**1. Introduction**

Freeze-drying (FD) or lyophilization is a dehydration method that has been proven to produce high-quality fruit and vegetable products, as compared with other drying processes. Since it is carried out at a low temperature (typically a maximum of 38 to 54 ◦C shelf temperature), product damage is minimized. Thus, it is used for delicate, high-sensitive, and high-value products to maintain their color, flavor, shape, and nutritional characteristics [1–3]. Examples of these kinds of foods are berry fruits such as blueberries, strawberries, maquiberries, and cranberries, among others. Freeze-dried berry fruits are generally consumed as they are, whole and without peeling or cutting, as conservation of their original shape and appearance is often desired for the final product [4]. However, most of these berries are naturally wrapped by a waxy outer skin that imparts barrier to water vapor flow during FD, as happens in all drying processes. This mass transfer limitation is often the phenomenological factor that controls

the processing time, energy consumption, and concomitant final quality. In some processes, such as FD, where fruits are exposed to high-energy inputs and/or a high-vacuum environment, a skin rupture or a busting process is frequently observed due to pressure lift just under the skin. As a result, besides the long processing time, many freeze-dried berries, while maintaining most of their original nutritional quality, cannot keep their original shape and appearance, diminishing their organoleptic characteristics significantly [5–8].

Several skin pretreatment methods have been tested to facilitate water vapor transfer in drying processes, most of them focused on reducing the processing time of Osmotic Dehydration (OD). Physical and chemical methods, such as maceration of the outer skin by knife blade, chemical additives, or needle perforation, have been reported [1,9–14]. Due to the presence of undesirable compounds (NaOH, HCl, ethyl oleate), the generation of waste material, and the low quality found in the final product, chemical treatments are not a real alternative to weaken the berry skin. For example, Grabowski and Marcotte [13], reported that chemical skin pretreatment in OD of cranberries resulted in the lowest value of taste acceptability. Different results were reported by Ketata et al. [15]: when liquid nitrogen pretreatment was applied to the OD of blueberries, the dewaxing of the fruit skin allowed a reduction in the drying time of 45% to 65%; however, this was accompanied by a significant loss of total phenolics. Mechanically cutting fruits is another alternative that has been tested in blueberries and tomatoes. However, due to the softness of these fruits, many problems have been observed, including significant damage to the final product, loss of nutritious fluids, and not having the whole fruit as the final product [1]. Alternatively, skin needle perforation has been tested on cranberries and cherry tomatoes. It has been reported that to obtain a significant mass transfer enhancement in OD, 20% to 30% of the total surface area of cranberries should be perforated [13]. Azoubel and Murr [10] perforated cherry tomatoes with 1 mm diameter needles with a pin hole density of 16 holes/cm<sup>2</sup> prior to OD and air drying, attaining a significant time reduction when more than 80 holes/cm<sup>2</sup> were used.

Scharschmidt and Kenyon [16] reported the results related to skin pretreatment in FD processing where blueberries were perforated by needles to a density of 2–3 punctures per berry. The main results were that the berries kept their original spherical shape without physical changes to their outer integument, which was declared to be a problem in nonperforated blueberries. In addition, the FD time was two-thirds or less than that of the control process without puncturing. However, no information was given on processing conditions (temperature and pressure), and also the needle diameter was not specified. Thus, even with some time-limited information, physical skin perforation of the whole berry fruit (i.e., blueberry) seems to be a suitable alternative to more efficiently carry out mass transfer processes that reduce the processing time, cost, and energy requirement and avoid product explosion or bust, so that the fruit can retain its basic shape.

Collaterally, puncturing has been tested by utilizing carbon dioxide (CO2) laser beam technology to carry out surface microperforations. This has important advantages, and it is already used in many fields such as medicine, cosmetics, and marking industries. Due to its superior accuracy, environmental cleanliness, and safety, applications can be carried out at precise locations, in multiple surfaces and arrangements, and within a size range of 50 to 300 μm [17,18]. Also, it is a noncontact technology that significantly mitigates the chance of physical and microbiological contamination of materials that are typically associated with traditional cutting or contact devices or fluids [19,20]. Fujimaru et al. [9] reported the application of CO2 laser microperforation of blueberry skin as a pretreatment in OD. The study demonstrated that CO2 laser microperforation can be a viable skin pretreatment which offers notable improvements in water removal; it almost doubled the moisture loss in the first 24 h of OD, from 7% (control) to 11% (2.5 mm square grid perforation), which was even better than the 9% water loss for blueberries that were mechanically cut. This already-proven technology, which the food industry has not completely incorporated, is an attractive alternative that can be applied to berry FD in order to overcome the problems associated with its outer skin.

From a theoretical point of view, the busting process is a consequence of a high initial mass transfer resistance due to the presence of the skin, with this being probably the most significant variable governing water vapor flow during FD and its implications. When the resistance to mass transfer needs to be estimated, the Manometric Temperature Measurement (MTM) methodology has been proven to be an effective tool. Moreover, Pikal et al. [21], concluded that a high resistance to mass transfer at the beginning of the sublimation process is due to the presence of a surface barrier resulting from a structure different from that of the dried layer, a conclusion that has been supported by scanning electron microscopy. This observation may be applied to berry FD, but it needs to be tested.

Thus, the objectives of this study were as follows: (1) to describe the busting process of blueberry freeze-drying through experimental and phenomenological approaches, and (2) to evaluate the use of CO2 laser microperforation technology to reduce the processing time as well as product explosion and its associated consequences.
