Abstract
Raspberries are both seasonal and highly perishable, and the industry continues to suffer substantial postharvest losses. The rapid growth in South African raspberry production demands innovative and sustainable ways to add value to the raspberry so as to minimize losses, promote its use in various sectors of the food industry, and realize its full economic potential. Encapsulation can be applied to raspberry juice to produce functional and shelf-stable value-added powder products. Therefore, the general aim of this research was to develop a nutrient-dense and shelf stable powder from raspberry juice (Rubus idaeous) encapsulated with different wall materials for multiple applications in the food industry.
The study reported in Chapter 3 investigated the potential of using gum Arabic (GA), maltodextrin (MT), and waxy starch (WS) as wall materials in producing raspberry juice powder (RBJP). The raspberry juice was mixed with 10% of each wall material and freeze-dried to produce RBJP. The encapsulated powders were characterized according to their physicochemical and phytochemical composition and their rheological and antioxidative properties. The study established that GA-encapsulated powder exhibited better yield (12.70%), oil holding capacity, total anthocyanin content (170.11 mg C3gE/g DM), total monomeric anthocyanin content recovery (18.00%), and total phenolic content (292.00 GAE/100 g DM). MT-encapsulated powder showed higher redness (56.86) and radical scavenging activity (54.55%). On the other hand, WS-encapsulated powder had a higher phenolic content recovery. However, the powder exhibited low redness (48.80), solubility (54.52%), and total soluble solids (6.25 ºBrix). Hygroscopicity and titratable acidity did not significantly (p > 0.05) vary among the powders. Cyanidin diglucoside was identified as the primary anthocyanin in the RBJP and was significantly higher in MT (2549.89 μg/g) compared to GA (1935.45 μg/g) and WS (1458.81 μg/g). In addition, the MT and GA powders were characterized by irregular-shaped and non-spherical particles, which were less agglomerated and relatively larger, while the WS powder exhibited more uniform, spherical particles, which agglomerated together. Therefore, it was demonstrated that GA and MT are
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valuable for the production of a nutrient dense RBJP that can be used as food additives or ingredients to formulate functional foods.
Chapter 4 presents a follow-up study conducted to investigate the storage stability of the RBJP produced from binary blends of GA and MT. The wall materials were blended using different ratios, which were 1GA:1MT, 2GA:1MT, 3GA:1MT, 1GA:2MT, and 1GA:3MT, before they were freeze-dried to produce powder. The RBJP was analysed for surface morphology using scanning electron microscopy (SEM), minerals using energy dispersive-ray spectroscopy (EDX), and functional groups using the fourier transform infrared (FTIR) technique. In addition, the RBJP was analysed for stability of total monomeric anthocyanin (TMA) content, antioxidant activity, and colour properties at 50 °C and 30% relative humidity for 6–10 days. The 1GA:2MT and 1GA:3MT powders were highly unstable, and their storage study terminated at day 6. GA produced RBJP with larger particles and an irregular shape, while increasing the concentration of MT produced RBJP with a spherical shape, smooth surfaces, and smaller particles. However, all RBJP parties showed agglomeration. Oxygen and carbon are the main elements observed in all the RBJP. The functional groups (alkene (C=C), carbon dioxide (C=O=C), carboxylic acid (C=O-OH), alkane (C-H), and amine (N-H)) varied among the RBJP. The study demonstrated that 2GA:1MT and 3GA:1MT encapsulated powders had a slow degradation rate for TMA, colour, and radical scavenging activity during storage. This suggests that a higher concentration of GA in the MT:GA blends could highly produce the shelf-stable powders desired by the food industry.
Overall, the study has demonstrated that GA and MT can be used individually or as binary blends to encapsulate raspberry juice to produce nutritionally rich and shelf stable RBJP that has the potential to be used in various sectors of the food industry. The successful encapsulation of raspberry juice into RBJP promotes diversified use of raspberry juice, food and nutrition security, healthy food consumption, and contributes to the achievement of some of the United Nations Sustainable Development Goals (SDGs) that focus on health and well-being.