Digital Re-print - January | February 2013

Yeast in aquaculture

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Yeast products are getting more and more popular in aquaculture. However many products (as a

whole or as fractions) are on the aquacul-ture market at the moment and differen-tiating between one from another can be difficult. This small article aims at shading some lights on the subject and explains that all yeast products are not equal.

Yeast is a unicellular organism belong-ing to the kingdom of Fungi. More than a thousand species have been found in two major phyla: Basidiomycota and Ascomycota in which belong species able to duplicate by budding such as Saccharomyces cerevisiae.

Due to their unique properties to grow under aerobic conditions and produce gas and ethanol under anaerobic conditions, some yeast (mostly S. cerevisiae) have been used for the manufacture of fermented foods such as bread , beer and wine for a long time. Yeasts are also used as single sell protein source in animal nutrition and in aquaculture under various forms and species (Torulaspora, Torulopsis, Kluyveromyces, Saccharomyces and caetera). They can be found for example in shrimp and marine fish larval feeds or included as a protein source in aquafeeds.

The reasons for this extensive use is its excellent nutri-tional contents, its easy supply in dried form or under liq-uid form when bak-ery yeast plants or breweries are near aquafeed plants, and nowadays a competi-

tive price in regards to other protein sources such as fish or soybean meal. Further appli-cations are being developed for yeast as functional feed additives as probiotic live yeast, yeast fractions (yeast cell walls, yeast extracts) or as a source for more purified products such as beta-glucans and nucle-otides. The production process of yeast can allow the possibility to incorporate trace minerals and then produce highly bioavail-able organic trace minerals, also known as selenium and chromium yeast.

The pink yeast Phaffia rhodozyma, is naturally rich in astaxanthin and has been used for some time as natural source of the pigment in salmonids. Although now it tends to be replaced by bacterial products which have a higher concentration and whose cell wall is more easily degraded. We will only refer in the following article on products coming from S. cerevisiae origin.

Nutritional properties of yeastTypical dry yeast composition is 93-97

percent dry matter and can contain from 40 to 60 percent crude protein nitrogen, 35-45 percent carbohydrates, and 5-9 per-cent lipids. A quite important fraction of the nitrogen is under the form on nucleic acids (up to 12%) that can lead to produce significant level of uric acid if consumed at high concentration, like meat. The amino acid profile of yeast is close to soybean meal and therefore well adapted to animal nutri-tion; it is rich in Glutamic acid and Lysine (up to 8%). Yeast is naturally rich in B vitamins such as biotin, thiamine and folic acid. It also produces niacin but contrary to some belief does not produce B12 Vitamin. Ergosterol which is a significant fraction of yeast cell wall, also is also a precursor of Vitamin D2 by using UV treatments.

Baker’s yeastEven if their name remains Saccharomyces

cerevisiae (cerevisiae for beer), most of the strains of Baker’s yeast have been selected for their high fermentative power, particu-

larly useful for bakers. Strains are specific to the type of bread and the region where it is sold, in order to respond to different bread making conditions (French bread, white bread, flat bread, croissant, etc.) and resist to different process conditions (osmotic pressure from high sugared bread, freezing, acid-ity of sour dough,…).

Baker’s yeast comes as a pure and primary culture grown on sugar substrate such as molasses. The pro-duction is performed under very strict conditions in order to maintain the genetic puri-ty, consistency, specificity and efficacy of the strains. (Figure 1). It can be sold under differ-

Yeast in aquacultureby Philippe Tacon PhD, Lesaffre Feed Additives, France

Figure 1: Yeast manufacturing process (primary grown culture)

Grain&feed millinG technoloGy26 | January - february 2013

FEATURE

Yeast products are getting more and more popular in aquaculture. However many products (as a

whole or as fractions) are on the aquacul-ture market at the moment and differen-tiating between one from another can be difficult. This small article aims at shading some lights on the subject and explains that all yeast products are not equal.

Yeast is a unicellular organism belong-ing to the kingdom of Fungi. More than a thousand species have been found in two major phyla: Basidiomycota and Ascomycota in which belong species able to duplicate by budding such as Saccharomyces cerevisiae.

Due to their unique properties to grow under aerobic conditions and produce gas and ethanol under anaerobic conditions, some yeast (mostly S. cerevisiae) have been used for the manufacture of fermented foods such as bread , beer and wine for a long time. Yeasts are also used as single sell protein source in animal nutrition and in aquaculture under various forms and species (Torulaspora, Torulopsis, Kluyveromyces, Saccharomyces and caetera). They can be found for example in shrimp and marine fish larval feeds or included as a protein source in aquafeeds.

The reasons for this extensive use is its excellent nutri-tional contents, its easy supply in dried form or under liq-uid form when bak-ery yeast plants or breweries are near aquafeed plants, and nowadays a competi-

tive price in regards to other protein sources such as fish or soybean meal. Further appli-cations are being developed for yeast as functional feed additives as probiotic live yeast, yeast fractions (yeast cell walls, yeast extracts) or as a source for more purified products such as beta-glucans and nucle-otides. The production process of yeast can allow the possibility to incorporate trace minerals and then produce highly bioavail-able organic trace minerals, also known as selenium and chromium yeast.

The pink yeast Phaffia rhodozyma, is naturally rich in astaxanthin and has been used for some time as natural source of the pigment in salmonids. Although now it tends to be replaced by bacterial products which have a higher concentration and whose cell wall is more easily degraded. We will only refer in the following article on products coming from S. cerevisiae origin.

Nutritional properties of yeastTypical dry yeast composition is 93-97

percent dry matter and can contain from 40 to 60 percent crude protein nitrogen, 35-45 percent carbohydrates, and 5-9 per-cent lipids. A quite important fraction of the nitrogen is under the form on nucleic acids (up to 12%) that can lead to produce significant level of uric acid if consumed at high concentration, like meat. The amino acid profile of yeast is close to soybean meal and therefore well adapted to animal nutri-tion; it is rich in Glutamic acid and Lysine (up to 8%). Yeast is naturally rich in B vitamins such as biotin, thiamine and folic acid. It also produces niacin but contrary to some belief does not produce B12 Vitamin. Ergosterol which is a significant fraction of yeast cell wall, also is also a precursor of Vitamin D2 by using UV treatments.

Baker’s yeastEven if their name remains Saccharomyces

cerevisiae (cerevisiae for beer), most of the strains of Baker’s yeast have been selected for their high fermentative power, particu-

larly useful for bakers. Strains are specific to the type of bread and the region where it is sold, in order to respond to different bread making conditions (French bread, white bread, flat bread, croissant, etc.) and resist to different process conditions (osmotic pressure from high sugared bread, freezing, acid-ity of sour dough,…).

Baker’s yeast comes as a pure and primary culture grown on sugar substrate such as molasses. The pro-duction is performed under very strict conditions in order to maintain the genetic puri-ty, consistency, specificity and efficacy of the strains. (Figure 1). It can be sold under differ-

Yeast in aquacultureby Philippe Tacon PhD, Lesaffre Feed Additives, France

Figure 1: Yeast manufacturing process (primary grown culture)

Grain&feed millinG technoloGy26 | January - february 2013

FEATURE

ent forms and packaging (instant dried yeast, active dry yeast, compressed, cream).

The primary grown culture controlled process makes also a very consistent base for the production of yeast extracts, autolysed yeast, yeast cell walls and their derivate: nucleotides and beta-glucans. Yeast cell walls produced from Baker’s yeast usually have a high content of mannans. They are recognised as good toxin binders. Fractions coming from baker’s yeast have a light beige colour.

The most popular aquaculture application of Baker’s yeast is in hatcheries where it is a major feed source for artemia and rotifer (see for example Couteau et al 1990).

Brewer’s yeastBrewer’s yeast can be identified either

as the ferment used in brewery industries (Yeast primary production) or the by-prod-uct of these industries which is the form mainly used in aquaculture. In the latter case, yeast biomass is harvested from the fermen-tation vats at the end of beer fermentation. It can be sold under liquid form (18-20% of dry matter) but preferentially as inactive yeast spray or drum dried. It can also been grown as a more controlled product and specific strains and find its way to human care as a food supplement and holistic thera-peutic, also known as natural brewer’s yeast.

Brewer’s yeast for aquafeed applications is sold by trading companies as a commodity based on the protein content, or by local breweries in need to dispatch their slurry. The quality and the supply of these products can be inconsistent and depends greatly on the source of supply.

The nutritional content is similar as the one in baker’s yeast, but contains more trace minerals such as selenium and chromium. The protein content of brewer yeast is relatively high and and its amino acid content is similar to baker’s yeast. Numerous works have shown the efficacy of Brewer’s yeast to replace partially or totally the proteins found in fish and vegetable meal in fish and shrimp. Shrimp feeds formulators typically incorporate brewer’s yeast in their formula at the rate of two to four percent.

Brewer’s yeast can be used to produce yeast fractions, however due to the nature of brewer’s yeast and the specificity of the production processes, the quality is less consistent than in baker’s yeast. Products coming from brewery yeast tend to have a distinctive bitter smell and taste and a darker colour than the ones coming from baker’s yeast.

Ethanol yeast Ethanol yeasts are harvested after having

performed alcoholic fermentation and distil-lation for the conventional production of Bioethanol from sugar-cane, beet sugar or grains syrup. In the first case, the yeast biomass is harvested and then dried with the recy-cled energy used to heat the vegetal material. The majority of ethanol yeast comes from Brazil.

Production prices and selling prices are very low, however the quality, such as the protein content is very inconsistent. The supply depends on the activity of the bioetha-nol plants and can also be inconsistent.

Another concern is the sanitary safety of these products. Antibiotics are sometimes added to the process in order to prevent bacteria competing with the yeast for nutri-ents and avoiding yield decrease. It is there-fore possible that some antibiotic residues and possibly other toxins might be left in the final dried product.

Autolysed yeast – Inactive Dried Yeast

Inactive and Autolysed yeast come from primary grown cultures or Brewer’s yeast.

Table 1: Effect of live yeast Actisaf on growth and survival parameters in tilapia under stress conditions. (n=3, P<0.05, measures with different letters are significantly different)

Treatment Survival (%) SGR FCR PER

CON 40% -10 fry 75.0ab 3.33a 3.11e 0.83ab

CON 40% -20 fry 64.8a 3.47a 3.26e 0.78ab

Act 40% - 10 fry 87.5bc 5.80d 1.43abc 1.89cd

Act 40% - 20 fry 92.6c 5.43c 1.01a 2.64d

Act 27% - 10 fry 91.7bc 5.46cd 1.62bc 2.26c

Act 27% - 20 fry 96.29c 5.24c 1.17ab 3.17eFigure 2: Schema of a process to produce yeast

extracts and yeast cell walls

Grain&feed millinG technoloGy January - february 2013 | 27

FEATURE

Eventsthe farmer. “That wide-ranging discus-sion should highlight some important issues about the potential of digital technology for our industry going for-ward,” says Gilbert.

“We will then open the discus-sion to questions and comments from attendees. This will be an interesting part of the session, where equipment suppliers can gain some feedback while feed manufacturers can gain a fuller understanding of the benefits that digital engineering is delivering in terms of cost and efficiency,” adds Gilbert.

The debate has been struc-tured along the lines of the production, chain from delivery, through storage and grinding to pelleting and processing, cooling and drying to product storage and dispatch. Companies participating include, Foss, Adifo, Amandus Kahl, Wenger and Andritz.

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and the chairman of the two-hour seminar. “There will be seven or eight very short presentations made by supply companies that have devel-oped products either using or that use digital technologies to improve the feed manufacturing process.

Yiannis Christodoulou, president, Agentis Innovations, Thailand will be speaking on behalf of his company. He explains the theme of his presentation, “the animal and aquafeed industries have developed into sophisticated processing systems requiring a high degree of either manual or automated control. Automation is often only par-tially utilised within the feed industry as a means to improve accuracy of pro-duction and reduce reliance on labour. Many businesses do not capitalise on the huge economic benefits of a holis-tic approach to automation which includes the complete process from order processing to farm delivery.”

Following the presentations, speak-ers will be encouraged to discuss their developments and how they have or will impact feed manufacturing for the benefit of the feed manufacturer and

summit is another best-practice of the value VIV adds to its events. Developed in close co-operation with loyal advisors to VIV, this is a premium-quality conference based on my personal initiative,” he says.

A very special seminarPerendale Publishers will be tak-

ing part in one of the CropTech-FeedTech Asia seminars on March 13, 2013. Called ‘Digital engineering in feed manufacturing’, this unique seminar is for those working in the area of mill technology and aims at providing background information on intelligent solutions that have been introduced to address processing chain dilemmas.

“What’s unique for our industry about this event is its format,” says Roger Gilbert, publisher of Grain and Feed Milling Technology magazine

VIV Asia presents three special features: CropTech-FeedTech Asia, focusing cost-effective feed production, MeatTech, highlighting the latest technologies to produce safe products that can be used eas-ily by the consumers, and the VIV Animal Health Summit Asia.

The summit is the first confer-ence in Asia to address the rapidly growing concerns about the use of antibiotics in animal protein produc-tion, both at CEO and technical level. On a personal note, Berculo is particularly excited about the VIV Animal Health Summit Asia. “The

Grain&feed millinG technoloGy January - february 2013 | 57

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experience in animal nutrition.

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www.yeast-science.com

They are major products within the food industry as flavour enhancers and in pet food as feed attractants. They are used in aquaculture feeds as a source of protein and nitrogen. Brewer’s yeast, and its ethanol equivalent, is mostly favoured as it is cheaper than baker’s yeast. They are also easier supplied as yeast suppliers prefer to sell the more controlled and tailored Baker’s yeast on food markets.

Inactive yeast is a yeast that has been deactivated by high temperature drying (often spray drying). The cells come as a whole and the cell wall is not ruptured mak-ing the access to intracellular material (amino acids, vitamins…) difficult. A way to access these materials is to partially hydrolyse the yeast cell wall to let the cellular content be partially released from the cell. This can be facilitated by activating the internal autolytic enzymes of the live yeast (autolysis), add-ing external enzymes (notably proteolysis) or playing on the osmotic pressure to rupture the cell wall (plasmolysis). Different grades of autolysed yeast can be obtained depending on the level of autolysis (from partial to total). The final product is a mix-ture of cellular content and yeast cell wall. Furthermore the autolysis process degrades

protein and forms peptides (dipeptides to tetra peptides) and oligonucleic acids which are readily digestible by the animal. Again here depending on the original yeast material used, autolysed and inactive yeast quality can be very different.

Live yeast as probiotics Live yeast helps regulate the gut micro-

biota. Its effects have been shown, first in human where it can reduce diarrhoea, espe-cially with children. Specific strains have then been developed and produced industrially such as S. cerevisiae boulardii or S. cerevisiae Sc 47 (Actisaf) for the animal nutrition market. It is a common practice now to sup-plement feeds to increase milk production in dairy cows or help piglets survival.

Live yeast are characterized by their living cells count, expressed by colony forming unit (cfu per gram), typically ten billions cfu/g. Dosages are made in the feeds as dilutions to get an efficient cfu count per g of feed, a 1000 fold dilution giving a 10.107 per g of feed for example. Viability of the yeast is mandatory for its effect and cfus should be checked before and after pelleting using plate counts.

Despite the increasing use of yeast as

a probiotic in terrestrial animals, there are only a few numbers of works studying its

effect in fish as a gut functions stabiliser. The major reason is that live yeast does not resist the severe condi-tions of the manufacturing processes of aquafeeds (high temperatures,

steam, long condi-tioning times, see Aguirre-Guzzman et al 2002). The

studies are then dif-ficult to transfer from

lab conditions to farm using commercial feeds.

All the work published so far was made with yeast either top dressed on

feeds or incorporated in pressed (uncooked) feeds. Nevertheless some direct effects to

the gut maturation have been found in sea bass with a species extracted from the rain-bow trout gut Debaryomyces hansenii (see the works from Tovar-Ramirez and also the reviews by Chi et al 2006 and Gatesoupe 2007). Marine yeasts and yeasts isolated from fish seem a very logical choice to use in species of aquaculture interest. However, such material is often difficult to grow under industrial conditions and did not lead to the development of an actual product yet. The products on the market are therefore often from S. cerevisiae origin. It has to be noted that up to now, no yeast products have been registered in EU as a probiotic in aquaculture.

As an example of S. cerevisiae effects, (Lara Flores et al 2003, 2010) Table 2 shows some works done in tilapia fry fed for 3 weeks with feeds supplemented with Actisaf (also knwn as Biosaf) at 1 kg/T in two diets (40% and 27% proteins) and at 2 crowded conditions (1 fry per L or 1 fry per 2L).

All the yeast treatments also increased the Alkaline Phosphatase activity, and we can see a better improvement of feed conver-sion ratio (FCR) and survival under stressful conditions (low protein percentage and crowded conditions). There is also a better

protein efficiency ratio (PER) and digestive enzyme activity when Actisaf is used.

Live yeast can be used directly on farm, where it has been showed (empirically) to improve water quality in shrimp and fish ponds. It is either used alone or mixed with bacteria. Farms producing mash feed onsite also add yeast in order to degrade cellulolytic material to ensure a better digestion.

Yeast culture or fermented yeastYeast culture is a particular product in

which yeast is allowed to ferment. Yeast biomass, substrate and fermented extracel-lular metabolites are then dried.

Yeast extractsYeast extracts (YE) come from the fur-

ther hydrolysis and purification of autolysed yeast. Insoluble yeast cell walls are separated from the cellular content by centrifugation. YE are very soluble, rich in peptides (up to

Figure 4: Cumulative mortality after immersion with L. Anguiilarum (blue line is control, orange line is

Pronady at 0.5g/kg. n=3, Pronady significantly decreases mortality at 120h. P<0.01)

Figure 3: Number of pellets remaining in the feeding tray one hour after feeding (n=4, YE are significantly different than

control at P<0.05).

Grain&feed millinG technoloGy28 | January - february 2013

FEATURE

They are major products within the food industry as flavour enhancers and in pet food as feed attractants. They are used in aquaculture feeds as a source of protein and nitrogen. Brewer’s yeast, and its ethanol equivalent, is mostly favoured as it is cheaper than baker’s yeast. They are also easier supplied as yeast suppliers prefer to sell the more controlled and tailored Baker’s yeast on food markets.

Inactive yeast is a yeast that has been deactivated by high temperature drying (often spray drying). The cells come as a whole and the cell wall is not ruptured mak-ing the access to intracellular material (amino acids, vitamins…) difficult. A way to access these materials is to partially hydrolyse the yeast cell wall to let the cellular content be partially released from the cell. This can be facilitated by activating the internal autolytic enzymes of the live yeast (autolysis), add-ing external enzymes (notably proteolysis) or playing on the osmotic pressure to rupture the cell wall (plasmolysis). Different grades of autolysed yeast can be obtained depending on the level of autolysis (from partial to total). The final product is a mix-ture of cellular content and yeast cell wall. Furthermore the autolysis process degrades

protein and forms peptides (dipeptides to tetra peptides) and oligonucleic acids which are readily digestible by the animal. Again here depending on the original yeast material used, autolysed and inactive yeast quality can be very different.

Live yeast as probiotics Live yeast helps regulate the gut micro-

biota. Its effects have been shown, first in human where it can reduce diarrhoea, espe-cially with children. Specific strains have then been developed and produced industrially such as S. cerevisiae boulardii or S. cerevisiae Sc 47 (Actisaf) for the animal nutrition market. It is a common practice now to sup-plement feeds to increase milk production in dairy cows or help piglets survival.

Live yeast are characterized by their living cells count, expressed by colony forming unit (cfu per gram), typically ten billions cfu/g. Dosages are made in the feeds as dilutions to get an efficient cfu count per g of feed, a 1000 fold dilution giving a 10.107 per g of feed for example. Viability of the yeast is mandatory for its effect and cfus should be checked before and after pelleting using plate counts.

Despite the increasing use of yeast as

a probiotic in terrestrial animals, there are only a few numbers of works studying its

effect in fish as a gut functions stabiliser. The major reason is that live yeast does not resist the severe condi-tions of the manufacturing processes of aquafeeds (high temperatures,

steam, long condi-tioning times, see Aguirre-Guzzman et al 2002). The

studies are then dif-ficult to transfer from

lab conditions to farm using commercial feeds.

All the work published so far was made with yeast either top dressed on

feeds or incorporated in pressed (uncooked) feeds. Nevertheless some direct effects to

the gut maturation have been found in sea bass with a species extracted from the rain-bow trout gut Debaryomyces hansenii (see the works from Tovar-Ramirez and also the reviews by Chi et al 2006 and Gatesoupe 2007). Marine yeasts and yeasts isolated from fish seem a very logical choice to use in species of aquaculture interest. However, such material is often difficult to grow under industrial conditions and did not lead to the development of an actual product yet. The products on the market are therefore often from S. cerevisiae origin. It has to be noted that up to now, no yeast products have been registered in EU as a probiotic in aquaculture.

As an example of S. cerevisiae effects, (Lara Flores et al 2003, 2010) Table 2 shows some works done in tilapia fry fed for 3 weeks with feeds supplemented with Actisaf (also knwn as Biosaf) at 1 kg/T in two diets (40% and 27% proteins) and at 2 crowded conditions (1 fry per L or 1 fry per 2L).

All the yeast treatments also increased the Alkaline Phosphatase activity, and we can see a better improvement of feed conver-sion ratio (FCR) and survival under stressful conditions (low protein percentage and crowded conditions). There is also a better

protein efficiency ratio (PER) and digestive enzyme activity when Actisaf is used.

Live yeast can be used directly on farm, where it has been showed (empirically) to improve water quality in shrimp and fish ponds. It is either used alone or mixed with bacteria. Farms producing mash feed onsite also add yeast in order to degrade cellulolytic material to ensure a better digestion.

Yeast culture or fermented yeastYeast culture is a particular product in

which yeast is allowed to ferment. Yeast biomass, substrate and fermented extracel-lular metabolites are then dried.

Yeast extractsYeast extracts (YE) come from the fur-

ther hydrolysis and purification of autolysed yeast. Insoluble yeast cell walls are separated from the cellular content by centrifugation. YE are very soluble, rich in peptides (up to

Figure 4: Cumulative mortality after immersion with L. Anguiilarum (blue line is control, orange line is

Pronady at 0.5g/kg. n=3, Pronady significantly decreases mortality at 120h. P<0.01)

Figure 3: Number of pellets remaining in the feeding tray one hour after feeding (n=4, YE are significantly different than

control at P<0.05).

Grain&feed millinG technoloGy28 | January - february 2013

FEATURE

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65%-70% of the product), free amino-acids like glutamic acid and vitamins. They also contain a high level of nucleic acid which can be further purified to increase the level of tasty 5’ nucleotides. They are used in aquac-ulture in functional feeds, and hatcheries, as a source of nucleotides complementing the de novo synthesis of cells in multiplication and helping boost immunity and anti-stress mechanisms.

Autolysed yeast and inactive yeast are commonly mistakenly sold under the label yeast extract in aquaculture. A good way to differentiate them is to look at the carbo-hydrate levels. Autolysed yeast has a level around 20-22% (mostly from the remaining YCW) whereas YE contain only three to six percent of carbohydrates.

The small peptides and free amino acids in YE can also prove to be a potent attractant for aquafeed in shrimp. In a trial performed in Thailand with white shrimp L. vannamei. Feed containing YE at 2 kg/T of feed was presented in feeding trays at the corner of hapas and the remaining feed was counted after one hour. We can see a faster

feeding when YE are included. (Tacon and Suyawanish 2011).

Yeast cell wallsYeast Cell Walls (YCW) represent the

shell of the yeast cell and are roughly 40-50 percent of the mass of the cell. YCW are composed mainly of fibrous polysaccharides glucans with beta 1,3 and beta 1,6 links, (50% and 8% respectively ), mannans under the form of Mannoproteins (40%) and chitin (2%) (see Lippke and Ovalle 1998). Further purification can lead to the production of either purified beta-glucans (50% and up) and mannoprotein (often used in wine mak-ing for clarification). The presence of these compounds often leads to the mislabelling of YCW as MOS or Beta-glucans.

These two carbohydrate types are very interesting for the aquaculture market, beta-glucans are direct stimulators of the immune systems in shrimp and fish, upon the stimula-tion of specific blood cells (granulocytes or macrophages). Mannans are involved in the binding to pathogenic bacteria (especially those with pili having mannose receptors)

and eliminate them from the intestine. It is also suspected that the mannanes act as prebiotics promoting the growth of benefi-cial bacteria.

YCW have been shown to be effective to improve the resistance to bacterial chal-lenges in numerous aquaculture species. Beta glucans have to be use carefully in aquaculture as some experiments report/negative effects in fish when used for prolonged periods at high concentrations. This can be avoided by careful choosing the source of YCW and using them either at high concentration (2 kg/T) only for a short period, or a low concentration continuously (0.5 g/Kg).

An example of sea-bass juveniles fed with Pronady (a YCW of the Lesaffre group) at 0.5 g/kg of feed for 8 weeks can be seen in Figure 4, showing a significant protection against L. Anguillarum without any growth difference with the control. However a minimal amount of YCW seems needed to be ingested before challenge in order to provide an efficient immunostimulation and so there might be a gap period when

Figure 5: Yeast rich in organic selenium manufacturing process

Grain&feed millinG technoloGy30 | January - february 2013

FEATURE

65%-70% of the product), free amino-acids like glutamic acid and vitamins. They also contain a high level of nucleic acid which can be further purified to increase the level of tasty 5’ nucleotides. They are used in aquac-ulture in functional feeds, and hatcheries, as a source of nucleotides complementing the de novo synthesis of cells in multiplication and helping boost immunity and anti-stress mechanisms.

Autolysed yeast and inactive yeast are commonly mistakenly sold under the label yeast extract in aquaculture. A good way to differentiate them is to look at the carbo-hydrate levels. Autolysed yeast has a level around 20-22% (mostly from the remaining YCW) whereas YE contain only three to six percent of carbohydrates.

The small peptides and free amino acids in YE can also prove to be a potent attractant for aquafeed in shrimp. In a trial performed in Thailand with white shrimp L. vannamei. Feed containing YE at 2 kg/T of feed was presented in feeding trays at the corner of hapas and the remaining feed was counted after one hour. We can see a faster

feeding when YE are included. (Tacon and Suyawanish 2011).

Yeast cell wallsYeast Cell Walls (YCW) represent the

shell of the yeast cell and are roughly 40-50 percent of the mass of the cell. YCW are composed mainly of fibrous polysaccharides glucans with beta 1,3 and beta 1,6 links, (50% and 8% respectively ), mannans under the form of Mannoproteins (40%) and chitin (2%) (see Lippke and Ovalle 1998). Further purification can lead to the production of either purified beta-glucans (50% and up) and mannoprotein (often used in wine mak-ing for clarification). The presence of these compounds often leads to the mislabelling of YCW as MOS or Beta-glucans.

These two carbohydrate types are very interesting for the aquaculture market, beta-glucans are direct stimulators of the immune systems in shrimp and fish, upon the stimula-tion of specific blood cells (granulocytes or macrophages). Mannans are involved in the binding to pathogenic bacteria (especially those with pili having mannose receptors)

and eliminate them from the intestine. It is also suspected that the mannanes act as prebiotics promoting the growth of benefi-cial bacteria.

YCW have been shown to be effective to improve the resistance to bacterial chal-lenges in numerous aquaculture species. Beta glucans have to be use carefully in aquaculture as some experiments report/negative effects in fish when used for prolonged periods at high concentrations. This can be avoided by careful choosing the source of YCW and using them either at high concentration (2 kg/T) only for a short period, or a low concentration continuously (0.5 g/Kg).

An example of sea-bass juveniles fed with Pronady (a YCW of the Lesaffre group) at 0.5 g/kg of feed for 8 weeks can be seen in Figure 4, showing a significant protection against L. Anguillarum without any growth difference with the control. However a minimal amount of YCW seems needed to be ingested before challenge in order to provide an efficient immunostimulation and so there might be a gap period when

Figure 5: Yeast rich in organic selenium manufacturing process

Grain&feed millinG technoloGy30 | January - february 2013

FEATURE

the product is not efficient. (data from Dr. Morgane Henry, Hellenic Center for marine Research , 2011)

YCW products, depending on the quality of the autolysed yeast separation, contain also significant percentages of proteins and lipids. It should be noted that the lower the level of proteins, the higher of level of carbo-hydrates, and then the better immunostimu-lation from the YCW is. Various quality of YCW are on the animal production market and major differences can be found between products depending on the strain, the sub-strate used to produce the yeast, and event the drying process.

Mannans represent as most 25-27 per-cent of YCW in good quality YCW from primary grown yeasts, but can be found as low as 9 percent in crude preparation coming from industry by-products. Glucans or poly-glucose can range from 18 To 40 percent. YCW Protein level remains the most convenient indicator of quality, the best products being those having lower nitrogen content. The variability between batches can also be very high. Texture should be checked first. Good YCW often have a smooth, fine texture, low granu-lometry and a light beige colour. There is also the tendency to believe that all YCW are the same and that differentiation of products must be done to the highest level of glucans (sometimes measured as both

alpha and beta forms)or mannans. Not all the YCW are equal. Efficiency should be checked as a prerequisite to use, or change, YCW.

At LFA we have conducted a survey of four YCW (2 bakery and 2 brewery yeasts) produced in 4 of our own facto-ries in the same L. Anguillarum challenge in sea bass supplemented at 0.5 g/kg of feed for 8 weeks. Only 2 responded significantly (1 bakery, 1 brewery), the remaining 2 had even negative results at 4 weeks (lower survival than control). This result shows first that not all is under-stood in the way these products work and that one particular YCW cannot be replaced by another.

Selenium yeastYeast can be induced to be a source

of organic selenium, mainly under the form of seleniomethionine, which is then stored in proteins. During the growth of baker’s yeast, selenium is added to the medium and is replacing sulphur in methionine. The excess of selenium is then eliminated by careful washing steps (see Figure 5) to ensure that the selenium left is 97-99 percent organic. Selenium yeast should be then checked for the highest percentage of selenomethionine and the consistency between batches. Seleniomethionine is the main carbon-

associated form of selenium in the ani-mal’s body and then allow making organic selenium which are readily available when oxidative stress reactions occur.

The main application would be in aqua-culture as fish meal is a main supply of selenium and the development of diets with less fish meal will require compensation of selenium in aquafeed formulae. Such an application could be useful in preventing the oxidation of poly unsaturated fatty acids (PUFA) in fish flesh. Chromium yeast is seldom used in aquaculture diets.

Conclusion Yeast products are getting more fre-

quently used in aquaculture. Some appli-cations are promising as the use as an alternative source of proteins or as a sanitary and welfare enhancer. However many products ranging from crude ethanol yeast by-products to more purified beta-glucans are available on the market. Therefore potential users must accurately select them in function of their targeted application. It is also as important to select a reliable source of the products to ensure a consistency of the supply and the quality.

More InforMatIon:Website: www.yeast-science.com

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