Blackberries are classified as aggregate fruits, not true berries. The familiar dark fruit is composed of numerous tiny, individual fruit structures called drupelets, all clustered around a central core. Each of these small, fleshy drupelets contains a single seed. This seed is the hard component that gives the fruit its characteristic slight crunch.
Physical Characteristics of the Seed
Blackberry seeds are small, typically measuring between 3.2 and 4.3 millimeters in length and 1.8 to 3.0 millimeters in width, making them comparable in size to a grain of rice or a poppy seed. When separated from the pulp, the seeds exhibit an oval or slightly teardrop shape, often appearing flattened. Their color ranges from light tan or beige to medium brown, sometimes reaching a dark purple-brown hue depending on the cultivar.
The surface of the seed features a distinct, rough texture. This texture is the result of intricate patterns of minute ridges and valleys known as reticulations or surface sculpturing. These formations on the seed coat are specific enough that they can be used to differentiate between various blackberry cultivars.
Internal Structure and Hardness
The pronounced hardness of the blackberry seed results from its specialized protective outer shell, known botanically as the pyrene. This stony layer is a highly lignified and sclerified endocarp, which is the innermost layer of the drupelet’s fruit wall. The sclerified tissue is composed of cells with thick, tough secondary cell walls, a process called lignification, which provides immense structural rigidity.
This dense encasement protects the delicate plant embryo housed within the seed. The hard shell acts as a physical barrier against mechanical damage from chewing and chemical degradation from stomach acids. This armor ensures the seed can survive being eaten by an animal, pass through the digestive tract, and remain viable for germination after being dispersed. The noticeable crunch when eating a blackberry is the sensation of fracturing this protective shell.
Passage Through the Digestive System
Blackberry seeds are entirely safe to consume, and their journey through the digestive system is a passive one. Due to the exceptionally hard, lignified outer layer, the body’s digestive enzymes are largely unable to break them down. Consequently, the seeds pass through the stomach and intestines mostly intact, surviving the acidic environment and mechanical churning.
The seeds function primarily as a source of insoluble fiber. This type of fiber does not dissolve in water and remains structurally unchanged as it moves through the gastrointestinal tract. By remaining whole, the seeds add bulk to the stool, which helps maintain regular bowel movements and supports digestive efficiency.