Crop Load Density Affects 'York' Apple Juice and Hard Cider Quality
To assess the impact crop load has on hard cider chemistry, 'York' apple (Malus3domestica Borkh. trees were hand thinned to three different crop loads: low [two apples per cm branch cross sectional area (BCSA) medium (four apples per BCSA) and high (six apples per BCSA) Higher crop loads produced smaller, less acidic fruit that were slightly more mature. In juice made from fruit from these treatments, the total polyphenol content did not differ at harvest, but, after fermentation, the medium crop load had 27% and the high crop load had 37% greater total polyphenol content than the low crop load. Yeast assimilable nitrogen (YAN) concentration in juice made from fruit from the low crop load treatment had 18% and 22% greater than the medium and high crop load, respectively. YAN concentrations in juice from the medium and high crop load treatments were similar. Our results provide apple growers and hard cider producers with a better understanding of how apple crop load impacts YAN concentrations in juice and total polyphenol concentrations in juice and cider. Hard cider is an alcoholic beverage produced from fermented apple juice or apple juice concentrate. Domestic cider consumption has increased more than 850% in the last 5 years and there are now over 550 cider producers in the United States (TTB, 2007 14; Brown, 2016) The vast majority of cider produced in the United States is made from apple cultivars that were originally planted for fresh or processing markets (Peck and Miles, 2015) Culinary apples do not have all of the fruit quality characteristics desired by cider producers, but many of the desired cider apple cultivars have not been documented as being widely planted in the United States (Miles et al. 2015; Peck, 2012; U.S. Apple Association, 2015) In the United States, where the production of traditional hard cider apple cultivars has lagged behind the increase in cider sales, methods to increase cider quality from existing apple cultivars are needed. Fruit quality attributes that are important for culinary apple production include low incidence of damage and decay, fruit shape and size (typically measured by fruit mass) peel color, flesh firmness, soluble solid concentration (SSC) titratable acidity (TA) and pH, and flavor (La Belle, 1981) Along with the starch pattern index (SPI) and internal ethylene concentration (IEC) fruit quality factors are often measured to gauge harvest maturity (Watkins, 2003) For cider production, fruit quality attributes also include polyphenol and YAN concentrations in the fruit, and juice yield, while cosmetic attributes such as color, shape, and size are much less important (Lea, 1996) Apple orchard management practices that focus on fruit quality characteristics that are desirable for cider production are needed. Specifically, most apples commercially grown in the United States have low YAN and polyphenol concentrations (Thompson Witrick et al. 2014) While both exogenous nitrogen and polyphenols (i.e. 'enological tannins' may be added to increase their concentration in cider, the sensory impact of addition of these products to cider warrants further investigation. For example, the addition of commercially available exogenous tannins to red wine has been shown to increase the measured total polyphenol concentration, but they did not always lead to improvement in sensory character (Harbertson et al. 2012) As such, increasing endogenous polyphenol concentration in fruit remains the generally preferred approach to achieve desired sensory characteristics for wine and cider. In European wine grape (Vitis vinifera L. production, measurable improvements in fruit quality have been achieved through adjusting the relationship between fruit yield and vegetative growth, often referred to as crop load. Grape cluster crop load has been shown to impact secondary metabolism in grape berries, which can in turn impact wine chemistry, aroma, and flavor. For example, SSC was greater in 'Chambourcin' grapes that were from vines with reduced fruit clusters (Dami et al. 2006) Similarly, lower crop loads for 'Sauvignon blanc' grapevines resulted in wine that had more favorable sensory scores (Naor et al. 2002) A study of 'Shiraz' grapevines under five training systems in the Barossa Valley of Australia demonstrated that grape berry anthocyanin and polyphenol concentrations decreased with increasing crop load (Wolf et al. 2003) However, a point is reached when continuing to decrease crop load results in decreased yield and increased production cost, but no further increase in wine quality (Berkey et al. 2011; Bravdo et al. 1985; Preszler et al. 2010) Although horticultural practices for apples and grapes are quite different, wine grape growers exert a tremendous amount of effort optimizing fruit quality to make their crop more desirable to their buyers. For these reasons, crop load targets are often specified in vineyard management with the goal of maintaining optimal fruit quality for winemaking (Wolf, 2008) With the increased utilization of apples for cider production, it is necessary to more fully understand how orchard management decisions, such as crop load density, impact cider quality. The development of crop load management practices can be used by orchard managers to improve cider produced from culinary apples. The goal of this project was to assess the impact of three different crop load densities on fruit and cider quality. Materials and Methods Field treatments were conducted in a 14year old 'York Imperial (Ramey)'M.9' orchard at the Alson H. Smith, Jr. Agricultural Research and Extension Center (Winchester, VA) in 2014. 'York' apples are primarily used for processing into products such as juice, vinegar, and applesauce. On 16 June 50 d after full bloom) five single tree replications of each of the three crop load treatments were implemented by hand thinning apples to the specified crop load density level on three branches per tree. The low crop load treatment was thinned to two apples per cm BCSA, the medium crop load was thinned to four apples per BCSA, and high crop load was thinned to six apples per BCSA. The rest of the tree was thinned to about the same crop load density by visually assessing the crop load on the three branches and replicating that spacing. Fruit was only sampled from the three branches that were hand thinned to the precise number of fruit per BCSA. The experiment was blocked based on a visual assessment of whole tree crop load before the implementation of the treatments. Treatmentswere randomly assigned Received for publication 23 May 2016. Accepted for publication 18 July 2016. We thankDavid Carbaugh, AbbyKowalski, Taylor Mackintosh, Hengjian Wang, Sihui Ma, Molly Kelly, Tina Plotka, Ken Hurley, Ann Sandbrook, and Brian Wiersema for technical assistance. Funding for this work was provided by the Virginia Department of Agriculture and Consumer Services, the Virginia Agricultural Experiment Station, and the Hatch Program of the National Institute of Food and Agriculture, U.S. Department of Agriculture. Corresponding author. E mail: gmp32@cornell.edu. 1098 HORTSCIENCE VOL. 51(9) SEPTEMBER 2016 to trees with similar crop load levels. The orchard was not irrigated or fertilized during the course of this experiment and weed, insect, and disease management was executed according to regional recommendations (Pfeiffer et al. 2014) An initial harvest was conducted on 29 Sept. to assess the relative maturity of the treatments using a pooled 10 fruit subsample from the treated branches on each tree which were analyzed for IEC, SPI, fruit firmness, SSC, and TA as described in ThompsonWitrick et al. (2014) Briefly, apples were weighed and visually assessed for percent red blush (0 100% Flesh firmness was measured on the same samples, after removing part of the peel at two locations along the equator of each apple, using a Fruit Texture Analyzer penetrometer [GEURuss Manufacturing (Pty) Ltd. Strand, South Africa] fitted with an 11.1 mm diameter Effegi tip. The SPI was determined by staining the stem side of an equatorial cross section of the apples with iodine solution (0.22% w/v iodine, 0.88% w/v potassium iodine) and rating patterns against a chart, where 1 100% staining and 8 0% staining (Blanpied and Silsby, 1992) IEC was measured on a 1 mL sample removed from the core cavity of the apple using a gas chromatograph (Agilent 7890; Wilmington, DE) equipped with a flame ion detector. The calyx half of each apple was juiced in a Champion Juicer (Lodi, CA) and SSC was measured using a digital PAL 1 refractometer (Atago U.S.A. Inc. Bellevue, WA) and reported as percent Brix. TA was measured by titrating a 5 mL juice aliquot against a 0.1 N NaOH solution to an endpoint of pH 8.1 with an autotitrator (848 TitrinoPlus, Herisau, CH) A separate 50 mL juice subsample was frozen 80 C) and shipped to the Enology and Fermentation Laboratory in the HABB1 Building at Virginia Tech (Blacksburg, VA) for total polyphenol analysis as described below. On 10 Oct. an additional 10 apples per tree were analyzed for the same parameters as mentioned above. The remaining apples from the treated branches were also harvested and transported to the Enology and Fermentation Laboratory and stored at 4 C for 1 week before processing. All other apples from each sample tree were harvested, counted (as were fruit that dropped prematurely) and reported as crop load on a square centimeter trunk cross sectional area (TCSA) basis. On 17 Oct. apples from four of the five field replications were cleaned in a rod and reel washer and ground into a pulp using a hammer mill (RH HM100; Herbold Meckesheim USA, Smithfield, RI) There was not enough fruit from the fifth replication to produce a sufficient volume of juice for fermentation. As the apple pulp was produced, it was layered evenly onto a custombuilt rack and cloth press and pressed until juice no longer