Hi patetc - tomatoes ripen from the inside out but periods of stressful weather like long dry spells followed by heavy rains, excessive heat over a long time, etc. during the early ripening process can cause nutrient transportation problems within the plant. The end result is sometimes the central core of a tomato becomes tough and remains green or greenish-white. The walls of the fruit can also become pale and seeds and seed cavities remain green in some varieties.

It is also a problem seen when there is insufficient nutrients available for the plants at that crucial time so review your soil needs and planting hole preparation for next year.

You can't control the weather but there are some things you can do to lessen the stress on the plants - heavy mulching helps maintain consistent soil moisture during dry spells, make sure the soil is well-draining if heavy rains are the problem, use soaker hose or drip irrigation systems for more even watering, and time feeding so that you aren't feeding during weather-stress times.

Some research claims that the problem is more common in the older varieties of tomatoes such as you are growing and that some of the newer hybrids have had the problem genetically bred out of them. I can't attest to that as I grow mostly heirloom varieties but you might consider some different varieties (of the thousands out there ;) for next year.

Hope this helps.

Dave