What a nasty surprise on the inside of your tomatoes! Weather conditions, over or underfertilization, or an insect pest could all be your culprit. Chances are the plants were under stress of some kind. In high-stress situations, such as extremely high or low temperatures, defoliation, drought, or underfertilization, tissues can become hard and white. Ocassionally green tissue also forms, says J. W. Scott, Ph.D., professor of horticulture at the University of Florida. An outside possibility is blotchy ripening, a disorder thought to be caused by a potassium deficiency. But this disorder is usually characterized by external and internal areas of the fruit that never ripen.
A southern ripening disorder that somewhat matches your description is the result of sweet potato whitefly (Bemisia tabaci) or silverleaf whitefly (B. argentifolii), Dr. Scott says. The whiteflies introduce a toxin into the fruit that causes irregular ripening. Parts of the skin do not ripen and remain yellow to white, while the inside of the fruit has severe white tissue formation. Sometimes the exterior discoloration disappears but there can still be a lot of white tissue inside, Dr. Scott explains.