Advantages of abrasive pre-treatment application in plum drying process

Cilj istraživanja je bio utvrditi utjecaj sorte i primjene abrazivnog predtratmana na vrijeme sušenja, stabilnost biološki aktivnih spojeva, senzorska svojstva i nutritivni sastav suhe šljive u odnosu na konvencionalne predtretmane. Istraživanje je provedeno na plodovima triju sorti šljive ꞌBistricaꞌ, ꞌPresidentꞌ i ꞌTopend plusꞌ ubranim u pokusnom nasadu Zavoda za voćarstvo Hrvatskog centra za poljoprivredu, hranu i selo u Donjoj Zelini. Nakon provedenih pomoloških (masa, visina, širina i debljina ploda) i fizikalno-kemijskih analiza (tvrdoća, topljiva suha tvar, ukupna suha tvar, pH vrijednost, ukupna kiselost i boja ploda) u vrijeme berbe, na plodovima namijenjenim sušenju izvršeno je sortiranje, pranje i otkoštičavanje plodova. Prije postupka sušenja plodovi šljive tretirani su s jedanaest različitih kombinacija predtretmana: abrazijom pri tri različita vremena vrtnje cilindra 5, 10 i 15 minuta obloženog inertnim abrazivnim materijalom PW 400 uz konstantnu rotaciju cilindra od 120 rpm, potapanjem plodova u lužnatu otopinu KOH različitih koncentracija (0,5, 1 i 1,5 %) pri temperaturi 22 i 60 °C u intervalu od 60 sekundi, kao i potapanje plodova u destiliranu vodu pri istim temperaturama i istom vremenskom intervalu. Plodovi su sušeni u komornoj sušnici pri konstantnoj temperaturi od 42 °C i brzini strujanja zraka 2,0 msˉ¹ do 35 % vode u plodu. Razlike između ocjene kvantitativne deskriptivne analize i fizikalno-kemijskih svojstva ploda različitih sorti s obzirom na predtretmane statistički su analizirane odgovarajućim modelima analize varijance (ANOVA). Sva analizirana pomološka svojstva razlikovala su se između istraživanih sorti šljive. Predtretman je značajno utjecao na vrijeme sušenja. Najkraće vrijeme sušenja utvrđeno je kod predtretmana abrazijom 15 minuta kod svih sorti, a slijede predtretmani s KOH na 60 °C svih triju koncentracija (0,5, 1 i 1,5 %) te abrazijom 5 i 10 minuta. Na vrijednosti morfoloških svojstava osušenih plodava značajno je utjecala sorta. Predtretman je značajno utjecao na boju osušenog ploda. Veća degradacija boje kožice ploda utvrđena je kod uzoraka koji su tretirani s KOH svih koncentracija na temperaturi od 60 °C, tj. ista je imala nižu kromatsku vrijednost L* i više kromatske vrijednosti a* i b*, plodovi su razvili tamnu boju. Plodovi tretirani abrazijom u svim vremenima vrtnje zadržali su poželjnu svojstvenu boju šljive, tj. kod njih je izmjerena viša kromatska vrijednost L* i niže kromatske vrijednosti a* i b*. Sorta 'Bistrica' u interakciji sa svim predtretmanima najbolje je ocijenjena za poželjna senzorna svojstva boje, okusa, sočnosti, arome i žvakavosti osim za svojstvo mirisa koje je najbolje ocijenjeno kod sorte 'Topend plus', dok su nepoželjna svojstva izražena kod sorte 'President'. Predtretmani abrazijom pozitivno su utjecali na poželjna senzorska svojstva: boju, okus, sočnost, aromu i gumoznost/žvakavost kod svih sorti u istraživanju. Udio ukupnih fenola, hidroksicimetnih kiselina i flavonola značajno se razlikovao s obzirom na sortiment i ovisio je o predtretmanima. Predtretmani s KOH koncentracije 1,5 % pri obje temperature pozitivno su utjecali na očuvanje bioaktivnih spojeva. Sorta 'Topend plus' imala je najveću količinu ukupnih fenola, hidroksicimetnih kiselina i flavonola kod svih predtretmana u odnosu na druge sorte.Plum (Prunus domestica L.) is mostly represented in the EU market as fresh or industrially processed as dried fruit. Production technology of dried plums is based on fruit drying at high temperatures between 85 and 90 °C, with less fruit water content between 20 and 26 %. This type of fruit dehydration at high temperatures has an influence on fruit quality decrease in a form of texture, colour, taste and bioactive components loss along with high energy consumption. In order to keep the texture, colour and taste as well bioactive components of dried fruits, drying at lower temperatures of 50 °C, with higher final fruit water content of 35 % is used. Essential parameters in cultivar selection intended for drying are fruit size, congruent ratio of total soluble solids, total acids, and easiness of stone removal as well initial fruit water content. Plum fruits intended for drying have to be picked at optimum harvest window which is determined during continuous monitoring of basic pomological characteristics such as fruit mass and size as well as physical and chemical parameters - fruit firmness, total soluble solids content, total acids and pH. Plum drying process is slow and energy demanding process due to waxy layer that covers plum fruit skin. Removal of waxy layer which enables free movement of water is therefore needed by means of fruit surface treatment. Pretreatments are dissolving the waxy layer which consequently creates microscopic fractures thus enabling water movement. Pre-treated fruits are drying significantly faster from untreated fruits. Cultivars used in this research – ‘Bistrica’, ‘President’ and ‘Topend plus’, were treated with 11 different pre-treatments: abrasion of 5, 10 and 15 minutes, KOH in solution concentration of 0.5, 1.0 and 1.5 % at temperature of 22 °C, KOH in solution concentration of 0.5, 1.0 and 1.5 % at temperature of 60 °C and with distilled water at two temperatures (22 and 60 °C). Results Higher precipitation during the vegetation season in which the research was conducted as well as lower than average temperatures during fruit ripening had influence on fruit firmness, pomological characteristics, dry matter content, total soluble solids content and total acids. Pomological characteristics of fresh plum fruit (weight, height, width and thickness of fruit as well as weight, length, width and thickness of stone) are cultivar dependant. Cultivars 'President' and 'Topend plus' had highest weight as well as height, width and thickness of fruits in comparison to cv 'Bistrica'. For all cultivars in the research a significantly shorter drying time of samples pre-treated with abrasion of 10 and 15 minutes, as well as with alkaline solution (KOH) of 1.0 and 1.5 % concentration at 60 °C was determined. Parameters such as temperature, solvent types and concentration have a significantly influence on drying time of plum fruits. Higher temperature (60 °C) and usage of solvent of higher concentration (1.5 %) had significantly reduced drying time of plum fruits of all cultivars in the research. Values of morphological characteristics of dried fruits were significantly influenced by pretreatment. Cv ‘Bistrica’ had higher weight, height, width and thickens of dried fruit at samples pre-treated with abrasion; for cv ‘President’ it was highest at KOH pre-treatments of all concentrations at temperature of 22 °C; for cv ‘Topend plus’ it was the highest at pre-treatment with KOH of all three concentrations at temperature of 60 °C. Abrasive pre-treatment had a positive influence on sensory characteristics of dried fruits, especially on intensity of dark blue/black colour, plum-like taste, sweetness, juiciness, plumlike aroma and chewiness of all three cultivars in the research. The highest sensory scores were noted for dried fruits treated with abrasive pre-treatment in duration of 10 minutes. Cv ‘Bistrica’ irrespective of pre-treatment had the highest scores for following sensory characteristics: intensity of dark blue/black colour, plum-like taste, sweetness, juiciness and plum-like aroma as well as for chewiness. Cv ‘Topend plus’ had the higher scores for flavour and firmness/texture sensory characteristics of dried fruits. The highest content of bioactive compounds (total phenols, total hidroxycinnamic acids and total flavonols) was determined in dried fruits of all three plum cultivars treated with chemical and thermal pre-treatments, whilst the lowest values of mentioned bioactive compounds were determined in fruits pre-treated with abrasion. KOH treatment of 1 and 1.5 % concentrations at temperatures of 22 and 60 °C had positive influence on the content of bioactive compounds. Cv ‘Topend plus’ had the highest content of all researched bioactive compounds irrespective of used pre-treatment in comparison to cv ‘Bistrica’ and ‘President’. Conclusions Advantages of chemical pre-treatment application are manifested in significant shortening of drying time and in preservation of bioactive compounds, whilst the main disadvantage lies in waste disposal of used chemical reagents and their negative environmental impact. Use of innovative and ecologically acceptable technique of abrasive pre-treatment of plums fruits shortens the drying time in comparison to conventional pre-treatments (chemical and thermal). Efficiency comparison of different pre-treatments used within this research leads to a conclusion that abrasive pre-treatments contribute to preservation of sensory and qualitative fruit characteristics as well as to nutritive values. This type of plum fruits pre-treatments can in the near future replace completely chemical pre-treatments of plum fruits