*2.2. Fruit Processing*

Kiwifruits (*Actinidia deliciosa* cv. "Hayward") at full maturity were purchased in November 2021 from a local supermarket in Galati County, Romania. The fruits were sorted and processed immediately. Once they arrived at the laboratory, the fruits were washed with distilled water, and peels were separated from the fruits. The fruits were further cut in half and squeezed using a juice extractor (Stainless Steel Fruit Vegetable Juice Extractor Juicer Squeezer, Guangdong, China). The fresh material, specifically peels and pomace, was used for initial phytochemical content and antioxidant activity analyses.

### *2.3. Extraction of Phytochemicals from Fresh Kiwi Samples*

In order to characterize the freeze-dried powders of the fresh material, a subsequent extraction was performed using a solid–liquid ultrasound-assisted method with two different solvents. The design of extraction procedures was based on a comprehensive comparison of both lipophilic and hydrophilic profiles by using two different combinations of solvents: a mixture of ethanol and water (ratio of 70:30, *v*/*v*) and *n*-hexane–acetone (ratio 1:3, *v*/*v*). The fresh material was extracted with the corresponding solvent solutions in a solid–liquid ratio of 1:10 by mixing and ultrasound treatment at 35 ◦C for 30 min. Each extraction was repeated twice to obtain enriched extracts.

### *2.4. Customized Design to Develop Kiwi-Based Ingredients Enriched with Probiotics*

About 50 g of fresh material (kiwi peels and pomace) was added to ultrapure water in a ratio of 1:2 and mixed by homogenization (ProBlend Crush Blender, Global Headquarters Netherlands, Eindhoven). Prior to inoculation, the mixtures were enhanced with different flours, i.e., black rice and buckwheat (20%), and allowed to hydrate, followed by pH adjustment to 5.0 with 1.0 N NaOH. The designed samples were coded as follows: KP (freeze-dried kiwi peels), KBR (freeze-dried kiwi pomace and black rice flour), KPB (freezedried kiwi pomace and buckwheat flour), and KPO (freeze-dried kiwi pomace). All samples were sterilized using a UV lamp and inoculated with 2% *L. casei* 431® and freeze-dried (CHRIST Alpha 1–4 LD plus, Osterode am Harz Germany) at −42 ◦C under a pressure of 10 Pa for 48 h. Afterwards, the powders were collected and packed in glass containers and kept at 4 ◦C until further analysis.

### *2.5. Total Polyphenol (TPC) and Total Flavonoid (TFC) Analysis*

Spectrophotometric methods were used for the TPC and TFC evaluation by using the Folin–Ciocâlteu reagen<sup>t</sup> and aluminum chloride methods, respectively, as described by Milea et al. [25]. For TP content, from each sample extract, a volume of 0.2 mL was diluted with 15.8 mL of distilled water, followed by the addition of 1 mL of Folin–Ciocâlteu reagen<sup>t</sup> and 3 mL of 20% Na2CO3. The mixtures were left to stand for one hour in the dark to react, followed by reading the absorbance at λ = 765 nm (Jenway Scientific Instruments, Essex, UK). TP was expressed as mg gallic acid equivalents/g dry weight (mg GAE/g

DW) using a calibration curve. For TF evaluation, a volume of 0.25 mL from each extract was sequentially mixed with 0.075 mL of sodium nitrite (5%), 0.15 mL of 10% AlCl3, and 0.5 mL of 1 M NaOH. The absorbance of the mixtures was immediately measured at 510 nm against the suitable blank. TF was expressed in mg catechin equivalents (CE) per g of dry powder (mg CE/g DW).

### *2.6. Total Monomeric Anthocyanin Content*

Due to the special formulation of the KBR sample, which contained freeze-dried kiwi pomace and black rice flour, the total monomeric anthocyanin content (TAC) was assessed using the official AOAC method [26] and expressed as mg cianidin-3-O-glucoside equivalents/g DW (mg C3G/g DW).
