Citation: Kami ´ nska-Dwórznicka, A.; Kot, A.; Jakubczyk, E.; Buniowska-Olejnik, M.; Nowacka, M. Effect of Ultrasound-Assisted Freezing on the Crystal Structure of Mango Sorbet. Crystals 2023, 13, 396. https://doi.org/10.3390/ cryst13030396 Academic Editor: David Cordes Received: 17 January 2023 Revised: 17 February 2023 Accepted: 23 February 2023 Published: 25 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). crystals Article Effect of Ultrasound-Assisted Freezing on the Crystal Structure of Mango Sorbet Anna Kami ´ nska-Dwórznicka 1, *, Anna Kot 1 , Ewa Jakubczyk 1 , Magdalena Buniowska-Olejnik 2 and Malgorzata Nowacka 1, * 1 Department of Food Engineering and Process Management, Faculty of Food Sciences, Warsaw University of Life Sciences (WULS-SGGW), Nowoursynowska 159C, 02776 Warsaw, Poland 2 Department of Dairy Technology, Institute of Food Technology and Nutrition, University of Rzeszów, ´ Cwikli ´ nskiej 2D St., 35601 Rzeszów, Poland * Correspondence: anna_kaminska1@sggw.edu.pl (A.K.-D.); malgorzata_nowacka@sggw.edu.pl (M.N.) Abstract: In this study, the effect of ultrasound-assisted immersion freezing (UAF) of mango sorbet in comparison to conventional freezer freezing, on freezing course and created crystal structure, was studied. The scope of work included the preparation of a sorbet mixture prepared on the basis of frozen mango fruit with the addition of locust bean gum (LBG), guar gum and a commercial mix of carrageenan without the addition of stabilizers, and freezing it using a conventional freezer and ultrasound-assisted freezing equipment, with variable operating parameters (21.5 kHz and 40 kHz—continuous or chopped mode). Then, the freezing time and the crystal structure of the frozen samples (a microscopy analysis) were examined. US-assisted freezing reduced the time of the process for stabilized samples of the sorbet. It was also proven that, proper stabilization with the combination of US treatment results in the formation of favorable crystal structure. Stabilized sorbet subjected to US action at a frequency of 21.5 kHz in chopped mode was characterized by the most uniform crystal structure, consisting of crystals with the smallest diameters among all the tested samples; the equivalent diameter was 9 μm, while for the stabilized control it was 25 μm. Keywords: US-assisted freezing; sorbet; recrystallization; structure 1. Introduction Nowadays, the food industry is focusing on innovative methods that are simple, fast, non-toxic for the final product and also economic. Ultrasound use has been the subject of research for many years. Based on the commonly accepted definition, ultrasound is an acoustic wave with a frequency greater than 20 kHz, the threshold of human hearing. The mechanism of ultrasound is based on acoustic cavitation [1]. It takes place on the basis of an interaction between ultrasonic waves, liquid and dissolved gas. US-assisted freezing cavitation bubbles could be most beneficial as they reduce both the heat and the mass transfer resistance at the ice/liquid interface. US-assisted techniques can also be considered green technology due to the creation of an environmentally friendly process [2]. Additionally, ultrasound can be used to minimize processing or increase quality and improve processing efficiency, ensuring food safety while extending product shelf life [3–5]. The cavitation and mechanical effects produced by ultrasound can improve food’s freezing rate and antibacterial effects, improving energy transfer and also reducing the costs of freezing [3,5]. Food drying and freezing are two of the most commonly used methods of preservation. Food freezing and cooling are essential not only in frozen food production but also as the main operation for ice cream production [6]. Crystallization is a process that leads to the formation of a crystalline lattice structure. When crystallization occurs at slow speed, the formed ice crystals are large—diameters cross the 50 μm range, causing cellular damage and great thawing loss in tissue products. For ice cream-type products, it results in coarse Crystals 2023, 13, 396. https://doi.org/10.3390/cryst13030396 https://www.mdpi.com/journal/crystals