Changes in the Fruit Quality Parameters of Medlar Fruit (Mespilus germanica L.) after Heat Treatment, Storage, Freezing or Hoarfrost

The present study deals with the comparison of traditional fruit processing methods on medlar fruits and their effects on sugar content, organic acids, and phenolic composition in the medlar fruit variety ‘Domača nešplja’. The study aimed to analyze which processing methods can be used to make technologically mature medlar fruits that are not yet suitable for consumption edible and to maintain their good chemical quality. The two major sugars in medlars are fructose (59.30 g/kg FW) and glucose (54.43 g/kg FW), and the most abundant organic acids present are malic (8.44 g/kg FW) and quinic acid (8.77 g/kg FW). A total of 38 different phenolic compounds were identified in the medlar fruits: 13 phenolic acids, 9 flavanols, 1 flavone, 3 flavanones, and 12 flavonol glycosides. To explicate: phenolic acids (532.85 mg/kg FW) and flavanols (375.21 mg/kg FW) predominated; neochlorogenic acid had the highest content among phenolic acids; and procyanidins were the most abundant flavanols. The analysis observed statistical differences in metabolite content amongst fruits treated differently (technologically ripe fruits (harvested from the three fruits), edible fruits (technologically ripe fruits stored at 8 °C for 25 days), fruits exposed to the hoarfrost (temperature −1 °C to −4 °C), fruits heated at 60 °C (3 h), and frozen fruits (at −20 °C for 2 months). The lowest levels of fructose (191.77–195.1 g/kg DW) and sorbitol (29.35–31.3 g/kg DW) were detected in the heated and edible fruits. Edible fruits had a 30% lower content of organic acids than technologically ripe fruits and a five times lower content of flavanols, whereas flavonols had an 18.7 times lower content of phenolic acids than technologically ripe fruits. Heating the fruits to 60 °C resulted in a 40% increase in total phenolic compounds in medlars. The results of the study indicate that exposure of medlar fruit to hoarfrost does not significantly affect the chemical quality of the fruit and only minimally alters the composition of sugars, acids, and phenolic compounds. The processing of medlar fruit with hoarfrost, therefore, remains the most suitable method of fruit bletting.