Anthocyanins (ATs) are water-soluble compounds found in different tissues of the plant, such as leaves, roots, flowers, and fruits [55]. These play an essential role in the sensory attributes of food products, as they are responsible for the characteristic red, purple, and blue coloration in fruits [56]. Anthocyanins can be found as aglycones or glycosylated derivatives. The six most frequently found in foods are pelargonidin, cyanidin, malvidin, delphinidin, petunidin, and peonidin [57]. The highest AT levels are found in berries, currants, grapes, and tropical fruits [58]. In the case of fresh strawberries, AT concentrations from 3.7 to 64.9 mg/100 g FW have been reported [3,6]. Numerous in vitro and in vivo studies have recognized the potential effect on preventing neurodegenerative and cardiovascular diseases and antioxidant, anti-inflammatory, anti-obesity, anti-diabetic, and chemopreventive properties [59,60,61].

For TT, 7 studies with 23 trials comprising temperatures from 70 to 100 °C for 1 to 15 min provided concentration data to calculate percentages of change of total ATs. Reductions (18 trials) from 5% to 44% were observed in pasteurized strawberry, strawberry purees and pulp [32,33,36,41,42,44]. On the contrary, other authors reported increments (5 trials) from 2% to 32% in strawberry puree [33,40].

Regarding HPP, a total of 6 studies, including 40 trials with pressurization conditions from 100 to 600 MPa for 1 to 20 min at 0–50 °C, were analyzed. Degradations (24 trials) from 7% to 28% were reported in strawberry pulp and puree [38,39,41,44]. In contrast, positive effects (16 trials), from no differences to 15% increase, were recorded in the same products [38,40,44].

In general, just after pressurization, the levels of ATs were preserved close to those found in fresh strawberries. In contrast, a more significant impact was observed in the degree of change of ATs due to thermal treatment (TT) with a downward trend in both cases.

Since ATs are very chemically sensitive compounds, most of the studies registered the degradation of ATs after processing and storage. For instance, pasteurization of entire strawberries at 90 °C for 5 min resulted in a reduction of 5–35% of cyanidin-3-glucoside, pelargonidin-3-rutinoside and pelargonidin-3-glucoside [31,32]. Higher degradations of 87–92% of these ATs were observed during 90 days of storage at 23 °C [32]. Similar results with losses of around 90–93% were observed after storage for eight weeks at 25 °C of strawberry puree treated at 100 °C for 10 min [35]. In another study, the storage of strawberry puree (heated at 90 °C for 15 min) during 12 weeks at 6 °C led to a mean degradation of about 19% of these ATs [43]. Degradation of total ATs (22%) was also observed in the strawberry pulp after heating at 70 °C for 2 min [44] and in strawberry puree pasteurized 90 °C for 15 min (44% of reduction) [41]. In general, degradation of ATs can follow three possible processes; cleavage of covalent bonds, polymerization and derivatization [32,36,41,43]. Moreover, this decrease could be partially caused by condensation reactions of ATs with other phenolics to produce colored polymeric pigments, resulting in strawberry pulp browning.

Interestingly, many authors have reported different degradation percentages depending on the strawberry variety, concluding that the effect of TT and subsequent storage on ATs also depends on the response of the fruit variety to the treatment [36,37,42]. The stability of ATs is also highly influenced by the food matrix properties. High pH and ascorbic acid content in the food matrix accelerates the degradation rate of ATs [43,62,63,64].

Although overall TT led to degradation on ATs, some studies reported a positive impact after processing. Garzoli et al. [33] reported a slightly higher content (2–18%) on total ATs in pasteurized puree (85 °C/3 min) compared with fresh puree. In agreement, Stübler et al. [40] showed that heat-treated (72 °C for 1 min) strawberry puree incremented the individual ATs as follows: cyanidin-3-O-glucoside (40%), pelargonidin-3-O-glucoside (26%), pelargonidin-3-O-rutoside (22%), pelargonidin-3-O-malonyl-glucoside (34%), and pelargonidin-3-O-acetylglucoside (39%). These increments in ATs might be attributed to a higher extraction of ATs from the matrix due to the heat treatment [40]. As mentioned before, the changes in ATs content after HPP were minor compared with TT. Some studies reported no significant changes after pressurization. For instance, Bodelón et al. [38] found no significant differences in ATs levels in strawberry puree after HPP at 100, 200, 300, and 400 MPa at 20 °C compared with the untreated puree. However, a slightly higher decrease in ATs was observed in the puree pressurized at 50 °C compared with the untreated control. In line with this, the levels of individual and total ATs in strawberry pulp showed no significant changes after HHP treatments, regardless of the applied pressures or treatment times [44]. These results further support the idea that ATs were stable after pressurization. Stübler et al. [40] reported minor non-significant increases (8–12%) on all the soluble individual ATs, in strawberry puree after pressurization at 600 MPa for 1 min at room temperature. One explanation for this apparent increase in ATs could be the release of these compounds from the intact cells to the surrounding matrix. In other works, slight but significant decreases of about 7% on total ATs were reported in strawberry puree after HPP at 300 and 500 MPa for 1, 5, and 15 min at 0 °C. Under the same pressurization conditions combined with 50 °C, there was a degradation of 14% on ATs [41]. In the samples pressurized without heat treatment, the oxidative enzyme activity (PPO and POD) did not change significantly, leading to oxidation reactions and thus ATs degradation. However, the higher degradation of ATs in the pressurized puree combined with heat treatment might be due to the formation of colorless chalcones and reduction in flavylium cations and quinoid bases as a consequence of the thermal treatment. In another study, significant losses of 15 and 21% of total ATs were recorded in strawberry puree from two-year crops treated with HPP at 300 and 600 MPa for 15 min at 50 °C [39]. In the same line, Terefe et al. [42] also reported losses of ATs after HPP at 600 MPa for 5 min at 20 °C at different percentages depending on the variety: Camarosa (22–28%), Ruby Gem (27–42%), and Festival varieties (17–20%). Similar behavior for all varieties was observed after three months of refrigerated storage with losses between 62% and 77%. Losses up to 69% were also observed when pressurized strawberry puree (500 MPa for 15 min at 50 °C) was stored for 12 weeks at 6 °C showing that even HPP in combination with temperature was not enough to inactivate enzymes and led to a significant degradation during storage [43].

Storage at 4–6 °C after TT was examined in three studies [37,42,43]. Two studies were discussed to analyze the effect of storage after HPP, but only one used HPP without heat treatment [42], and the other combined HPP with heat treatment of 50 °C [43]. The losses of ATs during TT and HPP storage were progressive with the time, observing, in general, higher losses in the storage of HPP products. Storage from three to six months after HPP was only reported in a study with clear and cloudy strawberry juices that were not included in the figure because samples were also subjected to a previous blanching process at 100 °C/1 min in order to inactive enzymes [45].

Ellagitannins (ETs) constitute complex molecules with variable water solubility structured by one or more hexahydroxydiphenoyl (HHDP) moieties, which could be hydrolyzed to release ellagic acid (EA) [65]. ETs and EA are predominantly found in pomegranates, berry fruits, oak-aged red wine, tropical fruits, and nuts. ETs concentration in strawberries ranges from 7.18 to 28.85 mg/100 g FW and EA between 0.9 and 14.8 mg/100 g FW [3,5] with a more significant contribution from achenes than from flesh [5,66]. The potential health benefits of ETs and EA are associated with the metabolites (urolithins) produced by the human gut microbiota [67,68]. The biological functions attributed to urolithins comprise anti-oxidant, neuroprotective, anti-microbial, anti-inflammatory, and anticancer properties [67,69,70]. Notably, there were no differences in urolithins’ production and urinary excretion between volunteers ingesting either fresh strawberries or thermally treated puree [71].

For TT, 5 studies with 10 trials including temperatures from 55 to 90 °C for 1 to 15 min provided concentration data to calculate percentages of change of EA. Increases (6 trials) from 8% to 66% were observed in pasteurized strawberries, strawberry pulp, and strawberry purees [32,40,41,44]. In contrast, only Álvarez-Fernández et al. [34] reported a slight EA degradation (4 trials) from 8% to 35% in strawberries. Whereas for HPP, 3 studies with 27 trials with pressurization conditions ranging from 100 to 600 MPa for 1 to 25 min at 0–50 °C were analyzed. Increments (13 trials) from non-differences to 28% were reported in strawberry puree [40,41], whereas 2 to 37% decreases (14 trials) were observed in strawberry pulp [44], showing that the increases observed were most probably due to the enhanced extraction from the achenes.

In general, TT and HPP influenced the level of EA both positively and negatively depending on the conditions. Still, the most significant increases were observed as a result of heat treatment.

For TT, Cao et al. [44] reported a 17% increment in EA content after heating strawberry pulp at 70 °C for 2 min. The rise of EA levels after processing could have resulted from the hydrolysis of ETs and the release from the cellular structures. In agreement, pasteurization of strawberry puree at 90 °C for 15 min led to an increase of 31% on EA [41], and only a limited increase of 8% was observed when a milder TT of 72 °C for 1 min was applied [40]. Likewise, pasteurized (90 °C/5 min) strawberries had 143% higher levels of EA when compared with fresh fruit. However, after 360 days of storage at −20 °C, EA decreased 65% compared to the control [31]. Another study showed a zero-order kinetic model for EA degradation, with a final reduction of 32% in pasteurized strawberries stored for 90 days at 23 °C. Degradation due to storage might be due to increased exposure of EA released from cell walls, leading to non-enzymatic oxidation reactions [32]. On the other hand, during storage of pasteurized strawberry pure (90 °C/15 min) at 6 °C for 12 weeks, EA levels raised until week 10 and then slightly decreased toward the end of storage [43]. An explanation for the increase in EA during storage might be attributed to progressive release from high molecular weight ETs present in the puree [43]. Although the general trend is to increase EA after TT, Álvarez-Fernández et al. [34] reported decreases in EA (8–35%) during the strawberry puree processing and in the final product in the 2011 and 2012 harvest.

For HPP, Cao et al. [44] reported significant losses from 2% to 37% on EA levels in strawberry puree after HPP at 400 MPa for 5, 10, 15, 20, and 25 min, and 500 MPa for 5, 10, and 15 min at room temperature. However, a minor and non-significant decrease of 3% EA was observed in strawberry puree pressurized at 600 MPa for 1 min at room temperature [40]. In agreement, Marszalek et al. [41] also reported that the levels of EA in fresh strawberry puree did not change significantly after HPP at 300 and 500 MPa for 1, 5, and 15 min at 0 °C. However, when combining the same pressurization conditions at 50 °C, a significant increase of 28% EA concentration was observed [41]. These increments could be due to a release of EA from ETs due to the combination of HPP with temperature [72]. An EA increase of 43% was reported when strawberry puree pressurized at 500 MPa for 15 min at 50 °C was stored for 12 weeks at 6 °C [43].