Berdasarkan uraian pada rujukan di bawah ini, salah satu tahap perkembangan buah biji-bijian adalah biji tsb mengalami apa yg disebut masak susu (milky ripe).

Berdasarkan pengalaman masa kecil saya waktu sekolah di desa dan sering main ke sawah nenek, buah padi juga mengalami tahapan seperti jagung. Ketika buahnya baru muncul dan masih sangat muda, isinya berupa cairan bening.

Pada tahap perkembangan selanjutnya, buah yg berupa cairan bening ini berubah menjadi cairan putih seperti susu.

Selanjutnya, cairan putih susu ini menjadi semakin keras sehingga terbentuklah buah padi yg matang.

Ref.:
The growth cycle of a cereal is divided into ten principal growth stages and each one is further divided into secondary growth stages. The milk stage is one of the principal growth stages in the growth cycle. In this stage of grain development, the grain yields a milky substance when pressed. This milky substance is the starch portion of the grain. The milk stage is also known as milk development or milky ripe.
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Puncture a kernel with your finger to evaluate the juice inside. Harvest the corn when the juicy has a milky appearance. Let it continue growing if the juice appears watery. A thick, pasty texture indicates the corn is too ripe.
https://homeguides.sfgate.com/tell-sweet-corn-ready-harvest-...