*4.1. Life-Cycle Accessment Method of Carbon Footprint*

The ISO 14,040 life-cycle assessment (LCA) is an internationally accepted methodology used to calculate a product's environmental footprint [103]. Life-cycle carbon footprints (CFs) of dairy products cover the direct emission from the dairy factory (scope 1); the energy carrier footprint for factory operations (natural gas, steam, power, nitrogen, and compressed air in scope 2); and the raw material, packaging, and logistics in scope 3. In addition, the life-cycle CF comprises the emissions from the dairy farm (upstream) and distribution center (downstream) [104]. The boundaries are set as

shown in Figure 5a. The CFs of MPL products were reported as equivalent CO2 emission for one kg of MPLs, according to the ISO 14,067 reporting standards [105].

**Figure 5.** (**a**) Boundary definition of the life-cycle carbon footprints (CFs) of dairy products and exemplary emissions from scope 1 (direction emission), scope 2 (energy carriers), and scope 3 (raw material procured, packaging material, and transportation). (**b**) Cascade of CFs of BMP, BMC, and MPLs using the following processes: "CO2-DEM" (supercritical CO2 and DME), "DME" (supercritical DME), and "Solvent" (hexane and ethanol extraction and acetone de-fatting, kg equivalent CO2/products). Scopes of BM CFs: adapted from References [45,106–109]. MPLs, milk phospholipids; BM, buttermilk; BMC, BM concentrate; DME, dimethyl ether; SFE, supercritical fluid extraction. \* Ultra High Temperature processing.

The CF of BM (baseline CF, 1.10 kg CO2/kg BM powder) was cited directly from data derived from the Unified Livestock Industry and Crop Emissions Estimation System (ULICEES) model in Canada [45]. The data abides by the Intergovernmental Panel on Climate Change (IPCC) methodology [106]; it covers emissions like methane [45], nitrous oxide [107], and carbon dioxide using the F4E2 model [108]; and uses an allocation matrix to partition six inventory flows (i.e., fuel, power, raw milk transportation, alkaline/acid, water, and waste water) into 11 major dairy products [109].

In this study, BM was assumed as the starting material for producing MPLs. Therefore, the CF for producing BM was set as the baseline. The CF of MPLs in Figure 5b and Table 4 is a sum of the CF of BM (as the baseline) and the CF for extracting MPLs from BM at dairy factories. The starting amount of BM was assumed to be 100 kg (1.3%, *w*/*w*, DM basis). Since MPLs are considered as the target products, CF of protein in the MPL fractions was not included in the estimations.

The CF of BM concentrate (BMC) via membrane separation (MS) was calculated using the equation: CFBMC = CFBM + CFMS, where CFBMC, CFBM, and CFMS were the CF of BMC, BM, and MS, respectively. The CF of MPL products using SFE or solvent extraction was calculated using the equation CFMPLs = CFBMC + CFSFE or CFMPLs = CFBMC + CFSol where CFMPLs, CFBMC, CFSFE, and CFSol were the CF for the MPL product, BMC, SFE, and solvent extraction process, respectively, as illustrated in Figure 5b. The CF for BMP (1.5% purity) was 1.10 kg CO2/kg MPL, and the CF of BMC (11.0% purity) was 87.40 kg CO2/kg MPL, as calculated in Table 5. Starting from BMC, the CFs of MPL products were 170.59, 159.07, and 101.05 kg CO2/kg MPL for processes of CO2-DME supercritical fluid extraction, DME SFE, and organic solvent extraction, respectively.


**Table 4.** Normalized carbon footprints of milk phospholipids (kg CO2/kg MPLs).

BM, buttermilk; BMC, BM, concentrate; MPLs, milk phospholipids; C6, hexane; DME, dimethyl ether; SFE, supercritical fluid extraction; UF/DF, ultra/dia-filtration; CF, carbon footprint. Membrane filtration power consumption: 1.486 kWh/kg products [110]; Canada power CF factor: 0.1567 kg CO2/kWh [111]; CF of reusable solvents (DME, hexane and ethanol): 0.16 kg CO2/kg solvent; reused acetone CF: 0.42 kg CO2/kg solvent [112]; DME CF 1.01 kg/kg; 84% reuse [113]; baseline of BM: 1.1 kg CO2/kg BMP [45]; SFE CO2 reuse 95% [114]; SFE CO2/DME power cost estimation 100 kWh/kg extract [115].
