Ultrasound Co-Extraction Strategies to Obtain Carbohydrates and Protein from Macroalgae Gracilaria birdiae

The growing demand for sustainable, functional food ingredients has increased interest in algae, particularly for their bioactive compounds. Gracilaria birdiae is mainly used for agar production, but its protein potential is underexplored. Conventional extraction methods require high temperatures and long durations, whereas emerging technologies, such as ultrasound (US), could be more efficient alternatives when combined with other strategies. This study is the first to evaluate the co-extraction of carbohydrates and proteins from G. birdiae using US combined with different extraction methods. The proximate composition of the algae was as follows: 63.97% carbohydrates, 5.20% proteins, and 19.65% lipids. Ethanol pretreatment did not improve US co-extraction but enabled phycobiliprotein recovery. Higher US power (500 W, 373 W·cm−2) and biomass concentration enhanced recovery, achieving up to 64.43 mg/g of carbohydrates and 10.28 mg/g of proteins. Sequential extraction using hot water and US at 60 °C produced 97.51 and 74.70 mg/g of carbohydrates and 5.67 and 5.08 mg/g of proteins, respectively. Acid treatment combined with US 60 °C achieved the highest recovery of 396.15 mg/g carbohydrate (1% v/v H2SO4) and 38.49 mg/g of protein (7% v/v H2SO4). Alkali extraction yielded lower amounts: 190.51 mg/g of carbohydrates and 33.20 mg/g of protein using 5% w/v NaOH. Microscopy revealed that the chemical treatments fully disrupted the cell wall, promoting compound release. Thus, combining the US with chemical extraction effectively enhances carbohydrates and protein recovery from G. birdiae.