Physical Chemical and Microbial Properties of Apples

Several investigators have shown that fruit cultivars differ in their susceptibilities to P. expansum rot and to patulin formation in the apple tissue. Jackson et al. [87] reported that cider pressed from ground-harvested Red Delicious apples had significantly higher levels of patulin than cider prepared from other apple cultivars (Golden Delicious, Granny Smith, Fuji, Gala, MacIntosh, Red Rome). Of the four cultivars studied by Spotts and Mielke [88], Royal Gala apples were found to be most resistant to decay from P. expansum, while Fuji apples were least resistant. These results, along with those published by others [65,70,76,86,89,90], indicate that apple cultivars differ substantially in susceptibility to blue mold rot. The differences in cultivars may be due to unique physical and chemical characteristics of each cultivar such as skin thickness and strength, flesh firmness, pH of flesh, sugar levels, levels of antimicrobial compounds, and other apple constituents [86,89,91]. Apple cultivars with an open calyx are at greater risk for patulin development within the apple core [92]. Since core rot is often not detected, juice or cider pressed from affected fruits may have high patulin levels.

Penicillium expansum can be isolated from the surface of unblemished fruit, although the fungus typically does not grow until it is able to make contact with the flesh of the fruit. The pathogen enters the fruit through skin breaks caused by bird, insect or weather-related damage, improper handling, and through damaged lenticels near bruised areas [73]. Several investigators reported greater patulin levels in apple juice pressed from damaged than from sound fruit [76,87,93,94]. Susceptibility to surface wounds and bruising is influenced by apple cultivar, but it also may be a function of degree of maturity of the fruit, since skin layers soften during the ripening process [3,67]. Other factors such as mineral imbalances (e.g., high nitrogen, low calcium) are believed to increase susceptibility of fruit to infection. Mineral imbalances are caused by improper fertilization, excessive or too little rain, and poor soil conditions [95].

At present, the effects of the chemical composition of fruit on patulin production are not well understood. Apples are composed of a complex mixture of sugars (primarily fructose, glucose, and sucrose), oligosaccharides, and polysaccharides, together with malic, quinic, and citric acids, polyphenols, amides and other nitrogenous compounds, soluble pectin, vitamins, minerals, water, and a variety of esters. The relative proportions of these components depend on the apple cultivar, the conditions under which the apples were grown, the state of maturity of fruit at the time of pressing, and extent of damage to the fruit. Patulin is produced over the range of pH values found in apple juice (3.2 to 3.8) and is stable at these pH values, but degrades at higher pH values [32,91,96]. McCallum et al. [36] found that the concentration of patulin formed in juice was correlated negatively with the pH value. Prusky et al. [97] reported that Penicillium spp. colonization and growth are enhanced by low pH in the host tissue. They also found that P. expansum actively reduced the pH during decay development by causing accumulation of fumaric and gluconic acid in the fruit tissue.

The mineral content of apples may influence degree of decay by postharvest pathogens. Calcium is believed to be the major mineral nutrient affecting apple quality and storage life [98,99]. The effect is thought to be partly due to the role of the mineral in preventing physiological disorders in the developing fruit [99-102]. Calcium is also believed to improve fruit firmness by forming complexes with pectic substances in the cell wall [101,103]. The influence of other minerals and antimicrobial compounds in apples (e.g., phenolic compounds) on their susceptibility to fungal rot and mycotoxin production is not known at this time. Research is needed to determine how the apple constituents affect patulin formation.

Microbiological factors can influence patulin formation. The microbiological flora on apples and other fruit differs according to geographic area, climatic conditions, pesticide or fungicide treatments, cultivar, presence of competitive microorganisms, harvest method, and postharvest treatments [69]. Spores of P. expansum are found in soil, on plant surfaces, and in air and are transferred to dump tank and flume water in packinghouses by contaminated wooden picking bins and fruit [104]. Early findings by Sommer et al. [74] indicate that the presence of a patulin-producing species does not necessarily imply patulin production in apples. Factors like incubation temperature, lesion size, and substrate, also play important roles. Substantial differences have been noted among P. expansum strains in terms of growth kinetics and patulin production [86]. McCallum et al. [86] found that highest patulin levels were those from isolated strains displaying aggressive growth and profuse mycelial development.

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