Determination of fruit size and weight is important in horticulture to estimate yield and the proportion of fruit in each grade (Zhang and Robson, 2002). Typically, the distribution of apple fruit weight fits a normal distribution (Webb et al., 1980); although some exceptions exist (Clarke, 1990; Judd et al., 1989), they appear to be tolerated statistically (Zhang and Robson, 2002).
Although fruit weight distributions are frequently reported in the literature, fruit firmness distributions have not been reported. This is particularly surprising given that fruit firmness is such an important quality attribute for both apples (Harker et al., 2008) and cherries (Dever et al., 1996). Means and sds may be sufficient to characterize firmness if the underlying distributions are normally distributed; however, the shape of the firmness distributions for cherry cultivars is unknown.
For apples, firmness, along with juiciness and crispness, is associated with freshness (Péneau et al., 2007). For cherries, the visual characteristics of the fruit and stem and absence of defects and injuries are key visual indices of freshness (Drake et al., 1989). However, the United States Department of Agriculture (USDA) uses firmness along with other quality factors to establish cherry grades (USDA, 2005).
Firmness and size are also selection criteria in the cherry breeding program at the Summerland Research and Development Center (SuRDC, Summerland, BC, Canada), along with self-fertility, rain-splitting resistance, sweetness, stem quality, and other traits (Hampson, 2014). In general, firmer fruit are preferred by consumers. Recently, Hampson et al. (2014) determined the ideal range for cherry firmness, using Just-About-Right methodology. Cherries with firmness values less than 2.56 N·mm−1 and greater than 4.71 N·mm−1 were considered “too soft” and “too hard,” respectively by consumers.
Texture measurements are influenced by the product water status, cell wall physical properties, and tissue structure—all of which act together in the characterization of firmness (Landahl et al., 2004). Research has investigated the mechanical properties of fruit cells to gain insight into the strength of tissues, by evaluating tensile properties (De Belie et al., 2000; Harker et al., 1997) and compression characteristics (Lin and Pitt, 1986); however, there is no research available for cherries.
Understanding the agronomic factors that contribute to size and firmness variation is important in agricultural research to establish appropriate cultural practices and understand environmental influences (Searle et al., 2003). Tijskens et al. (2007) have developed a model to relate harvest maturity of nectarines with firmness, using a “biological shift factor” to account for differences due to season, cultural practices, and growing conditions. Such work allows for fruit to be nondestructively graded, so that sorting and handling can proceed according to the firmness category; however, no such model is available for cherries.
Lechaudel et al. (2007) have developed models to show how environmental factors influence the accumulation of water, structural and nonstructural dry matter during fruit development of mangos. These models tracked changes in fruit growth using parameters linked to wall extensibility and yield threshold pressure, as well as evaluated changes in shrinkage/swelling using surface conductance of the fruit skin, the elastic modulus, and the hydraulic conductivity of the fruit; however, no such models are available for cherries.
Therefore this research was undertaken to 1) characterize sweet cherry firmness distributions with descriptive statistics, exponential models, and area-under-the-curve calculations, and 2) use the calculated parameters along with multivariate statistics to gain insight into the effects of cultivar, treatment, storage time, and year.
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Meansz of six parameters for characterizing the firmness distributions of sweet cherry samples from 2013 (n = 24) and 2014 (n = 24).
Climatic conditions (mean temperature and precipitation) over the growing season for the 2 years.z