Blood is a highly perishable product and must be processed as soon as possible after slaughter. Blood meal can be prepared by a small-scale operation. Blood meal is hydroscopic and needs to be dried to less than 10-12 % moisture and stored in a dry place in order for it not to deteriorate. There are different ways to prepare blood meal: solar drying, oven drying, drum drying, flash drying, spray drying. The drying method is important because there is an inverse relationship between the amount of heat applied and protein digestibility. Particularly, lysine content and lysine availability decrease when the amount of heat increases (Batterham et al., 1986). Overcooked blood meals are darker, due to destruction of the haemoglobin, and less palatable.
Solar and oven drying
Solar drying is well suited for small-scale operations or when advanced technical equipment is not affordable. Blood is collected in large pans and slowly boiled while stirring constantly. When moisture is sufficiently reduced (10-12 %), blood meal is spread on a clean cemented surface and then sun-dried. It can also be oven-dried. The blood may be spread on milling offals, rice bran or other plant products for better drying and that results in a complete feed.
For large scale operations the 3 processes detailed below are used.
The raw blood is finely comminuted to form a free-flowing slurry that is then deposited onto the descending side of the top of a heated drier drum and formed into a film by one or more spreader rolls. The film is rapidly dried and scraped in the form of a dried sheet which can either be flaked or pulverized to provide a high grade blood meal product. Vapors above the drying cylinder are scrubbed before being released to the atmosphere and represent the only effluents from the process (Overton, 1976).
Ring and flash drying
The blood is dispersed into the high velocity venturi section of the system. The blood first comes into contact with the hot drying airstream and the bulk of the evaporation occurs. The product is then dried as it is conveyed up through the drying column. The presence of a "manifold" or "internal classifier" in the ring drying system is what differentiates it from the flash dryer (GEA, 2009a; GEA, 2009b).
The blood is spray dried as whole blood, or after-separation into plasma and red albumin (GEA, 2010). Blood products have to be dried at low temperatures in order to prevent heat coagulation (GEA, 2009a; GEA, 2009b). Spray dried blood meals are also called spray dried blood powder or blood floor (Dipanjali Konwar et al., 2005).
Spray-dried porcine plasma is prepared as follows: the blood from slaughtered pigs is added to an anticoagulant (generally sodium citrate) and then centrifuged to remove erythrocytes. The plasma obtained is subsequently spray-dried and used for production of animal feeds (van Dijk et al., 2001).
Products resulting from the 3 processes have an overall higher quality than sun-dried blood meals since the duration of the heating period is lower than with cooking. Proteins and amino acids are better preserved and lysine content is higher (Cromwell, 2009).
Blood can be coagulated to aid in the removal of water by adding 1 % unslaked or 3 % slaked lime. However, this method of water removal increases the amount of dry matter losses by 10–15 %, which includes many of the minerals.
In some situations, blood needs to be stored prior to being processed and dried. Raw blood can be stabilized and stored for one week by adding 0.7 % sulphuric acid or an equivalent amount of another acid. A method for preparing blood meal by adding 3 % sulphuric acid and storing for 72 h before sun-drying has been described (Divakaran, 1987; Divakaran et al., 1988).