How Tempering and Degerming Enhance Germ Recovery in Maize Processing

When maize kernels are ground without any preparatory treatment, the result is a product with poor shelf life, uneven milling, and a high fat content that quickly turns rancid. The tempering-degerming (T-D) method solves all of these problems in a systematic way. It is the backbone of modern maize dry milling, and understanding how it works helps explain why commercially milled maize products look, taste, and last the way they do. This post breaks down the two core stages of the T-D method – tempering and degerming – and traces what happens to the germ, bran, and endosperm once they are separated.

Why the maize kernel needs preparation before milling 🔗

A maize kernel is made up of three main components: the pericarp (bran), the endosperm (the large starchy interior), and the germ (the oil-rich embryo nestled at the base). Each of these components has a different chemical composition, density, and structural role – and that is precisely what makes separating them both necessary and achievable.

The germ contains most of the fat in the kernel. Studies show that oil content in maize germ ranges from 33 to over 50% of its dry weight. If the germ is not removed before milling, that fat remains distributed throughout the flour or meal. According to ScienceDirect, full-fat maize products have a much shorter shelf life than degermed products because the germ oil oxidizes over time, causing rancidity. Research published in Cereal Chemistry confirms that fat rancidity is primarily driven by lipid-degrading enzymes – lipases and lipoxygenases – released from the germ and aleurone during milling, which act on the free oil and rapidly degrade product quality.

The tempering-degerming method addresses this directly. According to cereal technology literature from CUTM, the major objectives of the T-D method are to remove essentially all germ and hull so that the endosperm contains as little fat and fibre as possible, to recover the maximum amount of endosperm as large, clean grits, and to recover the germ as large, pure particles.

Tempering: conditioning the kernel for clean separation 🔗

Tempering – also called conditioning – is the first active step in the T-D process. It involves adding a precise, controlled quantity of water to the cleaned maize and allowing the dampened grain to rest in a temper tank for a set period before any mechanical processing begins.

Applied Milling Systems describes the objective clearly: the goal of tempering is to moisten the outer bran layers for just the right amount of time so that moisture penetrates the bran coat but does not pass through into the endosperm. This differential moisture uptake is the key. The bran becomes pliable and begins to loosen from the endosperm surface, while the endosperm itself remains relatively dry and firm.

What happens inside the kernel during tempering 🔗

As Wikipedia’s entry on dry milling and grain fractionation explains, the germ becomes more flexible and resilient during tempering due to differential swelling, while material does not actually move out of the kernel. The germ swells and toughens, the pericarp softens, and the boundary between germ and endosperm becomes easier to fracture cleanly. This differential swelling is what makes efficient mechanical separation possible at the next stage.

Industrial practice, as documented on ScienceDirect, typically brings moisture content to between 18 and 24% for the degerming process, using either cold or hot water depending on the facility and product target. Moisture level at the degerminator stage is commonly cited at 21-25%. Tempering time is usually kept short – typically 10 to 30 minutes – to prevent the moisture from migrating too deep into the kernel.

Getting this balance right is critical. Too little moisture makes the kernels brittle, causing excessive breakage and dust during mechanical degerming. Too much moisture makes the kernels gummy and difficult to process efficiently. Temperature during conditioning also plays a role, as it governs the rate at which moisture diffuses through the kernel layers.

Degerming: mechanical separation of the kernel fractions 🔗

Once tempering is complete, the conditioned maize enters the degerminator – a machine designed to fracture the kernel in a controlled way that dislodges the germ and bran without crushing them into fine particles.

How the degerminator works 🔗

According to Applied Milling Systems, the degerminator peels the bran layers from the grain while dislodging a large portion of the germ; these components are then removed through the screen of the degerminator itself or in a separate sifting and aspiration step. The largely intact endosperm – now debranned and degerminated – proceeds to subsequent processing.

The most widely used machine for this purpose is the Beall degerminator. The Beall operation separates kernels from the tempering section into two streams – tailings and throughs – and one of its advantages is the ability to adjust the tailgate weight to control residence time and hold back material for more complete processing. The machine uses controlled impact and attrition rather than grinding, which is important: the goal is to crack the kernel along natural structural lines where the germ connects to the endosperm, not to shatter everything indiscriminately.

Post-degerming separation: sifting, aspiration, and gravity tables 🔗

The output from the degerminator is a heterogeneous mixture of germ pieces, bran (pericarp) fragments, endosperm chunks, and fine particles. These need to be sorted into clean product streams. As ScienceDirect describes, the resulting material is sieved, aspirated, gravity-separated, and roller-milled to separate the germ, pericarp, and endosperm pieces of different sizes.

Each separation technique works on a different physical property:

• Sifting (screening) separates particles by size. Coarse endosperm grits and intact germ pieces are separated from finer particles and flour.
• Aspiration uses air flow to exploit differences in terminal velocity. Since aspiration separates by terminal velocity – which is affected by particle size, shape, and density – it is well suited to pulling light bran flakes away from denser endosperm and germ.
• Gravity separation exploits density differences between germ and endosperm. The germ is oily and lighter; endosperm pieces are denser and starchy. Gravity tables tilt and vibrate to cause these components to segregate across the deck surface.

Together, these steps produce distinct product streams: cleaned germ, endosperm grits of various sizes, fine meal, flour, and bran fractions.

Drying and cooling after degerming 🔗

After degerming and separation, the products still carry excess moisture from the tempering step. Industry practice involves drying the degermed stock to approximately 15% moisture content, then cooling and grading the material into fractions of different particle sizes ranging from large hominy grits to fine flour. Cooling before packaging is important – hot products packaged immediately can generate condensation inside the package, creating moisture that promotes spoilage and mould growth.

The recovered fractions and their uses 🔗

The T-D method does not just improve the endosperm products – it recovers valuable co-products from every separated fraction.

The germ fraction 🔗

The germ is the most commercially valuable co-product. Corn germ oil’s main fatty acid is linoleic acid at 55-62%, followed by oleic acid at 22-28%, and it also has a high tocopherol content which gives the oil better oxidative stability than its polyunsaturated fatty acid content would suggest. The extracted oil is used as cooking oil, in food processing, and in industrial applications. The oil-depleted germ cake, which remains after oil extraction, is rich in protein and is used as animal feed.

The endosperm fractions 🔗

Large grits – called flaking grits – are the most commercially prized endosperm product, used in flaked breakfast cereals; smaller grits are used in extruded snacks and brewing; meal fractions go into food mixes and fermentation feedstock; and flour is used in food coatings, breadings, and tortillas. The particle size and fat content of each fraction vary, and milling parameters are adjusted depending on which products are prioritised.

The bran fraction 🔗

Pericarp recovered through aspiration and sifting is primarily used as animal feed and as fibre-enriched food ingredients. It is also being explored as a feedstock for bioethanol production given its high fibre content.

Impact on shelf life and product quality 🔗

Research published via PubMed Central confirms that because of the high fat content in undegermed products, whole or partially degerminated maize products are not particularly shelf-stable, whereas proper degermination results in dry, shelf-stable products with the majority of both germ and fibre removed.

The improvement in shelf life is substantial. A study on degermed maize products found that wholegrain flour from maize varieties stored at 35°C exceeded safe fat acidity levels within 30 days, whereas degermed maize flour remained within acceptable limits even after 90 days of storage. For markets in tropical and developing regions where cold chain infrastructure is limited, this difference is particularly significant.

Product quality benefits go beyond shelf life. When properly degerminated, the larger grits contain only 0.45-0.55% oil; as product size decreases toward flour, oil content rises to 1.5-2.5%. Monitoring fat content in the endosperm fractions is therefore a reliable quality control indicator: if fat content in the overtail product from the degerminator climbs above 2.2-2.5%, it signals that germ separation is incomplete and product stability will be compromised.

The T-D method versus non-degerming systems 🔗

It is worth contrasting the T-D approach with traditional whole-kernel milling to understand what is actually gained. In the non-degerming system, whole maize is ground directly – typically using a stone grinder – producing a meal that retains the germ, bran, and endosperm together. After grinding, some germ and hull can be removed by sifting, but the separation is incomplete and the resulting product has high fibre and fat content. The meal has a richer flavour but deteriorates rapidly.

The T-D method produces cleaner, more consistent products, enables germ oil recovery as a separate high-value stream, and dramatically extends product shelf life. These advantages explain why it has become the industry standard for large-scale maize dry milling worldwide.

What do you think? Given that the germ contains the bulk of maize’s nutritional oil and several bioactive compounds, is it worth exploring ways to reincorporate germ-derived nutrients into degermed flour products – and what processing challenges might that create? And as demand grows for shelf-stable fortified maize products in tropical regions, do you think the T-D method can be made more accessible to small-scale millers without compromising separation efficiency?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC7551338/
2. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/dry-milling
3. https://onlinelibrary.wiley.com/doi/full/10.1002/cche.10750
4. https://courseware.cutm.ac.in/wp-content/uploads/2020/06/Corn-Milling.pdf
5. https://www.appliedmillingsystems.com/cornprocessing
6. https://en.wikipedia.org/wiki/Dry_milling_and_fractionation_of_grain
7. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/corn-germ
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC4260129/
9. https://www.researchgate.net/publication/227809149_Small-scale_production_and_storage_quality_of_dry-milled_degermed_maize_products_for_tropical_countries