The global demand for food that promotes health with high nutritional and nutraceutical values has been increasing. It is estimated that, by 2020, the global market for integral and functional food will reach 800 billion dollars [1]. The consumption of fruits and vegetables has been linked to health benefits, such as protection against diseases associated with oxidative stress, for example, chronic diseases. These effects have generally been attributed to the wide range of polyphenols, such as anthocyanins, flavonoids, proanthocyanidins, and flavonols present in fruits and vegetables [2,3]. Phenolic compounds are secondary metabolites of plants that constitute one of the most numerous and widely distributed groups of natural products in the plant kingdom. In general, these compounds can be classified into two main groups, namely: flavonoids (flavonols and anthocyanins) and non-flavonoids (stilbenes and phenolic acids). These compounds are important determinants in the sensory and nutritional properties of fruits and vegetables and have been attracting attention due to their antioxidant capacity [4,5].

Berries are highly prized for their intense colour, delicate texture, and unique flavour. Despite having several common attributes, the group is quite diverse and includes, for example, fruits such as blueberries, blackberries, raspberries, strawberries, elderberries, black and red currants, which have been postulated as a good source of phenolic compounds [6,5]. The blackberry (Rubus spp.) is an excellent source of natural antioxidants, particularly anthocyanins and phenolic acids [7], which have been associated with health benefits, such as antioxidant, anticonvulsant, and anti-inflammatory properties [8,3]. Antioxidants are substances that, in small quantities, have the ability to donate an electron or a hydrogen atom to a free radical. They can chelate the metal cations and thus form a stable molecule [9].

Blackberries are very perishable fruits and should be consumed in very short times (days). Over-maturation, excessive softening, and attack of pathogens are the main causes of post-harvest losses [6]. Therefore, the processing of fruits into products such as juices, purees, jams, canned fruits, and marmalades can be an alternative to prolong the accessibility and the consumption of fruits such as blackberry [3]. On the other hand, processing and storage can have notable effects on the phenolic content of fruits, and can reduce or even increase their health-benefit properties [10]. Gil et al., [11] demonstrated that the processing of pomegranate into juices increased their antioxidant capacity and their total phenolic content (TPC). On the contrary, the processing of strawberries into marmalades affected the flavonoid content with a decrease of 20% [12]. Similarly, the flavonoid content of red raspberries decreased slightly with processing, and more markedly during six months of storage [13,3]. Based on these findings, it is of great importance to review and determine the best treatment, given that, as mentioned above, the type of process might affect the composition and sensory characteristics of blackberries.

The concept of food preservation has changed gradually over the years. Initially, its purpose was to obtain safe products with a long lifetime. Today, fresh characteristics with a high content of nutrients and antioxidants are some of the attributes most requested by consumers, without neglecting food safety [14]. In traditional processes, the berries are crushed and pressed. The solid parts are discarded after extracting the juice, which generates an intensive accumulation of by-products, consisting of seeds, peels, and pulp remains. Through common processing, only part of the phenolic compounds are transferred to the juice, and the by-products generated are rich in anthocyanins and other compounds [15,5]. Thermal sterilisation/pasteurisation treatments ensure an effective reduction of microorganisms; however, they cause a significant loss of thermolabile compounds, and negatively affect the sensory, physicochemical, and nutritional properties of food [16,14].

For the food industry, pasteurisation is one of the most used methods for fruit drinks, such as blueberry products. Some authors have reported that this process leads to a substantial loss of bioactive compounds in berries due to oxidation and thermal degradation [17-19]. Therefore, it is necessary to adjust the intensity of the thermal treatment to a necessary minimum level that can achieve enzymatic and microbiological inactivation [20]. That is the reason why-in accordance with the aforementioned-the growing demand of consumers of minimally processed nutritious foods, together with the recent regulatory requirements, has led to a greater application of non-thermal technologies for the processing of puree, jams, marmalades, juices, and/or nectars. Such technologies have been investigated due to their potential to minimise changes in nutritional and organoleptic properties [21].

Hydrodynamic cavitation (HTD) is a new food processing technology that uses cavitation phenomena in liquids to crush and homogenise solid food particles [1]. It has a lower cost and a relatively simple configuration easily adaptable in food processing schemes, compared with other alternative technologies [22,23,42]. Cavitation is defined as the generation, formation, growth, and collapse of microbubbles due to variations in pressure in a fluid [24,1]. This process is generally produced by the mechanical rotation of a liquid at a specific speed. It can be achieved using a specialised rotor with holes known as hydrodynamic cavitators. When the rotor rotates, the action produces hydrodynamic cavitation away from the metal surface inside the holes. The cavitation effect is completely controlled within the system that facilitates the protection of the surface against damage. Subsequently, microscopic cavitation bubbles generate collapse with the discharge of shock waves that can mix together and inhibit incrustation. As a result, heat is produced with a uniform temperature distribution throughout the liquid, even without heat transfer surfaces [25]. Regarding the process equipment, the reactors used in the food industry generally consist of high pressure- and homogenisation-type reactors. They have a feed tank and two throttle valves that represent a first and second stage. The reactor is a high pressure positive displacement pump with a throttling device that follows the principle of the high pressure relief technique. By using a pump, the liquid from the feed tank is directed to the valve of the first stage. A pressure of up to 1,000 psi can be obtained in the first stage. Then, using second stage valves, the pressure can be increased to 10,000 psi with the use of different pressure relief valves. The liquid of the second stage is released into the low pressure region, and then recirculates back into the storage tank. As the throttling pressure increases, the temperature also increases. This way, cold water is recirculated in the feed tank in order to control this temperature increase. Cavitation begins at certain critical discharge pressure, which depends on the geometry of the regulation and the type of application [23].

Because of the internal heat generation, cavitation allows a single-stage processing, in which unitary operations-such as crushing, homogenisation, and heating-are performed in a closed system with limited oxygen exposure [26,23]. These process characteristics generate competitive advantages in the final product, mainly associated with a minimal degradation of food quality [27]. For example, the combination of uniform temperature with high pressure gradients was found to create favourable conditions for a single-stage processing of berries into puree-type products [28]. Similarly, this technology has been successfully used for the inactivation of enzymes (peroxidases and polyphenol oxidases) and microorganisms in cranberry puree [29,30], or to study the stability of bioactive compounds in tomato juice [23]. At other industries, HTD has been used successfully, in combination with for degradation of benzene present in wastewater [40], or to optimize the extraction of algae-oil from microalgae with different temperature and time [41]. At the scientific level, there are not many records about the advantages of using this technology; however, it is important to highlight that, by performing several unitary operations in a single stage, multiple operational and economic advantages can be generated.

The potential of this technology has not been fully explored for general fruit processing. There are records of the processing of blueberry puree and tomato juice with studies addressing physicochemical, functional, and microbiological properties; however, there are no records about the processing of blackberry jam. That is why, taking into account the changes registered in some fruits according to the type of process, the goal of the present study was to determine the antioxidant capacity of blackberry (Rubus glaucus) processed by hydrothermodynamic cavitation, compared with traditional technology (evaporation).

Fig. 3 illustrates the antioxidant capacity of blackberry jam processed by TDE and HTD, beginning at 25 °C and ending at 75/90 °C. Blackberry jam processed by TDE exhibited lower values at the initial temperature in comparison to HTD. As the temperature increased (ending at 75/90 °C), the antioxidant capacity exhibited the same behaviour using TDE or HTD. Likewise, the results were very similar to those of TPC in the two processes. There was a decrease of 10% in TDE, and 16% in HTD. The results of the antioxidant capacity had a direct correlation with the results of TPC, which indicates that the antioxidant activity depended on all the phenolic compounds, and the anthocyanins were partially responsible for this activity. This finding may be related to the fact that TPC is the main potential responsible for the antioxidant properties reported in fruit jams, for example. The antioxidant activity is not limited to this type of compounds but is related to the degradation of antiocyanins [31,3]. Koca and Karadeniz [38] observed that the antioxidant activity in blueberries was highly correlated with phenolic content, and less linearly correlated with anthocyanin content. A relationship between antioxidant capacity and TPC has been suggested; however, it should be complemented with the anthocyanin content for final differentiation to observe the actual effect between different treatments. Some studies [39] have suggested that TPC other than anthocyanins contributed positively to total antioxidant activity [5].

Figure 3 Source: The Authors.

The present study can lead to a general conclusion, according to which the reduction in size caused by HTD releases bio-compounds, thus making them more vulnerable to light and oxygen. This way, the lowest temperature did not favour these bio-compounds, and the protection of the compounds from light and oxygen might be a priority. Similarly, the type of heat transfer used in each process might have directly affected the stability of these compounds.

Multiple unitary operations and thermal conservation stages are used for preparing a fruit jam by traditional technology. On the other hand, the use of the principle of cavitation generates the integration of the process stages (extraction, grinding, homogenisation, and conservation). With this integration, operational savings in factories and operating costs (labour) can reach up to 50%, according to the equipment and process line. These savings were precisely observed during the conduction of the present study. These results confirm the industry interest for considering both processing costs and implementing greener processing technologies [42].

The present study determined the antioxidant capacity of blackberry jam processed by traditional evaporation technology (TDE) and hydrothermodynamic cavitation (HTD). The results indicated that blackberry (Rubus glaucus) is a significant source of phenolic compounds, with a high antioxidant capacity. The effect of processes of TDE and HTD on antioxidant capacity did not indicate significance differences. HTD shows a notable decrease in phenolic and antioxidant activity at temperature higher than 65 °C, in comparison to TDE. However, at a range from 20 to 60 °C phenolic compounds and antioxidant activity were more stable with HDT. Therefore, it is suggested to keep the HTD process up to a maximum temperature of 65 °C. Nevertheless, greater understanding of the antioxidant capacity directly associated with the anthocyanin content is still required, in order to determine the final effect that can be achieved by HTD processing.