This represents28x10°x50%=14x10° g protein This represents 1. 4 x 10"/70=200 days worth of protein for one person. After 72 hours there would be 2" or 4.7 1021 cells This represents 4.7x10 0-4.7x 10 g dry weight cells This represents 4.7x 10x50%=2.35x 10 gprotein. This represents 2.35x 10/70=ca 3 x 10 meals, or enough protein to feed the entire population of China for 3 days! You have now demonstrated the capability of exponentially increasing microbial populations to rapidly produce protein. However, although such outputs of protein are possible in theory, they cannot be achieved in practice, since exponential growth cannot be maintained for such periods because bioreactors are limited in size 4.3.2 The disadvantages of micro-organisms as a protein source Let us consider some of the disadvantages of micro-organisms compared to of pr amiprofigd Are hey n tritis] guidelines on th n utritional qualit of sap have been given by profiles and feeding trials in animals. While most microbial cells are rich in protein, many do not contain sufficient quantities of essential amino adids. For instance, algal and fungal cells tend to lack methionine Microbial cells may not be as easy to digest as conventional protein sources, for instance algae have cellulose cell walls which must be digestibility broken up if the proteins within the cell are to be easily digested by humans. The requirements for food are more strict than those for feed like to eat? Humans are their food. Microbial cells have little taste or smell, or even smell or taste unpleasantly to some people. The texture may not be the same as in conventional foods, particularly with unicellular organisms. These draw-backs can be overcome by adding a proportion of SCP to manufactured foods. However, even when SCP is incorporated into manufactured foods it may not have suitable characteristics such as stability, ability to bind water or fats, or ability to form gels, emulsions or foams. SCP for feed does not have to meet such strict requirements adverse What happens after you have eaten them? Even if a micro-organism is palatable it may effects not necessarily be acceptable to the human digestive system, and if eaten in quantity can produce indigestion, flatulence, nausea, vomiting or diarrhoea. As little as 15 g yeast cells per day can produce such effects in humans cost How much do they cost? sCP must compete in price with conventional protein foods and feeds. In itries where protein foods are readily available they can be relatively cheap. It has not always been possible to produce SCP at competitive prices Are they safe to eat? Micro-organisms which are pathogenic or toxic obviously can not be used as sCP sources. In addition most microbial cells have a higher content of nucleic acid, particularly RNA, than conventional foods. When such cells are digested by animals these nucleic acids are metabolised to uric acid Unlike most other mammals humans do not possess uricase, which oxidises uric acid to soluble allantoid for64 Chapter 4 This represents 2.8 x 1d x 50% = 1.4 x ldg protein. This repments 1.4 x 1$/70 = 200 days worth of protein for one person. After 72 hours there would be 2n or 4.7 x Id' cells. This represents 4.7 x Id' x lo-'' = 4.7 x 1O"g dry weight cells. ms represents 4.7 x IO" x 50% = 2.35 x 1o"g protein. This represents 2.35 x 10"/70 = ca. 3 x lo9 meals, or enough protein to feed the entire population of China for 3 days! You have now demonstrated the capability of exponentially increasing microbial populations to rapidly produce protein. However, although such outputs of protein are possible in theory, they cannot be achieved in practice, since exponential growth cannot be maintained for such periods because bioreactors are limited in size. 4.3.2 The disadvantages of micro-organisms as a protein source Let us consider some of the disadvantages of micro-organisms compared to conventional sources of protein. amino acid profiles digestibility importance of organoleptic properties edverse effects cost =w uric acid Are they nutritious? Guidelines on the nutritional quality of s8 have been given by the Protein Advisory Group (PAG) of the United Nations, and are based on amino acid profiles and feeding trials in animals. While most microbial cells are rich in protein, many do not contain sufficient quantities of essential amino acids. For instance, algal and fungal cells tend to lack methionine. Microbial cells may not be as easy to digest as conventional protein sources, for instance algae have cellulose cell walls which must be broken up if the proteins within the cell are to be easily digested by humans. The requirements for food are more strict than those for feed. What are they like to eat? Humans are particular about the organoleptic properties of their food. Microbial cells may have little taste or smell, or even smell or taste unpleasantly to some people. The texture may not be the same as in conventional foods, particularly with unicellular organisms. These draw-backs can be ovexrome by adding a proportion of SCP to manufactured foods. However, even when SCP is incorporated into manufactured foods it may not have suitable characteristics such as stability, ability to bind water or fats, or ability to form gels, emulsions or foams. SCP for feed does not have to meet such strict requirements. What happens after you have eaten them? Even if a micro-organism is palatable it may not necessarily be acceptable to the human digestive system, and if eaten in quantity can produce indigestion, flatulence, nausea, vomiting or diarrhoea. As little as 15 g yeast cells per day can produce such effects in humans. How much do they cost? SCP must compete in price with conventional protein foods and feeds. In countries where protein foods are readily available they can be relatively cheap. It has not always been possible to produce SCP at competitive prices. Are they safe to eat? Micmrganisms which are pathogenic or toxic obviously can not be used as SCP sources. In addition most microbial cells have a higher content of nucleic acid, particularly RNA, than conventional foods. When such cells are digested by animals these nucleic acids are metabolised to uric acid. Unlike most other mammals, humans do not possess uricase, which oxidises uric acid to soluble allantoid for