But according to the researchers, who have detailed their finding in an upcoming issue of the Journal of Archaeological Science, proteins do survive under extreme conditions. Furthermore, in contrast to commonly analyzed lipids, they may bear the hallmarks of technological processes used to prepare the food. Altogether, they can be highly informative molecules.
"Our work opens new perspectives in the analysis of ancient material. But most importantly, it shows the technology behind ancient cheese-making," Germany team leader Andrej Shevchenko, an analytical chemist at Germany's Max Planck Institute of Molecular Cell Biology and Genetics, told Discovery News.
Indeed, the analysis revealed Xiaohe's cheese wasn't made with rennet, an chymosine containing enzyme complex from calf intestine which was widely used since ancient times for curdling ruminant milk.
It was instead produced by combining milk with a mix of Lactobacillus kefiranofaciens and other lactic acid bacteria and yeasts.
The technique is still used today to make kefir cheese, similar to cottage cheese, and a kefir probiotic lactose-free beverage, food with a slightly sour taste first mentioned by Marco Polo in 13th century.
"It's the earliest known dairy practice that persists until present times in an almost unchanged way. The discovery moves the mysterious history of kefir as far as to the second millennium B.C., making it the oldest known dairy fermentation method," archaeologist Yimin Yang at the University of Chinese Academy of Sciences, Beijing, told Discovery News.
Edible for the lactose-intolerant inhabitants of Asia, the mummies' cheese was very simple to make. Kefir fermentation did not require slaughtering the livestock to obtain the curdling enzyme.
Furthermore, milk fat might have been physically removed in kefir cheese production, as now is commonly practiced in rural areas across the Eurasia steppe and also in Tibet.
"It's the first direct evidence that milking spread to Eastern Eurasia," Wang said.
Kefir production could have been scaled up or down according to the actual demand: dried kefir starter grains can be stored for years without losing their fermentation capacity. Fermented milk could be either consumed as a probiotic beverage or curdled protein mass strained into a cheese with extended shelf life and high nutrition value.
"This is a technology with the potential for mass production. It could have changed the nutritional habits of ancient populations of Eastern Eurasia," Andrej Shevchenko said.