Effect of Dipping and Vacuum Impregnation Pretreatments on the Quality of Frozen Apples: A Comparative Study on Organic and Conventional Fruits

Abstract

The effect of dipping and vacuum impregnation (VI) pretreatments with lemon juice solution on the quality and stability of organic and conventional frozen apples was investigated. Fresh apples were characterized; organic apples showed, at equal starch and ripeness index, a lower sugar content, and higher acidity than conventional ones. The polyphenol content was slightly higher in organic apples than in conventional ones while polyphenoloxidase activity was similar. No differences in color and firmness were evidenced. Dipping affected organic and conventional apples’ color by determining an increase of lightness (ΔL* ≈ 4) and h° (Δh° ≈ 6) parameters. VI reduced the lightness of apples (ΔL* ≈ −3) but the addition of lemon juice counterbalanced the lightness reduction by increasing ΔL* from 3 to 1.5 ca. Pretreatments did not affect the firmness of organic fruits while impaired that of the conventional ones ( 26% on average), likely due to different fruit matrix porosity and cell wall composition. Freezing (−40 °C) and frozen storage (up to 300 days) dramatically reduced the firmness of organic (42%) and conventional products (58%). At equal pretreatment and storage time (that is, 15, 30, 300 days), higher firmness retention was evidenced in organic apples than in conventional ones. All through frozen storage, VI was shown to better preserve the mechanical properties of organic apples than that of conventional ones. Both freezing and frozen storage reduced the hue of frozen apple products by up to 8% due to browning reactions. Lemon juice addition increased the hue of both frozen samples and thawed samples by about 2% all through storage time.

Response of organic and conventional apples to freezing and freezing pre-treatments: Focus on polyphenols content and antioxidant activity

“…Moreover, caffeic acid was shown to react with PPO by forming a caffeoyl-PPO complex, which is very slowly transformed in quinone, leaving a small part of the enzyme available for the binding with other phenol substrates (Janovitz-Klapp et al, 1990). Finally, as reported by Neri et al (2019), ORG apples showed after processing a higher firmness than CONV ones, which implies differences in mechanical strength between the tissues and, consequently, different cell responses toward physical damages induced by VI and/or freezing and frozen storage. Freezing may determine, in fact, cellular disruption, thus improving the extraction of analytes (Blanda et al, 2008).…”

“…The reduction of the PPO activity is due to the organic acids contained in the impregnant/dipping solution and, in particular, to the citric acid, which exerts a specific inhibitory effect on PPO (Zhou et al, 2016). In general, the ORG apples showed a decrease of the PPO activity higher than the CONV ones due to their higher citric acid uptake during pre-treatments (3.5 vs 3.1 mg g -1 dm in DIP samples and 4.8 vs 4.5 mg g -1 dm in VI_L ones) due to their higher porosity (Neri et al, 2019).…”

“…The extraction and the analysis of the organic acids (oxalic acid, citric acid , tartaric acid, ascorbic acid, malic acid and succinic acid) were performed on the lemon juice and on the apple samples before and after DIP and VI pre-treatments as described by Neri et al (2019).…”

“…Hence, the need to deepen the knowledge and comprehension of the role of cultivation and farming practices not only on the quality of fresh products but also on their technological properties. To this regards, even though numerous studies have sought to determine whether significant differences in postharvest quality exist between organic and convention plant foods (Mditshwa, Magwaza, Tesfay, & Mbili, 2017), very scarce are the information related to their response to processing (Arlai, Nakkong, Samjamin, & Sitthipaisarnkun, 2012;Neri et al, 2019). Thus, investigations aimed at defining the suitability of organic products at processing, and/or at optimizing the technologies to apply in order to preserve or improve the quality, and, in particular, the nutritional and functional properties of organic products are needed.…”

Antioxidant Activity in Frozen Plant Foods: Effect of Cryoprotectants, Freezing Process and Frozen Storage

“…These discrepancies were also found irrespective of the method of analysis used to determine the AOA and could depend on process-related factors such as the thawing methods and wounding [139,140] and/or on factors related to the raw material. In fact, the cultivar, the cultivation method, the ripening degree, the harvest season and pre-harvest or postharvest chilling stresses can significantly affect the composition of plant tissues and, thus, their mechanical strength [141] and response to physical stresses induced by processing, with effects on the release and oxidation of polyphenols after freezing and thawing [22,73,142]. In particular, these variables influence the composition of plant foods by affecting (i) the content of proteins and of sugars, which act as natural cryoprotectants [143], (ii) the phenolic content and composition due to the shift of the pathways involved in their biosynthesis [139,[144][145][146] and the concentration of oxidase enzymes, such as polyphenoloxidase, whose activity is affected not only by the total polyphenol content but also by the content of single polyphenols and/or by their oxidation products [147].…”