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

Abstract

The antioxidant activity (AOA) of plant foods is recognized as an index of the potential health benefits resulting from their consumption. Due to their high perishability and seasonality, plant foods are largely consumed or used as processed products and freezing is one of the technologies used for the production of high-quality foods. However, cell breakages occurring during freezing and frozen storage can lead to the release of antioxidant compounds and their degradation due to chemical and enzymatic oxidation reactions, and thus, they could present a lower antioxidant activity compared to the corresponding fresh product. In this context, process conditions, freezing pre-treatments and the use of cryoprotectants can limit the extent of freeze-induced damages and preserve the antioxidant activity of plant foods. This review collects and discusses the state-of-the-art knowledge on the single and combined effect of freezing and frozen storage conditions on the antioxidant activity of fruits and vegetables as well as the role of cryoprotectants. Classes of compounds responsible for the antioxidant activity of plant foods and the most common methods used for the evaluation of the antioxidant activity in vitro are also presented. The freezing principles and the effects of ice nucleation and crystallization on fruits, vegetables and their main derivatives (juices, pulps) have been addressed to highlight their impact on the AOA of plant foods. The effect of freezing and frozen storage on the AOA of plant foods resulted dependant on a series of intrinsic factors (e.g., composition and structure), while the role of extrinsic processing-related factors, such as freezing and storage temperatures, is ambiguous. In particular, many conflicting results are reported in the literature with a high variability depending on the method of analysis used for the AOA evaluation and data expression (fresh or dry weight). Other intrinsic raw material properties (e.g., cultivar, ripening degree), post-harvest conditions, as well as defrosting methods that in the majority of the studies are scarcely reported, contribute to the aforementioned discrepancies. Finally, due to the limited number of studies reported in the literature and the high variability in product processing, the effect of cryoprotectants on the AOA of plant foods remains unclear.

Impact of Processing Approach and Storage Time on Bioactive and Biological Properties of Rocket, Spinach and Watercress Byproducts

“…Vegetables' and fruits' short shelf life and seasonal nature result in massive losses and wastes [1,2]. The last report of the Food and Agriculture Organization of the United Nations accounts that in 2016, 21.6% of worldwide fruits and vegetables were lost [3].…”

“…Post-harvest conditions such as transport and storage conditions are also critical factors for the phytochemical profile of vegetables and fruits. In addition, food-processing approaches are one of the most critical parameters that may promote chemical modification or destruction of the natural bioactive metabolites [2,8,9,13]. However, plant foods' seasonality and perishability require processing and preservation strategies to prolong their shelf-life.…”

“…However, plant foods' seasonality and perishability require processing and preservation strategies to prolong their shelf-life. Thus, the availability of quality and safe products during off-seasons is guaranteed [2].…”

“…Freezing is one of the most used processing technology for the long-term preservation of plant foods by industrial companies and households [2,14]. The frozen time and temperature may influence the vegetable and fruit properties due to the biochemical and physical events occurring during freezing storage [14,15].…”

“…The ice crystals formed during freezing may damage cellular membranes depending on the location and size, promoting the loss of physical integrity and consequent decompartmentalization of antioxidant compounds. In addition, some authors described another effect: the possible release of bound phenolic acids and anthocyanins and the resultant increase in antioxidant capacity [2,15]. However, although the kinetics of some cellular events, such as the enzymatic and chemical reactions, are slowed during freezing, these are not entirely interrupted [2,14].…”