in the PNW because PHW has been shown to cause quality losses in other regions of North America. The most troubling characteristic that has been observed by other researchers is a loss of test weight (TWT) because TWT is an important factor in wheat grading and therefore pricing. Despite this, there have been no previous studies specific to SWW grown in this region. In this study, five SWW wheat varieties were harvested in timely and delayed harvests at one location in Oregon in 2015. The delayed harvest happened after 19 mm of rain had fallen. However, the grain did not suffer PHS. Wheat from the two harvests was compared based on its grain traits, milling quality, flour quality, and the quality of food products traditionally associated with PNW SWW. Mild PHW was not observed to substantially impact the majority of grain, milling, and end-use quality characteristics. The statistically significant impacts of PHW on grain traits included decreased TWT, increased kernel moisture, minor softening, and slightly decreased kernel polyphenol oxidase (PPO) activity. Variety significantly affected all grain traits. Of all quality traits observed in this study, only TWT was unambiguously negatively affected by PHW, enough to cause significant practical or economic consequences. The TWT losses experienced by the samples in this study after mild PHW would be enough to put a significant portion of SWW grown in the PNW at risk to be reduced to a grade of U.S. #3, essentially excluding it from the export market. This carries serious economic risk for farmers. The increased moisture was not of practical concern because it was not enough to cause storage problems although it could harm the PNW’s reputation for delivering high quality, low moisture wheat. The impact of softening and reduced PPO were considered too small to have practical impact. Milling and flour traits were not seriously affected by mild PHW. However, variety affected all of the measured milling and flour traits. Weathered flour was slightly brighter and less yellow. This is a positive outcome but the magnitude of the change was likely too small to be perceived by humans. This result is consistent with bleaching observed in previous research including Czarneki et al (1986). Sucrose solvent retention capacity was slightly decreased and mixograph manual peak time was slightly increased after PHW. These are considered desirable and undesirable changes respectively, but once again were likely too small to have a practical impact. End-use quality of flour was analyzed for products traditionally associated with PNW SWW. Weathering appeared to have very little impact on end-use quality. Variety affected all measured traits except for alkaline-noodle texture. The observation of varietal effects indicates that end-use product testing was able to discern flours of varying quality even if it was slightly less sensitive than the analytical methods used to measure grain, flour quality, and milling quality. After PHW, noodle darkening over 24 hours was increased and wire-cut cookie diameter (WC CODI) was slightly decreased. The effect on noodle darkening was of small concern because unweathered and weathered samples were still within a range acceptable to wheat breeders when making selections. The WC CODI results were somewhat ambiguous because of the presence of a weathering x variety interaction and a wide range of results for the individual varieties. This indicates that PHW may negatively impact some varieties more than others. All of the traits measured for sugar-snap cookies and the batter-based products (Japanese sponge cake and pancakes) were unaffected by weathering, which is a positive outcome for the soft wheat growers of the PNW.
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