A Rat Study to Evaluate the Protein Quality of Three Green Microalgal Species and the Impact of Mechanical Cell Wall Disruption

Abstract

The present study was conducted to evaluate the protein quality of microalgae species Chlorella vulgaris (CV), Chlorella sorokiniana (CS), and Acutodesmus obliquus (AO) and assess the impact of mechanical cell wall disruption. Male Sprague–Dawley rats, around 156 g after adaptation, were placed in metabolic cages and fed experimental diets that were either protein-free or contained 10% protein solely from one of the undisrupted or disrupted CV, CS, and AO. After 3 days, feces were collected for a period of 5 days and analyzed together with diet samples for crude protein contents. Apparent protein digestibility, true protein digestibility, amino acid score, and protein digestibility-corrected amino acid score were calculated. In vitro protein digestibility was measured using the pepsin–pancreatin method and the in vitro protein digestibility-corrected amino acid score was calculated. The crude protein contents of CV, CS, and AO were 53.5, 50.2, and 40.3%, respectively. The amino acid score of the first limiting amino acid was 1.10, 1.27, and 0.86, true protein digestibility was 64.7, 59.3, and 37.9% and protein digestibility-corrected amino acid score was 0.63, 0.64, and 0.29, respectively, for CV, CS, and AO. Mechanical cell disruption significantly improved protein digestibility without a substantial impact on the amino acid profile and score, resulting in the increase of protein digestibility-corrected amino acid score to 0.77, 0.81, and 0.46, respectively, for disrupted CV, CS, and AO. There was a strong correlation between in vitro protein digestibility and apparent protein digestibility (r = 0.986), and also between in vitro protein digestibility-corrected amino acid score and in vivo protein digestibility-corrected amino acid score (r = 0.994). The results suggest that the CV and CS are acceptable sources of protein for humans and animals and quality can be markedly improved by mechanical cell wall disruption. Additionally, in vitro protein digestibility measured using the pepsin–pancreatin method may be used to screen protein product candidates, save animals, reduce cost, and accelerate product development.

Microalgae as Sources of High-Quality Protein for Human Food and Protein Supplements

“…Accurately determining protein content in the raw material of microalgae is critical to the valorization of algal biomass for food and other forms of application, as microalgae are currently used predominantly in the form of raw biomass, while increased use of the purer forms of microalgal protein products may be achieved in the near future. In this regard, the method of elemental analysis is well-accepted and widely used for its multiple advantages, including, but not limited to, its simplicity, rapidity, and inexpensiveness, while the accuracy depends on the use of an appropriate N-to-P conversion factor [ 81 ]. Therefore, there exists a demand to establish an N-to-P conversation factor in reference to the value generated with a more reliable method, such as the summation of anhydroamino acids [ 48 ].…”

“…For example, casein has the AAS values of 1.03–1.32 based on several studies [ 19 , 125 , 126 ]. Microalgae Chlorella vulgaris and Chlorella sorokiniana had AAS values of 1.10 and 1.16, respectively, while Acutodesmus obliquus had an AAS value of 0.86 [ 81 ]. Kent et al (2015) reported that the microalgae Nannochloropsis sp., Scenedesmus sp., Dunaliella sp., and Chlorophyta sp.…”

“…In Canada, PER is still the official method for protein quality assessment while PDCAAS is also accepted [ 132 ]. Most protein products have PDCAAS values of lower than one, especially the plant [ 125 ] and microalgal proteins [ 81 , 133 ].…”

“…Wang et al, (2020) evaluated the protein quality of Chlorella vulgaris , Chlorella sorokiniana , and Acutodesmus obliquus using a rat bioassay [ 81 ]. The raw biomass of these 3 species had PDCAAS values of 0.63, 0.64, and 0.29, respectively, which are lower than that of animal proteins, such as egg, milk, and meat proteins, but the value of the first 2 species were comparable to, or better than, wheat and pulse proteins [ 134 , 135 ].…”

“…The PDCAAS values of Chlorella vulgaris and Chlorella sorokiniana are higher than that of cooked pulses [ 135 ]. The observed low PDCAAS of microalgal protein is primarily a result of low TPD, as Chlorella vulgaris and Chlorella sorokiniana had AAS values of over 1.0 [ 81 ]. Indeed, while ADP values for Chlorella vulgaris , Chlorella sorokiniana , and Acutodesmus obliquus were 60.6%, 54.8%, and 32.6%, TPD values were all higher, at 64.7%, 59.3%, and 37.9%, respectively.…”