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.

This work aimed to study the physical, structural, and sensory properties of a traditional full-fat mayonnaise (≈ 80% oil) enriched with an olive leaf phenolic extract, added as either free extract or encapsulated in alginate/pectin microparticles. Physical characterization of the mayonnaise samples was investigated by particle size, viscosity, lubricant properties, and color; a sensory profile was also developed by a quantitative descriptive analysis. The addition of the extract improved the dispersion degree of samples, especially when the olive leaf extract-loaded alginate/pectin microparticles were used. The encapsulated extract affected, in turn, the viscosity and lubricant properties. In particular, both of the enriched samples showed a lower spreadability and a higher salty and bitter perception, leading to a reduced overall acceptability. The results of this study could contribute to understanding the effects of the enrichment of emulsified food systems with olive by-product phenolic extracts, both as free and encapsulated forms, in order to enhance real applications of research outcomes for the design and development of healthy and functional formulated foods.