It is always a good idea to make a yeast starter solution to introduce yeast to your wine must. There are several reasons to do this, but the most obvious are:
Packets of dry active yeast, averaging 5 grams of yeast, contain hundreds of millions of yeast cells. There are many reasons those yeast cells may not be viable.
The packet may be old. I have seen yeast packets in local homebrew shops that were 7 years old. Never buy an old packet. If it is 2 years old you should not expect more than half the yeast cells to still be viable, especially if the yeast was sitting on a shelf at room temperature.
Yeast should be stored refrigerated, but at the local homebrew shop and at your home. The yeast will be viable many times longer if refrigerated.
Yeast should be transported refrigerated, but usually is not. The metal trailer of a 16-wheeler can get mighty hot in the summer. Even parcel shipped via aircraft ar at risk. I have seen hundreds of parcels sitting on blistering hot tarmacs in June, July, August and September. Yeast are at risk above 105° F. and certainly won't survive long on a hot tarmac.
Even after purchase, yeast are at risk if they are not taken straight home and refrigerated. I, myself, have bought yeast and then stopped to have lunch, leaving the yeast in a hot vehicle interior. After lunch, I've entered the vehicle only to find the interior at an uncomfortable 120+° F. The yeast left in the vehicle had very low viability when used, but making a starter solution with them 24 hours in advance resulted in good fermentations. Here's why.
Yeast bud (reproduce) about every two hours under the right conditions. We create those conditions in the yeast starter solution. When you add the yeast to the starter solution you begin with X number of viable yeast cells, whether only 10 or 150,000,000. Let's call that number, whatever it really is, "1". After 2 hours, that number is approximately "2" (it doubled). After another 2 hours that number is "4" (it doubled again). This doubling can be seen in the chart below:
As you can see, if you husband the starter solution for 24 hours before you intend to add it to your must, you will be adding over 4,000 times as many active yeast cells to the must than you would if you just sprinkled the packet of yeast into the must. If the packet of yeast is old (I have packets of yeast that have been in my refrigerator for 5 years and I expect some of the yeast within to be viable), you can husband the starter for 48 hours, in which case the active yeast will have expanded to 16,777,216 times as many. This is the power of doubling.
Making the Starter Solution
There are many ways to make and husband (care for) a starter solution. Here's one way:
In a sanitized 1-quart mason jar, combine 1/2 cup of preservative free* white grape juice, apple juice or pulp-free orange juice, 1/2 teaspoon of sugar and a pinch** of yeast nutrient. Stir until sugar is dissolved completely and then sprinkle a packet of yeast onto the liquid. Do not stir. The yeast may float or sink. It makes no difference, but it is easier to check the viability of yeast that float.
*Preservative free juice means juice that does not contain sorbate or sorbic acid or benzoate or benzoic acid in the ingredients listed on the label. Sufites and/or ascorbic acid are okay.
**A pinch is just that. Reach into the container of yeast nutrient and pinch a small amount of nutrient between your thumb and forefinger. That amount is sufficient.
Cover the mason jar with a piece of clean linen, paper napkin or paper towel secured with a rubber band. You want air to pass through the covering but want to stop airborne mold, bacteria or dust from entering the jar.
Set a timer for 2 hours and when it rings look at the starter solution. If the yeast are viable, the grains of yeast will have expanded to about twice their original size and turned a light yellowish-gray or yellowish-brown -- similar to the color of Dijon mustard. If not, all the grains of yeast will look exactly as they originally did -- slightly larger perhaps because they absorbed water, but their color will be the same.
If they are not viable, throw them out and start over with another yeast or simply add another packet of yeast to the solution and wait another 2 hours to see if these are viable.
Assuming the yeast are viable, add another 1/2 cup of preservative free white grape juice, apple juice or pulp-free orange juice, 1/2 teaspoon of sugar and a pinch of yeast nutrient. Stir until the sugar is dissolved completely, cover the jar again and reset the timer.
You repeat this procedure every two hours to keep feeding the yeast. You could use water instead of fruit juice, but fruit juice is better because it contains acid, additional nutrients and sugar. Pouring the juice into the starter and stirring to dissolve the sugar puts oxygen in the solution and yeast need oxygen to reproduce. The pouring and stirring every 2 hours recharges the oxygen and keeps the yeast happy. Happy yeast are a good thing.
When you add 1/2 cup of juice every 2 hours the volume increases. We start with a 1-quart jar because at 24 hours we have 6 cups (3 pints) of starter solution to add to the must. The jar will be 3/4 full and that extra airspace above the solution is an oxygen source for the yeast.
Except for watermelon or melon wines, it is a good practice to add the juice from your must at the 16th, 18th, 20th and 22nd hours. In total, you are only adding 2 cups (1 pint) of the must to the starter solution, which acclimates the yeast to the environment you'll be adding them to later. This is only 1/3 of the starter solution's total volume, but usually sufficient. If the must is vastly different than the juice used in the starter solution, you might start adding juice from the must earlier -- say at the 8th hour. I do this for mustang and several other wild grapes, starfruit, pineapple, blueberry, and berries from extreme northern latitudes (e.g. Newfoundland, Scandinavia). If the must-juice being added has been sulfited, so much the better, as it allows the yeast to become accustomed to sulfites and eliminates any potential shock later.
When you use a starter solution you always know your yeast is viable, the quantity of yeast you are adding to the must is several thousand times more than if you just sprinkled a packet of yeast to the must, and you can expect an almost immediate fermentation of your must. It may take another day for a truly vigorous fermentation to develop, but that is much quicker than the 3 days it usually takes.