After-cooking darkening (ACD) is an undesirable potato tuber trait, problematic in processed potato products (Wang-Pruski and Nowak, 2004). It is characterized as a change from a tuber's normal flesh color to gray, blue, purple, or black (Hughes, 1962). ACD is most common in boiled or steamed potatoes but is also problematic in processed products such as oil-blanched French fries, dehydrated potatoes, canned potatoes, prepeeled potatoes, and reconstituted dehydrated potatoes (Armstrong, 1963; Dale and Mackay, 1994; Smith, 1987). ACD is caused by the oxidation reaction of the ferrous–chlorogenic acid complex, resulting in a bluish gray compound, ferridichlorogenic acid (Wang-Pruski, 2006). Organic acids such as citric acid are believed to play a role in competing with chlorogenic acid for iron (Fe) and, as a result, decreasing the severity of ACD (Hughes and Swain, 1962a). In general, the degree of ACD is highest at the stem end compared with the bud end of the potato tuber, whereas the center has the lowest degree of ACD (Wang-Pruski, 2006). Previously, it has been suggested that Fe, phosphorus (P), and calcium (Ca) may be linked to the occurrence and severity of ACD (Hughes and Swain, 1962b), but there has been limited investigation to confirm this theory.
Elemental analyses of potato leaves and tubers has been used to provide information on the geographical origin of tubers (Anderson et al., 1999; Di Giacomo et al., 2007) as well as on their sensory (Whittenberger and Nutting, 1950; Zaehringer et al., 1969) and processing quality (Arteca et al., 1980; Hughes and Swain, 1962b, Mohr et al., 1984). However, only a limited amount of research has been done to determine the distribution of elements within each tuber and correlate these to tuber quality. Earlier studies on the distribution of elements in tubers were conducted in the 1960s and early 1970s by Hughes and Swain (1962a), Macklon and DeKock (1967), Johnston et al. (1968), and Bretzloff (1971) and yet detailed information regarding the concentration and distribution of elements in tubers remains incomplete.
Inductively coupled argon plasma (ICAP) analysis has traditionally been the technique used to study element concentrations of plant tissue (Anderson et al., 1999; Cubbada and Raggi, 2005). This technique yields, reliably and accurately, simultaneous quantifications of many elements (Andrasi et al., 1998). However, this method is destructive and involves extensive sample preparation before the ICAP measurements. Another method to study surface distribution of chemical elements, energy-dispersive x-ray spectrometry (EDS) coupled to the scanning electron microscope (SEM), has also been used to measure elements in plant tissue (LeRiche et al., 2006; Mohr et al., 1984; Takahashi et al., 2006). Although SEM/EDS has the potential to quantify elements, it has been mainly used to determine the spatial distribution and relative concentrations of elements in plant tissues and is thus useful for measuring gradients of elements within portions of a plant (LeRiche et al., 2006; Takahashi et al., 2006). The current study used both variable pressure (VP)-SEM/EDS and ICAP as complementary analytical techniques to measure the effects of cultivar, fertilizer, and tuber segment on the element distribution of tubers.
The concentration of elements in tubers can be affected by both agronomic treatments (Haase et al., 2007; Mondy and Ponnampalam, 1986; Warman and Havard, 1998; White et al., 2009; Wszelaki et al., 2005) and cultivar (Randhawa et al., 1984). Therefore, the first objective of this study was to present a broad evaluation of the concentration and spatial distribution of most of the essential plant nutrients [P, Ca, magnesium (Mg), potassium (K), sulfur (S), Fe, copper (Cu), sodium (Na), zinc (Zn), boron (B), manganese (Mn), aluminum (Al), silicon (Si), and chlorine (Cl)] in potato tubers of ‘Shepody’ and ‘Russet Burbank’. The second objective was to determine if the concentrations of these elements could be correlated with the ACD.
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