Some (many) bread machines don't mix well, so you might not get a good mixture whatever you do. But it's worth trying.
If you are making the bread immediately after adding the ingredients, it probably doesn't matter much how you add the ingredients; just put the yeast and the sugar and such in the centre.
If you use the timer, it is important that the yeast stay dry up until the moment the machine starts mixing. What gave me the best result is this:
- add water, sugar, salt, egg, fat;
- mix this by moving the pan around until the ingredients are mostly dissolved (or stir with a wooden implement);
- then put all the flour in a heap in the middle of the pan;
- make a hollow in the centre of this heap, put the yeast in this hollow;
- then set the timer.
Another tip: if you put the pan on a kitchen scale while adding ingredients, and you press "tare" on the scale after each ingredient, it's much easier to weigh everything. This only applies if you use ingredients by weight, of course.
Lastly, experimenting with amounts that differ from the recipe may greatly improve your bread. It depends on the machine. I also found that the "French" setting on the machine (= longer baking) gave the best crust.
There are many "natural" or "clean label" ingredients that work as mold inhibitors. Most of them work by adding some acid and thereby creating a more inhospitable environment for mold. Some of the popular mold inhibitors (at least is some U.S. bakeries) are: cultured whey, vinegar, and raisin juice.
I think vinegar and raisin juice are good options for home use.
Vinegar
Works by adding Acetic acid. The white vinegar you buy in the grocery store is typically 5% acetic acid (at least in the U.S.).
To inhibit mold, the recommended amount is 0.5-2.0% of the flour weight of 5% acetic acid vinegar. Add with your other liquids.
Raisin Juice Concentrate
Raisin juice concentrate has been shown to be effective at retarding mold and bacteria growth in bread. It works by adding Propionic acid and Tartaric acid. Further, it seems that a home baker should be able to purchase this in reasonable quantities and at a reasonable price.
The recommended amount is 5-10% of flour weight, by weight. Add with your other liquids.
My personal experience is positive: I used to purchase a whole-wheat bread with raisin juice concentrate and it lasted forever.
This study from the Journal of Food Science, Application of raisin extracts as preservatives in liquid bread and bread systems, found very dramatic increase in shelf life.
The mean mold-free shelf life of the bread containing 7.5% water extract [of raisins] was 18.1 +/- 3.3 d at room temperature while the negative control was mold free for 9.4 +/- 2.4 d. The antifungal efficacy of the extracts in bread was equivalent to 0.24% calcium propionate in 21 d of storage. Doubling the concentration of the extract did not improve the mold-retarding property in bread. The bread containing raisin paste, the percentage of which in dough was equivalent to 15% raisin extract, exhibited a stronger antifungal activity than did the extracts in bread.
Notes
Both of these can inhibit yeast activity which you may need to compensate for by increasing yeast or proofing time.
Sources: some personal experience, this really interesting article from Oklahoma State University, Clean Label Mold Inhibitors for Baking, and the study linked above.
Best Answer
In baking, doubling sometimes is accurate, sometimes not. Yeah, that is a non-answer, but is true. It is not unsafe to double, or halve, but sometimes results will vary. You hit a point which could cause a slight issue, even distribution. Changing volume of a recipe with an ingredient which is distributed over the surface might affect the distribution ratio, though in this case I think it would likely be minor. Other issues can arise in doubling would be if the dough needs to breath. Thermal reactions can often affect results when doubling or halving recipes as well. The cooking time adjustments are often obvious, but cooling can be less obvious but disastrous at times, and any resting times needed may be skewed.
All of that said, with allowing for baking time, and a little experimentation, most recipes can be safely doubled or halved, but tend to start breaking down beyond that, going for tripled, 4 times, etc tends to get progressively poor results without a lot of trial and error. It is certainly worth a try though. And alternate or fallback if you do not like the results of doubling is to make two separate batches, which you know will be correct.