Combined substrate, enzyme and yeast feed in simultaneous saccharification and fermentation allow bioethanol production from pretreated spruce biomass at high solids loadings

Table 2 Summary of simultaneous saccharification and fermentation (SSF) experiments

SSF	Substrate	Water insoluble solids (WIS)		Yeast loading amount, g	Enzyme loading, FPU	Glucose^e, g	Ethanol
SSF	Substrate	% (w/w)	Amount, g	Yeast loading amount, g	Enzyme loading, FPU	Glucose^e, g	g L^-1	%^f
B^*	Slurry	19	285	7.5	2138	68.0	2.1	2
B	Solid fraction^a	20	300	7.5	2250	0.1	9.5	13
FB	Solid fraction^a	10 to 20^b	50 to 300	7.5	1125 to 2250^d	0.1	27.9	37
FB	Solid fraction^a	10 to 20^b	50 to 300	3.5 to 7.5^c	2250	0.5	33	44
FB	Solid fraction^a	10 to 20^b	50 to 300	3.5 to 7.5^c	1125 to 2250^d	0.5	40	53

*Batch SSF was carried out with prehydrolysis of 24 h. ^aThe solid fraction with a WIS content of 31% (w/w) was not washed and therefore, free sugars and inhibitors exist in it but in lower amounts compared to the slurry. ^bFed-batch experiments started as a batch at 10% WIS with a working weight of 500 g. After 8 h, substrate was fed every 4 or 8 h until 65 h to a final WIS content of 20%. ^cFed-batch experiments started as a batch with an initial yeast loading of 3.5 g (7 g L^-1). After 8 h, cell suspension was fed every 4 or 8 h until 65 h to a final yeast amount of 7.5 g (5 g L^-1). ^dFed-batch experiments started as a batch with an initial enzyme loading of 1125 filter paper units (FPU) (22.5 FPU g of WIS^-1). After 8 h, enzyme preparation was fed every 4 or 8 h until 65 h to a final enzyme loading of 2250 FPU (7.5 FPU g of WIS^-1). ^eResidual amounts of glucose at the end of 96 h of SSF. ^fYield of theoretical maximum based on the total amount of glucose, mannose and galactose in the pretreated spruce. Yields are calculated for the concentrations after 96 h of SSF.