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Foliar Urea Applications Increase Yeast Assimilable HORTSCIENCE 55(8):1356–1364. 2020. https://doi.org/10.21273/HORTSCI15029-20 Borkh.) producers an opportunity to expand and diversify into a niche commodity with a high price premium (Peck and Knicker- Foliar Urea Applications Increase bocker, 2018). To ensure the success of these new enterprises, additional research is needed Yeast Assimilable Nitrogen to determine how orchard management af- fects cider quality. In particular, there has Concentration and Alcoholic been minimal research on how preharvest nitrogen fertilizer management in apple or- chards might affect economic returns, fruit Fermentation Rate in ‘Red Spy’ Apples quality, fermentation dynamics, and the sen- sory attributes of the finished product. Used for Cider Production Nitrogen is a macronutrient that is essen- tial for plant metabolism, and its role in fresh- Adam D. Karl and Michael G. Brown market apple production is well studied School of Integrative Plant Science, Horticulture Section, Cornell University, (Cheng and Fuchigami, 2002; Lea and 121 Plant Science Building, Ithaca, NY 14853 Beech, 1978; Merwin and Stiles, 1994; San- tos et al., 2016; Wargo et al., 2003). From Sihui Ma, Ann Sandbrook, and Amanda C. Stewart budbreak to just after bloom, apple trees use Department of Food Science and Technology, Virginia Polytechnic Institute stored nitrogen reserves to support vegetative and State University, HABB1 Room 401F, 1230 SW Washington Street, growth, flowering, and fruit set (Cheng and Blacksburg, VA 24060 Raba, 2009). For the remainder of the grow- ing season, apple trees acquire nitrogen from Lailiang Cheng the soil or from nitrogen applied to the leaf School of Integrative Plant Science, Horticulture Section, Cornell University, surface (referred to as foliar applications). Nitrogen-deficient orchards are found in 120 Plant Science Building, Ithaca, NY 14853 many regions of the world, and it is a com- Anna Katharine Mansfield mon practice to apply nitrogen fertilizers through both ground and foliar means Department of Food Science, Cornell AgriTech, 107 Food Research Lab, (Merwin and Stiles, 1994). Foliar nitrogen Geneva, NY 14456 applications in the late summer and fall ensure sufficient reserves for the following Gregory M. Peck spring. During the fall, Dong et al. (2005) School of Integrative Plant Science, Horticulture Section, Cornell University, found that leaf nitrogen uptake is more effi- 121 Plant Science Building, Ithaca, NY 14853 cient than root uptake. When nitrogen is limited in apple trees, Additional index words. amino acid, foliar fertilization, hard cider, hydrogen sulfide, Malus there is often a decrease in fruit size, yield, 3 domestica Borkh., primary amino nitrogen, yeast assimilable nitrogen and soluble solid concentrations (Xia et al., Abstract. Yeast assimilable nitrogen (YAN) can be a limiting nutritional factor for Saccharo- 2009). Conversely, excessive nitrogen fertil- myces cerevisiae yeast when fermenting apple (Malus 3domestica Borkh.) juice into hard cider. ization in mature orchards can lead to de- Endogenous YAN concentrations in apples are often below the recommended thresholds to creased fruit quality by reducing flesh completely use all of the fermentable sugar and minimize the production of off-flavors, such as firmness and red peel color (Fallahi et al., hydrogen sulfide. Cider producers supplement apple juice with exogenous nitrogen to increase 1997; Wang and Cheng, 2011). As a diag- YAN. Urea, commonly applied to apple orchards to increase fruit size and yields, was tested for nostic tool, total nitrogen concentration is its ability to increase endogenous apple juice YAN. Starting 6 weeks before harvest in 2017 and measured from mature apple leaves once 2018, a 1% urea solution was applied to ‘Red Spy’ apple trees one, three, or five times to create terminal shoot growth has subsided. A target low-, medium-, and high-rate treatments, respectively. Relative to the control, the high leaf nitrogen content between 2.2% and 2.4% treatment increased YAN by 229% in 2017 and by 408% in 2018. More than 90% of the is recommended for mature hard-fleshed and YAN in all juice samples was composed of primary amino nitrogen (PAN). Among all processing cultivars (Stiles and Reid, 1991). treatments, PAN mostly comprised asparagine, and as urea applications increased, the relative Currently, there are no standard nitrogen concentration of asparagine also increased. Aspartic acid and then glutamic acid were the fertilization guidelines for cider apple pro- second and third most abundant amino acids in all treatments, respectively, but comprised less duction in the United States. of the total PAN as the number of urea applications increased. Soluble solid concentration, pH, Increasing fruit nitrogen concentration titratable acidity, and total polyphenol concentration were not different among treatments. may be beneficial for cider fermentation. There was a positive correlation between increased urea application rate and the maximum Specifically, YAN comprises the nitrogen fermentation rate, which resulted in a shorter fermentation duration. Increasing the number of compounds that are metabolized by Saccha- urea applications was also correlated with greater hydrogen sulfide (H S) production in juice romyces cerevisiae yeast for cell division and 2 growth during alcoholic fermentation of fruit fermented from fruit harvested in 2017 but not for fruit harvested in 2018. No residual H2Swas found in the finished cider from any treatment. Increasing the number of urea applications was juice (Bell and Henschke, 2005). Yeast as- estimated to be less expensive than supplementing the juice with Fermaid OTM. There would similable nitrogen is composed of free amino have been no cost savings if Fermaid KTM was used as an exogenous nitrogen source. Foliar urea nitrogen (FAN), ammonia ions, and some applications were estimated to be more expensive than supplementing juice with diammonium short oligopeptides (Bell and Henschke, phosphate. This study demonstrated that foliar urea applications can effectively increase YAN 2005). Unlike wine grapes (Vitis vinifera concentration in cider apples while not negatively affecting other juice quality attributes. L.) that contain a variable, but large, propor- tion of YAN as ammonia, FAN comprises the vast majority of YAN in apples (Bell and Henschke, 2005; Ma et al., 2018). Fur- Production of hard cider (fermented apple ing beverage sector in the country during this thermore, although composition of amino juice) in the United States has grown from period (Alcohol and Tobacco Tax and Trade acids vary by apple cultivar, asparagine is 23.1 million liters in 2007 to 173.4 million Bureau, 2008, 2018). The growth of the cider typically the most abundant amino acid in the liters in 2017, making cider the fastest grow- industry offers apple (Malus ·domestica fruit (Ma et al., 2018). However, ammonia is 1356 HORTSCIENCE VOL. 55(8) AUGUST 2020 preferentially used and more rapidly metab- producers prefer adding yeast nutrient sup- Table 1. Treatment timing of foliar urea applications olized by S. cerevisiae than most organic plements with greater PAN than ammonia on cv. ‘Red Spy’ apple trees grown in Lansing, nitrogen sources (Crepin et al., 2012). There concentrations to limit initial alcoholic fer- NY. are no studies that specify the target YAN mentation rates, temperature increases from Weeks before harvest concentration for the cider industry; how- increased metabolic rates that can stress yeast Treatment 65432 ever, recent studies have shown that apple cells, and changes to sensory character imparted Control juice is often deficient in YAN according to by high ammonia additions (Charoenchai Low · standards developed for the wine industry. et al., 1998; Tahim and Mansfield, 2019). Medium ··· Although, recommendations vary by region, These high PAN supplements are typically High ····· cultivar, and wine style, 140 mg·L–1 is often composed of inactivated dry yeast cells and cited as the minimum YAN concentration for contain minerals, sterols, and vitamins im- successful fermentations (Bell and Henschke, portant for yeast metabolism (Angeles Pozo- orchard was planted in 2012 with 1.2 m 2005; Ma et al., 2018; Peck et al., 2016; Tahim Bayon et al., 2009). between trees and 3.7 m between rows. Over and Mansfield, 2019). Preharvest foliar nitrogen applications in the course of the experiment, the trees were Yeast assimilable nitrogen deficiencies vineyards have been found to be effective uniformly managed using standard pest con- can result in slow or incomplete wine and means of increasing grape YAN concentra- trol and pruning practices for the region cider fermentations (Bell and Henschke, tions. For example, Moss (2016) found a (Agnello et al., 2019). 2005; Vilanova et al., 2007). The composi- cumulative application of 30 kg·ha–1 of ni- Experimental design. Three foliar nitro- tion of YAN can impact chemical and sen- trogen applied as urea to foliage was capable gen treatments were applied starting 6 weeks sory attributes of the finished product of more than doubling the YAN concentra- before harvest in both years of the study. The (Boudreau et al., 2017a; Herraiz and Ough, tion in ‘Sauvignon blanc’ and ‘Petit Man- study was a randomized complete block de- 1993). In particular, low YAN is known to seng’ grapes. These foliar applications were sign, with six replications of single tree ex- increase the production of hydrogen sulfide also more effective in increasing must YAN perimental units. Different trees within the –1 (H2S), a malodorous
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