A dog's natural response to food is to try to grab it -- Food on the Ground = mine. As trainers, our job is to modify that response somehow. There are basically two ways to do that -- correction/punishment, and conditioning the response of food on the ground = look at trainer for chance of food from trainer instead.
In the old days, correction was all we did. In the first week of training, we would drop food on the floor in front of the dog, and if he went for it, we would nail him with a leash correction strong enough to (hopefully) make him think twice about doing it again. We changed his response from Food on the Ground = mine to Food on the Ground = something that is better avoided because sometimes when food is on the ground this person goes crazy and snaps my leash.
Leash corrections, like spanking children, are much less acceptable these days, both in dog training circles and in the public eye. For that reason, teaching an alternative response to food on the ground is now (mostly) taught in a more positive way. Trainer sets food on the ground and waits for the dog to take his eyes off the food and look at his trainer instead, and when he does, dog gets praise and a food reward from his trainer. He is never allowed to have the food on the ground -- he will be physically prevented from getting it by the trainer -- but the association we want him to make is Food on the Ground = possibility of food from trainer.
Both of these methods work to an extent. But both generally require a lot of maintenance. The desire to eat food off of the ground is, as I said before, strong, hard-wired, and -- I must point this out -- not wrong to the dog. Being a guide dog requires that the dog set aside its instincts in favor of doing what is important to us, another species. While I think we can all agree that scavenging is a problem that needs to be dealt with if safe guide work is going to happen, I also think a little understanding on our part is called for at the same time.
So! Using either Method #1 (leash correction) or Method #2 (positive reinforcement for choosing to ignore food on the ground), or some combination of both, the trainer ends up with a dog that sees food on the ground and reliably refrains from lunging for it. The dog is matched with a client who cannot see. All of a sudden the dog starts scavenging again. What happened? Wasn't the dog trained not to grab food?
Well... yes. But dogs' behaviors tend to drift back toward what their instincts are telling them to do. Say we have a finished, non-scavenging guide dog who is matched to a client in class. At dinner in class, the client finishes eating, takes his napkin off his lap, stands up, and, in the process of this, a crumb falls off the napkin and right in front of the dog. The client is pushing his chair in, the dog is standing up getting ready to go, and in the middle of this the dog quietly scarfs down the crumb. The client doesn't notice, the instructors don't see, and the dog has just been reinforced for grabbing something off the floor. Or say the class bus is driving into town. The dog is lying quietly at his person's feet. The person sitting next to him is eating chips or something and one of them falls. It lands in front of the dog. Maybe the dog resists for a few seconds because of all of his prior training, but gradually as time goes by and his obedient ignoring of the chip is not being reinforced by his person, who has no idea what's happening because he can't see, the dog delicately extends his neck and eats the chip. Reinforced again. In the real world this happens all the time, and there is nothing that can be done about it. It is a fact that if you can't see, you won't necessarily know that there is food on the ground. It is also a fact that some dogs learn to be very sneaky about getting food. They know that big, obvious lunges result in leash corrections, but they also learn that small, silent movements will either not be noticed or will be ignored (because the handler will read those movements as the dog shifting position or something else innocuous), and will modify behavior accordingly.
So what can be done about scavenging in the real world? I hate to say it, and no one will like this answer, but in the real world, if you have a dedicated scavenger, I really think the only way that you can keep the scavenging behavior to a minimum is by doing real-world setups. No one likes to do these. They are a pain and also there is a feeling that you're supposed to have a trained dog already and it shouldn't do these kinds of things. But suppose you have the most awesome guide dog in the world. Always gets you where you're going safely, works like a dream, pace and pull perfectly matched to you, doesn't care about any other distractions... but is a scavenger. Would you trade that super guide for a mediocre guide, or one that is not well-matched to you, but doesn't scavenge? Or would you rather keep this guide and put a little bit of work into the scavenging? Or would you rather keep this guide, not work on the behavior at all, and just manage it with a Gentle Leader, which will physically prevent it from getting the food most of the time, unless you're at a restaurant and the food is under the table where the dog is; in that case the dog will just eat it and you can't stop them. That last option is one that many people choose in the real world, and I can't say I blame them. I'm not sure that's not the option that I would choose, myself. It all depends which form of management you find more of a pain -- the setups, or the Gentle Leader combined with the certainty that sometimes the dog will still be able to scavenge with the Gentle Leader on. No judgment here, I promise!
The key to setups is that they have to be realistic. I will venture a guess that all or almost all dogs that have made it through a reputable guide dog program will understand that if they see you put a piece of food on the table at nose level, they had better then be pretty careful because something is up. They will know that you're paying close attention to them and will sit there like little angels with all their attention on you. ("Who, me? Lunge for food? No way. See me looking at you like a very good dog?") It's when you're not expecting the food on the ground that the dog can catch you unawares. There's never food on the ground coming up to the first intersection on your daily route -- except for the day that someone has dropped half a sandwich on the ground at the ramp, and it's down your dog's throat before you even know what happened. Those are the kinds of places that you have to put food if you want to do a real-world setup. The key is that you have to know exactly where the food is so you can be aware when the dog first notices it. So, for example, you could have your friend throw down an open bag with chips spilling out of it five steps to the left after you descend the stairs from your apartment building. (It has to be in a place where it will be obvious to you that the dog is trying to go for it, and where you will have time to react appropriately, otherwise it will backfire if the food is so close that the dog gets away with eating it.) When you know the food is in place, you head out with the dog and either give it a firm leash correction as soon as you feel the dog notice the food and begin to head in that direction, or else you verbally "drive" the dog past the food and then, once it has passed, praise the dog and give it treats yourself, whichever method you choose to use. How often you have to do this and how many different variations you come up with really depends on your dog's level of interest in food.
Which brings me to another point. Dogs have varying levels of interest in food. While it is very true that Labs overall have a very high level of interest in food, I have also worked with a very small number of Labs who were not interested in food. I have worked with shepherds who view food (sometimes even the food in their own bowl that I just gave them!) with suspicion, as if the world was trying to poison them with every piece of food that comes across their path. I have worked with dogs who, when scolded once for trying to get a piece of food on the floor, have never tried again, not with me or with a client who can't see. I have seen dogs who will lie under a table with a piece of bread inches from their nose for the duration of a meal and not try to grab it. If you have one of these dogs, enjoy it while you have it, but don't expect another one. I personally think this type of dog is unnatural -- which is not to say I don't enjoy it when I get one of them in my string! The much more typical dog response to food on the ground is to grab it. So don't feel bad if you are doing everything you can, and are being as consistent a handler as you possibly can, and your dog still tries to grab food when it gets a chance. Setups will help, but even they won't fix the behavior completely. If any school only put out dogs that never ever went for food on the ground in any situation, people would be waiting two or three years for a dog, or maybe even longer than that.
I guess the point of writing this is to say that, while of course we will all continue to valiantly fight the good fight of conditioning alternative responses to food on the ground, and maintaining those conditioned responses with clients in the field, sometimes instinct will win. Sometimes the dog is just going to be a dog and get the food no matter what we do. The most important thing is that the scavenging doesn't prevent the dog from being a safe guide. If the dog will lunge into the street when cars are moving or bang the handler into a pole diving for something on the ground, then scavenging has moved beyond the level of annoyance and into a safety issue.