Effects of electric infrared heating with light source penetration on microbial and entomological loads of dried currants and their organoleptic characteristics

A series of field bioassays was conducted to evaluate the effects of electric infrared (IR) penetration on microbial disinfection and insect control as well as on the organoleptic characteristics of dried black currants (Corinthian currants, Vitis vinifera L. var. Apyrena). A continuous IR processor was placed in the production line of an industrial facility and was used for the field assays. In a series of bioassays, currants were fed into the IR processor and examined with and without process agitation at different treatment parameters including retention times (ranging from 30 to 35 s) and various peak temperatures (45, 50, 55, 60, 65, 70 and 75 °C). Samples of processed currants were randomly selected for microbial analysis, as well as for determination of organoleptic characteristics. Infrared treatment resulted in a significant reduction of total viable counts, yeasts and moulds by factors ranging from 10 to 300. Increase of treatment time and peak temperature was positively correlated with microbial reduction. Sensory attributes were not affected up to 60 °C, while crystalline texture and overall acceptance were affected by IR treatment at 70 °C. With respect to currant phenolics, antioxidants that have potential benefits to human health, IR heating resulted in almost the same effect up to 65 °C, i.e. approximately 20 % reduction; higher peak temperatures further lowered total phenolic values. 5-hydroxymethylfurfural (HMF) formation was low up to 55 °C, while increased at higher temperatures; however, values still remained low as compared with dried fruit HMF levels. In another series of bioassays, golden sultanas were artificially infested with larvae of Ephestia elutella and adults of Oryzaephilus surinamensis and Tribolium confusum and treated with IR in agitating flow process at different peak temperatures (45, 50, 55, 60, 65 and 70 °C). Finally, a batch series of non-agitated flow process tests were conducted in glass Petri dishes. Briefly, O. surinamensis and T. confusum adults were exposed to IR penetration for different time exposure intervals (1, 2, 3, 4, 5, 8, 10 and 20 s) with and without the presence of currants and sultanas. In the bioassays with infested sultanas, all IR treatments with agitation provided complete control of the insect species tested, since IR-treated sultanas were found free of insects even 1 year after treatment. For exposure intervals higher than 4 s in the non-agitated flow process, all adults of O. surinamensis and T. confusum were killed in glass Petri dishes consisting of insects only and without the presence of currants and sultanas. In contrast, the presence of currants and sultanas in the non-agitated flow process reduced significantly the killing rate efficacy of IR against the insect species tested, especially on the individuals located beneath the sultanas and currants and where the infrared light could not penetrate them. Our results suggest that electric infrared heating with light source penetration could be a valuable tool for rapid and successful reduction of the microbial load and post-harvest insect disinfestation of dry currants and sultanas, provided that adequate agitation within the infrared process chamber is achieved so that sultanas and currants could be penetrated consistently in a three-dimensional manner.