Every time an animal is slaughtered, a significant portion of its body – bones, organs, condemned tissue, and other inedible parts – cannot enter the human food chain. Rather than discarding this material, the rendering industry converts it into carcass meal: a concentrated, protein-rich product used in animal feed, organic fertilizers, and several industrial applications. The preparation of carcass meal is a tightly controlled thermal process, and the method chosen – dry or wet rendering – directly determines the quality, safety, and yield of the final product.
Table of Contents
What is carcass meal?
Carcass meal is an umbrella term for rendered products derived from animal by-products unfit for human consumption. The most common forms are meat meal and meat and bone meal (MBM). According to Feedipedia, the distinction between the two is largely based on phosphorus content: when phosphorus exceeds 4.5%, the product is classified as meat and bone meal; below that threshold, it is called meat meal. Wikipedia’s entry on MBM notes a typical composition of 48-52% protein, 33-35% ash, 8-12% fat, and 4-7% moisture.
The raw materials feeding into this process are broad. Franklin Miller’s overview of rendering lists slaughterhouse trimmings, fatty tissue, bones, offal, and even entire carcasses of animals that died on farms or in transit as common inputs. Beef, pork, poultry, and sheep are the most frequently processed species.
Step 1: Collection and size reduction
Carcass meal preparation begins well before any heat is applied. Raw materials – including livestock and poultry carcasses and slaughterhouse wastes – carry high microbial populations and must be handled carefully to prevent cross-contamination. A review in ScienceDirect on pathogen inactivation in rendered products notes that the carcass collection area in a rendering facility has the highest probability of pathogen contamination, making rapid processing essential.
Before cooking, raw materials undergo size reduction. Larger livestock such as cattle and swine are crushed into smaller, uniform pieces, while smaller animals like chickens can be fed directly into the cooker. This grinding step is critical: uniform particle size ensures consistent heat penetration throughout the batch, maximizing both fat extraction and pathogen destruction. On a wet-weight basis, raw livestock carcasses typically contain roughly 60% water, 20% fat, and 20% mineral and protein content – proportions that will shift dramatically after thermal processing.
Step 2: Thermal treatment – rendering and sterilization
Rendering is the core of carcass meal production. The process simultaneously dries the material and separates the fat from the bone and protein, yielding a fat commodity (such as tallow or grease) and a protein meal. There are two principal methods: dry rendering and wet rendering. Both achieve sterilization, but they differ significantly in how they apply heat and what the finished product looks like.
Dry rendering
Dry rendering is currently the dominant method in most modern rendering facilities. The ground raw material is loaded into a steam-jacketed, horizontal cylindrical vessel fitted with an agitator. Crucially, no water or steam is introduced directly into the material – heat transfers indirectly through the vessel walls. As temperatures rise, cellular water evaporates and fats melt and drain away from the solid protein fraction. Continuous agitation prevents burning and promotes even moisture removal.
According to Kansas State University’s carcass disposal review, both dry and wet methods bring the material to a final cooker temperature of 120-135°C (250-275°F), which destroys harmful pathogens and yields usable end products. ScienceDirect’s engineering overview of rendering further describes how the cooked solids – at this stage called cracklings or crax – are then passed through screw presses to squeeze out residual fat, leaving behind a pressed cake. This cake is subsequently dried, ground to a uniform particle size, and packaged as meat and bone meal.
The key advantages of dry rendering include higher protein concentration in the final meal (since no added water dilutes the product) and relatively lower energy demand per unit of finished product. However, temperature control is more critical: without water’s moderating effect, materials can overheat, which degrades heat-sensitive amino acids like lysine.
Wet rendering
In wet rendering, live steam is injected directly into the rendering tank rather than being confined to a surrounding jacket. This pressurized environment acts like a large industrial pressure cooker. The ScienceDirect rendering overview explains that steam penetrates tissues more effectively than dry heat alone, and the combination of heat, moisture, and pressure breaks down cellular structures while achieving thorough pathogen elimination.
After cooking, the resulting slurry of fat, water, and protein solids is separated. The fat rises to a floating phase and is skimmed off, while the protein-rich solids are pressed, dried, and ground into meal. The liquid fraction – called stickwater or tank water – contains dissolved proteins and can be concentrated and added back to the solids before final drying, recovering otherwise lost nutrients. Alfa Laval, a major equipment supplier, notes that modern wet rendering systems can reduce fat levels in protein meal to below 7% and simultaneously boost protein content through improved fat separation.
Despite these benefits, the Kansas State review points out that wet rendering has largely been replaced in many facilities due to high energy consumption, potential loss of meal yield into wastewater (up to 25%), and adverse effects on fat quality. It remains in use where lower-temperature, gentler processing is required for specialty products.
Step 3: Fat separation and meal finishing
Regardless of which rendering method is used, the solids after cooking and pressing go through further finishing steps. The ScienceDirect pathogen review outlines the sequence clearly: the pressed cake is dried to reduce moisture, ground to reduce particle size, stored in structured feed bins or silos, and finally packaged for distribution. The fat stream separated during pressing is sent through a centrifuge to remove any remaining fine solids before being stored as a finished fat product – tallow in the case of ruminant-derived material, or poultry fat from poultry processing.
In the dry rendering continuous system, the USDA’s rendering training module describes how a drainer conveyor first separates liquid fat from the cooked solids, which then move to screw presses for further fat reduction. Solids from both the drainer and the centrifuge are combined on a discharge conveyor before pressing, ensuring maximum fat recovery and a leaner final meal.
Quality parameters of finished carcass meal
The quality of carcass meal varies considerably depending on the raw material composition and the processing conditions applied. Feedipedia reports that protein digestibility in meat and bone meal is normally quite high, ranging from 81 to 87%, making it well-suited for inclusion in monogastric diets. Standard-grade MBM typically runs 48-52% crude protein, with calcium content between 8.8-12% and ash between 28-36%.
The University of Georgia’s Poultry Extension notes that high-quality MBM is guaranteed to contain a minimum of 50% protein, while lower-grade products may carry only a 45% protein guarantee. Processing temperature has a direct negative correlation with lysine availability – a key amino acid for growth – meaning that overheating during dry rendering reduces the nutritional value of the final meal even if protein percentage remains high on paper. This is why temperature monitoring throughout the process is non-negotiable.
Moisture content in the finished meal must be kept below 10% for long-term storage stability. Above this threshold, mold growth and nutrient degradation become significant risks. Properly processed meal should also present a uniform color, granular texture, and minimal offensive odor – sensory indicators that thermal treatment was complete and consistent.
Regulatory context and safe use
Because meat and bone meal was identified as a vector in the spread of bovine spongiform encephalopathy (BSE) in the 1980s and 1990s, its use in animal feed is now tightly regulated in most countries. The European Union banned MBM from livestock feed in 2002 under Regulation (EC) No. 1774/2002. In many regions today, MBM from ruminant sources cannot be fed back to ruminants, though its use in monogastric feeds (pigs, poultry) and aquaculture continues where permitted. Pathogen screening – particularly for Salmonella – remains a critical quality control step, as studies have found Salmonella in a notable share of carcass meal samples taken from rendering plants.
Beyond animal feed, carcass meal also serves as an organic nitrogen fertilizer, valued for its high nitrogen content and slow-release characteristics. This dual-use potential makes it one of the more versatile outputs of the rendering industry, supporting both intensive animal production systems and sustainable crop cultivation.
Dry vs. wet rendering: which method wins?
There is no universally superior method – the right choice depends on the intended product, available infrastructure, and raw material type. Dry rendering produces a higher-protein meal with lower moisture content and is the industry standard for most commercial facilities today. Wet rendering offers gentler processing temperatures and superior fat separation, making it preferable when fat quality is the priority or when processing softer, higher-moisture tissues. In practice, many modern facilities use hybrid approaches or have shifted to continuous dry rendering systems that incorporate elements of both methods to optimise yield, energy efficiency, and product quality.
What do you think? Given that processing temperature directly affects lysine availability in carcass meal, how should feed formulators account for batch-to-batch variability when incorporating MBM into monogastric diets? And as BSE-related regulations continue to evolve, do you think processed animal proteins from non-ruminant sources should be more widely permitted in livestock feeds?
References
- https://en.wikipedia.org/wiki/Meat_and_bone_meal
- https://www.feedipedia.org/node/222
- https://www.franklinmiller.com/applications/rendering
- https://www.sciencedirect.com/science/article/pii/S2666833520300149
- https://krex.k-state.edu/bitstreams/9e813817-b26d-407e-ac13-bd23b2425ccb/download
- https://www.sciencedirect.com/topics/engineering/rendering-process
- https://www.alfalaval.us/industries/food-dairy-and-beverage/food-processing/protein-processing/meat-poultry-processing/wet-rendering-process/
- https://www.aphis.usda.gov/sites/default/files/7-rendering.pdf
- https://poultry.extension.org/articles/feeds-and-feeding-of-poultry/feed-ingredients-for-poultry/animal-by-products-in-poultry-feeds/meat-and-bone-meal-in-poultry-diets/
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